diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/ErrorReport.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/ErrorReport.java
index 71059260d..930c7fe48 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/ErrorReport.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/ErrorReport.java
@@ -13,6 +13,9 @@ import java.util.function.Function;
 import java.util.logging.Level;
 import java.util.stream.IntStream;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.Bukkit;
 import org.bukkit.Location;
 import org.bukkit.plugin.Plugin;
@@ -44,6 +47,7 @@ public class ErrorReport<T extends Throwable> {
     private T throwable;
     private File file;
 
+    @ParametersAreNonnullByDefault
     public ErrorReport(T throwable, SlimefunAddon addon, Consumer<PrintStream> printer) {
         this.throwable = throwable;
         this.addon = addon;
@@ -51,6 +55,7 @@ public class ErrorReport<T extends Throwable> {
         Slimefun.runSync(() -> print(printer));
     }
 
+    @ParametersAreNonnullByDefault
     public ErrorReport(T throwable, Location l, SlimefunItem item) {
         this(throwable, item.getAddon(), stream -> {
             stream.println("Block Info:");
@@ -83,6 +88,7 @@ public class ErrorReport<T extends Throwable> {
         });
     }
 
+    @ParametersAreNonnullByDefault
     public ErrorReport(T throwable, SlimefunItem item) {
         this(throwable, item.getAddon(), stream -> {
             stream.println("SlimefunItem:");
@@ -97,6 +103,7 @@ public class ErrorReport<T extends Throwable> {
      * 
      * @return The {@link File} for this {@link ErrorReport}
      */
+    @Nonnull
     public File getFile() {
         return file;
     }
@@ -106,6 +113,7 @@ public class ErrorReport<T extends Throwable> {
      * 
      * @return The {@link Throwable}
      */
+    @Nonnull
     public T getThrown() {
         return throwable;
     }
@@ -119,7 +127,7 @@ public class ErrorReport<T extends Throwable> {
         return count;
     }
 
-    private void print(Consumer<PrintStream> printer) {
+    private void print(@Nonnull Consumer<PrintStream> printer) {
         this.file = getNewFile();
         count++;
 
@@ -180,7 +188,7 @@ public class ErrorReport<T extends Throwable> {
         }
     }
 
-    private static void scanPlugins(List<String> plugins, List<String> addons) {
+    private static void scanPlugins(@Nonnull List<String> plugins, @Nonnull List<String> addons) {
         String dependency = "Slimefun";
 
         for (Plugin plugin : Bukkit.getPluginManager().getPlugins()) {
@@ -201,6 +209,7 @@ public class ErrorReport<T extends Throwable> {
         }
     }
 
+    @Nonnull
     private static File getNewFile() {
         String path = "plugins/Slimefun/error-reports/" + dateFormat.format(LocalDateTime.now());
         File newFile = new File(path + ".err");
@@ -215,7 +224,7 @@ public class ErrorReport<T extends Throwable> {
         return newFile;
     }
 
-    public static void tryCatch(Function<Exception, ErrorReport<Exception>> function, Runnable runnable) {
+    public static void tryCatch(@Nonnull Function<Exception, ErrorReport<Exception>> function, @Nonnull Runnable runnable) {
         try {
             runnable.run();
         }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunAddon.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunAddon.java
index a8bb04e3e..610791493 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunAddon.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunAddon.java
@@ -5,6 +5,10 @@ import java.util.Locale;
 import java.util.logging.Logger;
 import java.util.stream.Collectors;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
+import org.apache.commons.lang.Validate;
 import org.bukkit.NamespacedKey;
 import org.bukkit.plugin.Plugin;
 import org.bukkit.plugin.PluginDescriptionFile;
@@ -35,6 +39,7 @@ public interface SlimefunAddon {
      * 
      * @return The instance of your {@link JavaPlugin}
      */
+    @Nonnull
     JavaPlugin getJavaPlugin();
 
     /**
@@ -42,6 +47,7 @@ public interface SlimefunAddon {
      * 
      * @return The URL for this Plugin's Bug Tracker, or null
      */
+    @Nullable
     String getBugTrackerURL();
 
     /**
@@ -50,6 +56,7 @@ public interface SlimefunAddon {
      * 
      * @return The Name of this {@link SlimefunAddon}
      */
+    @Nonnull
     default String getName() {
         return getJavaPlugin().getName();
     }
@@ -60,6 +67,7 @@ public interface SlimefunAddon {
      * 
      * @return The version of this {@link SlimefunAddon}
      */
+    @Nonnull
     default String getPluginVersion() {
         return getJavaPlugin().getDescription().getVersion();
     }
@@ -70,6 +78,7 @@ public interface SlimefunAddon {
      * 
      * @return The {@link Logger} of this {@link SlimefunAddon}
      */
+    @Nonnull
     default Logger getLogger() {
         return getJavaPlugin().getLogger();
     }
@@ -85,7 +94,9 @@ public interface SlimefunAddon {
      * 
      * @return Whether this {@link SlimefunAddon} depends on the given {@link Plugin}
      */
-    default boolean hasDependency(String dependency) {
+    default boolean hasDependency(@Nonnull String dependency) {
+        Validate.notNull(dependency, "The dependency cannot be null");
+
         // Well... it cannot depend on itself but you get the idea.
         if (getJavaPlugin().getName().equalsIgnoreCase(dependency)) {
             return true;
@@ -101,6 +112,7 @@ public interface SlimefunAddon {
      * 
      * @return A {@link Collection} of every {@link Category} from this addon
      */
+    @Nonnull
     default Collection<Category> getCategories() {
         String namespace = getJavaPlugin().getName().toLowerCase(Locale.ROOT);
         return SlimefunPlugin.getRegistry().getCategories().stream().filter(cat -> cat.getKey().getNamespace().equals(namespace)).collect(Collectors.toList());
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunBranch.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunBranch.java
index 75b7ec483..1ce4e654e 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunBranch.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/SlimefunBranch.java
@@ -1,5 +1,9 @@
 package io.github.thebusybiscuit.slimefun4.api;
 
+import javax.annotation.Nonnull;
+
+import org.apache.commons.lang.Validate;
+
 import io.github.thebusybiscuit.slimefun4.utils.PatternUtils;
 
 /**
@@ -40,7 +44,8 @@ public enum SlimefunBranch {
     private final String name;
     private final boolean official;
 
-    SlimefunBranch(String name, boolean official) {
+    SlimefunBranch(@Nonnull String name, boolean official) {
+        Validate.notNull(name, "The branch name cannot be null");
         this.name = name;
         this.official = official;
 
@@ -55,6 +60,7 @@ public enum SlimefunBranch {
      * 
      * @return The name of this {@link SlimefunBranch}
      */
+    @Nonnull
     public String getName() {
         return name;
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IdConflictException.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IdConflictException.java
index da75b5c55..b7fb9aceb 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IdConflictException.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IdConflictException.java
@@ -1,5 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.api.exceptions;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import me.mrCookieSlime.Slimefun.Objects.SlimefunItem.SlimefunItem;
 
 /**
@@ -21,6 +23,7 @@ public class IdConflictException extends RuntimeException {
      * @param item2
      *            The second {@link SlimefunItem} with this id
      */
+    @ParametersAreNonnullByDefault
     public IdConflictException(SlimefunItem item1, SlimefunItem item2) {
         super("Two items have conflicting ids: " + item1.toString() + " and " + item2.toString());
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IncompatibleItemHandlerException.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IncompatibleItemHandlerException.java
index ff4ccd753..e91845bc5 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IncompatibleItemHandlerException.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/IncompatibleItemHandlerException.java
@@ -1,5 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.api.exceptions;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.plugin.Plugin;
 
 import io.github.thebusybiscuit.slimefun4.core.handlers.BlockUseHandler;
@@ -35,6 +37,7 @@ public class IncompatibleItemHandlerException extends RuntimeException {
      * @param handler
      *            The {@link ItemHandler} which someone tried to add
      */
+    @ParametersAreNonnullByDefault
     public IncompatibleItemHandlerException(String message, SlimefunItem item, ItemHandler handler) {
         super("The item handler type: \"" + handler.getIdentifier().getSimpleName() + "\" is not compatible with " + item + " (" + message + ')');
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/MissingDependencyException.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/MissingDependencyException.java
index d20c7b57a..d88ef290e 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/MissingDependencyException.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/MissingDependencyException.java
@@ -1,5 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.api.exceptions;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import io.github.thebusybiscuit.slimefun4.api.SlimefunAddon;
 
 /**
@@ -24,6 +26,7 @@ public class MissingDependencyException extends RuntimeException {
      * @param dependency
      *            The dependency that is required ("Slimefun")
      */
+    @ParametersAreNonnullByDefault
     public MissingDependencyException(SlimefunAddon addon, String dependency) {
         super("Slimefun Addon \"" + addon.getName() + "\" forgot to define \"" + dependency + "\" as a depend or softdepend inside the plugin.yml file");
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/PrematureCodeException.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/PrematureCodeException.java
index d02c773aa..1918fdb5d 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/PrematureCodeException.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/PrematureCodeException.java
@@ -1,5 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.api.exceptions;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import io.github.thebusybiscuit.slimefun4.api.SlimefunAddon;
 
 /**
@@ -20,6 +22,7 @@ public class PrematureCodeException extends RuntimeException {
      * @param message
      *            An error message to display
      */
+    @ParametersAreNonnullByDefault
     public PrematureCodeException(String message) {
         super("Slimefun code was invoked before Slimefun finished loading: " + message);
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/UnregisteredItemException.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/UnregisteredItemException.java
index 1ac0de5f4..e8e5d7a58 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/UnregisteredItemException.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/UnregisteredItemException.java
@@ -1,5 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.api.exceptions;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.plugin.Plugin;
 
 import me.mrCookieSlime.Slimefun.Objects.SlimefunItem.SlimefunItem;
@@ -25,6 +27,7 @@ public class UnregisteredItemException extends RuntimeException {
      * @param item
      *            The {@link SlimefunItem} that was affected by this
      */
+    @ParametersAreNonnullByDefault
     public UnregisteredItemException(SlimefunItem item) {
         super(item.toString() + " has not been registered yet.");
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/WrongItemStackException.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/WrongItemStackException.java
index b853a16e2..3167c7de7 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/WrongItemStackException.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/exceptions/WrongItemStackException.java
@@ -1,5 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.api.exceptions;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.inventory.ItemStack;
 
 import io.github.thebusybiscuit.slimefun4.core.attributes.DamageableItem;
@@ -29,6 +31,7 @@ public class WrongItemStackException extends RuntimeException {
      * @param message
      *            An error message to display
      */
+    @ParametersAreNonnullByDefault
     public WrongItemStackException(String message) {
         super("You probably wanted to alter a different ItemStack: " + message);
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/geo/ResourceManager.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/geo/ResourceManager.java
index 896f133ab..8a47e6745 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/geo/ResourceManager.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/geo/ResourceManager.java
@@ -7,6 +7,8 @@ import java.util.Locale;
 import java.util.OptionalInt;
 import java.util.concurrent.ThreadLocalRandom;
 
+import javax.annotation.Nonnull;
+
 import org.apache.commons.lang.Validate;
 import org.bukkit.Bukkit;
 import org.bukkit.ChatColor;
@@ -26,7 +28,6 @@ import io.github.thebusybiscuit.slimefun4.implementation.items.geo.GEOMiner;
 import io.github.thebusybiscuit.slimefun4.implementation.items.geo.GEOScanner;
 import io.github.thebusybiscuit.slimefun4.utils.ChestMenuUtils;
 import io.github.thebusybiscuit.slimefun4.utils.HeadTexture;
-import io.github.thebusybiscuit.slimefun4.utils.SlimefunUtils;
 import me.mrCookieSlime.CSCoreLibPlugin.general.Inventory.ChestMenu;
 import me.mrCookieSlime.Slimefun.api.BlockStorage;
 
@@ -46,7 +47,7 @@ public class ResourceManager {
     private final int[] backgroundSlots = { 0, 1, 2, 3, 5, 6, 7, 8, 9, 17, 18, 26, 27, 35, 36, 44, 45, 46, 48, 49, 50, 52, 53 };
     private final Config config;
 
-    public ResourceManager(SlimefunPlugin plugin) {
+    public ResourceManager(@Nonnull SlimefunPlugin plugin) {
         config = new Config(plugin, "resources.yml");
     }
 
@@ -57,7 +58,7 @@ public class ResourceManager {
      * @param resource
      *            The {@link GEOResource} to register
      */
-    void register(GEOResource resource) {
+    void register(@Nonnull GEOResource resource) {
         Validate.notNull(resource, "Cannot register null as a GEO-Resource");
         Validate.notNull(resource.getKey(), "GEO-Resources must have a NamespacedKey which is not null");
 
@@ -94,7 +95,7 @@ public class ResourceManager {
      * 
      * @return An {@link OptionalInt}, either empty or containing the amount of the given {@link GEOResource}
      */
-    public OptionalInt getSupplies(GEOResource resource, World world, int x, int z) {
+    public OptionalInt getSupplies(@Nonnull GEOResource resource, @Nonnull World world, int x, int z) {
         Validate.notNull(resource, "Cannot get supplies for null");
         Validate.notNull(world, "World must not be null");
 
@@ -109,7 +110,7 @@ public class ResourceManager {
         }
     }
 
-    public void setSupplies(GEOResource resource, World world, int x, int z, int value) {
+    public void setSupplies(@Nonnull GEOResource resource, @Nonnull World world, int x, int z, int value) {
         Validate.notNull(resource, "Cannot set supplies for null");
         Validate.notNull(world, "World cannot be null");
 
@@ -117,7 +118,7 @@ public class ResourceManager {
         BlockStorage.setChunkInfo(world, x, z, key, String.valueOf(value));
     }
 
-    private int generate(GEOResource resource, World world, int x, int z) {
+    private int generate(@Nonnull GEOResource resource, @Nonnull World world, int x, int z) {
         Validate.notNull(resource, "Cannot generate resources for null");
         Validate.notNull(world, "World cannot be null");
 
@@ -157,7 +158,7 @@ public class ResourceManager {
      * @param page
      *            The page to display
      */
-    public void scan(Player p, Block block, int page) {
+    public void scan(@Nonnull Player p, @Nonnull Block block, int page) {
         if (SlimefunPlugin.getGPSNetwork().getNetworkComplexity(p.getUniqueId()) < 600) {
             SlimefunPlugin.getLocalization().sendMessages(p, "gps.insufficient-complexity", true, msg -> msg.replace("%complexity%", "600"));
             return;
@@ -172,7 +173,7 @@ public class ResourceManager {
             menu.addItem(slot, ChestMenuUtils.getBackground(), ChestMenuUtils.getEmptyClickHandler());
         }
 
-        menu.addItem(4, new CustomItem(SlimefunUtils.getCustomHead(HeadTexture.MINECRAFT_CHUNK.getTexture()), ChatColor.YELLOW + SlimefunPlugin.getLocalization().getResourceString(p, "tooltips.chunk"), "", "&8\u21E8 &7" + SlimefunPlugin.getLocalization().getResourceString(p, "tooltips.world") + ": " + block.getWorld().getName(), "&8\u21E8 &7X: " + x + " Z: " + z), ChestMenuUtils.getEmptyClickHandler());
+        menu.addItem(4, new CustomItem(HeadTexture.MINECRAFT_CHUNK.getAsItemStack(), ChatColor.YELLOW + SlimefunPlugin.getLocalization().getResourceString(p, "tooltips.chunk"), "", "&8\u21E8 &7" + SlimefunPlugin.getLocalization().getResourceString(p, "tooltips.world") + ": " + block.getWorld().getName(), "&8\u21E8 &7X: " + x + " Z: " + z), ChestMenuUtils.getEmptyClickHandler());
         List<GEOResource> resources = new ArrayList<>(SlimefunPlugin.getRegistry().getGEOResources().values());
         Collections.sort(resources, (a, b) -> a.getName(p).toLowerCase(Locale.ROOT).compareTo(b.getName(p).toLowerCase(Locale.ROOT)));
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/GPSNetwork.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/GPSNetwork.java
index 9fd78d6b1..d57d1bb20 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/GPSNetwork.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/GPSNetwork.java
@@ -7,6 +7,9 @@ import java.util.Map;
 import java.util.Set;
 import java.util.UUID;
 
+import javax.annotation.Nonnull;
+
+import org.apache.commons.lang.Validate;
 import org.bukkit.Bukkit;
 import org.bukkit.ChatColor;
 import org.bukkit.Location;
@@ -66,7 +69,7 @@ public class GPSNetwork {
      * @param online
      *            Whether that {@link GPSTransmitter} is online
      */
-    public void updateTransmitter(Location l, UUID uuid, boolean online) {
+    public void updateTransmitter(@Nonnull Location l, @Nonnull UUID uuid, boolean online) {
         Set<Location> set = transmitters.computeIfAbsent(uuid, id -> new HashSet<>());
 
         if (online) {
@@ -87,7 +90,7 @@ public class GPSNetwork {
      * 
      * @return The network complexity for that {@link UUID}
      */
-    public int getNetworkComplexity(UUID uuid) {
+    public int getNetworkComplexity(@Nonnull UUID uuid) {
         if (!transmitters.containsKey(uuid)) {
             return 0;
         }
@@ -113,7 +116,7 @@ public class GPSNetwork {
      * 
      * @return The amount of transmitters
      */
-    public int countTransmitters(UUID uuid) {
+    public int countTransmitters(@Nonnull UUID uuid) {
         if (!transmitters.containsKey(uuid)) {
             return 0;
         }
@@ -122,7 +125,7 @@ public class GPSNetwork {
         }
     }
 
-    public void openTransmitterControlPanel(Player p) {
+    public void openTransmitterControlPanel(@Nonnull Player p) {
         ChestMenu menu = new ChestMenu(ChatColor.BLUE + SlimefunPlugin.getLocalization().getMessage(p, "machines.GPS_CONTROL_PANEL.title"));
 
         for (int slot : border) {
@@ -179,7 +182,8 @@ public class GPSNetwork {
      * 
      * @return An icon for this waypoint
      */
-    public ItemStack getIcon(String name, Environment environment) {
+    @Nonnull
+    public ItemStack getIcon(@Nonnull String name, @Nonnull Environment environment) {
         if (name.startsWith("player:death ")) {
             return HeadTexture.DEATHPOINT.getAsItemStack();
         }
@@ -194,7 +198,7 @@ public class GPSNetwork {
         }
     }
 
-    public void openWaypointControlPanel(Player p) {
+    public void openWaypointControlPanel(@Nonnull Player p) {
         PlayerProfile.get(p, profile -> {
             ChestMenu menu = new ChestMenu(ChatColor.BLUE + SlimefunPlugin.getLocalization().getMessage(p, "machines.GPS_CONTROL_PANEL.title"));
 
@@ -246,7 +250,10 @@ public class GPSNetwork {
      * @param l
      *            The {@link Location} of the new waypoint
      */
-    public void createWaypoint(Player p, Location l) {
+    public void createWaypoint(@Nonnull Player p, @Nonnull Location l) {
+        Validate.notNull(p, "Player cannot be null!");
+        Validate.notNull(l, "Waypoint Location cannot be null!");
+
         PlayerProfile.get(p, profile -> {
             if ((profile.getWaypoints().size() + 2) > inventory.length) {
                 SlimefunPlugin.getLocalization().sendMessage(p, "gps.waypoint.max", true);
@@ -270,7 +277,11 @@ public class GPSNetwork {
      * @param l
      *            The {@link Location} of this waypoint
      */
-    public void addWaypoint(Player p, String name, Location l) {
+    public void addWaypoint(@Nonnull Player p, @Nonnull String name, @Nonnull Location l) {
+        Validate.notNull(p, "Player cannot be null!");
+        Validate.notNull(name, "Waypoint name cannot be null!");
+        Validate.notNull(l, "Waypoint Location cannot be null!");
+
         PlayerProfile.get(p, profile -> {
             if ((profile.getWaypoints().size() + 2) > inventory.length) {
                 SlimefunPlugin.getLocalization().sendMessage(p, "gps.waypoint.max", true);
@@ -309,7 +320,8 @@ public class GPSNetwork {
      * 
      * @return A {@link Set} with all {@link Location Locations} of transmitters for this {@link UUID}
      */
-    public Set<Location> getTransmitters(UUID uuid) {
+    @Nonnull
+    public Set<Location> getTransmitters(@Nonnull UUID uuid) {
         return transmitters.getOrDefault(uuid, new HashSet<>());
     }
 
@@ -319,6 +331,7 @@ public class GPSNetwork {
      * 
      * @return The {@link TeleportationManager} for this {@link GPSNetwork}
      */
+    @Nonnull
     public TeleportationManager getTeleportationManager() {
         return teleportation;
     }
@@ -329,6 +342,7 @@ public class GPSNetwork {
      * 
      * @return The {@link ResourceManager} for this {@link GPSNetwork}
      */
+    @Nonnull
     public ResourceManager getResourceManager() {
         return resourceManager;
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/TeleportationManager.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/TeleportationManager.java
index a455639a5..bfa60c7d9 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/TeleportationManager.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/TeleportationManager.java
@@ -4,6 +4,10 @@ import java.util.HashSet;
 import java.util.Set;
 import java.util.UUID;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.Bukkit;
 import org.bukkit.ChatColor;
 import org.bukkit.Location;
@@ -31,6 +35,7 @@ public final class TeleportationManager {
     private final int[] teleporterBorder = { 0, 1, 2, 3, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 26, 27, 35, 36, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53 };
     private final int[] teleporterInventory = { 19, 20, 21, 22, 23, 24, 25, 28, 29, 30, 31, 32, 33, 34, 37, 38, 39, 40, 41, 42, 43 };
 
+    @ParametersAreNonnullByDefault
     public void openTeleporterGUI(Player p, UUID uuid, Block b, int complexity) {
         if (teleporterUsers.contains(p.getUniqueId())) {
             return;
@@ -78,6 +83,7 @@ public final class TeleportationManager {
         });
     }
 
+    @ParametersAreNonnullByDefault
     public void teleport(UUID uuid, int complexity, Location source, Location destination, boolean resistance) {
         teleporterUsers.add(uuid);
 
@@ -85,6 +91,7 @@ public final class TeleportationManager {
         updateProgress(uuid, Math.max(1, 100 / time), 0, source, destination, resistance);
     }
 
+    @ParametersAreNonnullByDefault
     public int getTeleportationTime(int complexity, Location source, Location destination) {
         if (complexity < 100) return 100;
 
@@ -92,6 +99,7 @@ public final class TeleportationManager {
         return 1 + Math.min(4 * distanceSquared(source, destination) / speed, 40);
     }
 
+    @ParametersAreNonnullByDefault
     private int distanceSquared(Location source, Location destination) {
         if (source.getWorld().getUID().equals(destination.getWorld().getUID())) {
             int distance = (int) source.distanceSquared(destination);
@@ -102,11 +110,11 @@ public final class TeleportationManager {
         }
     }
 
-    private boolean isValid(Player p, Location source) {
+    private boolean isValid(@Nullable Player p, @Nonnull Location source) {
         return p != null && p.isValid() && p.getWorld().getUID().equals(source.getWorld().getUID()) && p.getLocation().distanceSquared(source) < 2.0;
     }
 
-    private void cancel(UUID uuid, Player p) {
+    private void cancel(@Nonnull UUID uuid, @Nullable Player p) {
         teleporterUsers.remove(uuid);
 
         if (p != null) {
@@ -114,6 +122,7 @@ public final class TeleportationManager {
         }
     }
 
+    @ParametersAreNonnullByDefault
     private void updateProgress(UUID uuid, int speed, int progress, Location source, Location destination, boolean resistance) {
         Player p = Bukkit.getPlayer(uuid);
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/Waypoint.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/Waypoint.java
index 58ed699c6..d1e9407c1 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/Waypoint.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/gps/Waypoint.java
@@ -2,6 +2,9 @@ package io.github.thebusybiscuit.slimefun4.api.gps;
 
 import java.util.Objects;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.apache.commons.lang.Validate;
 import org.bukkit.Location;
 import org.bukkit.entity.Player;
@@ -31,6 +34,7 @@ public class Waypoint {
     private final String name;
     private final Location location;
 
+    @ParametersAreNonnullByDefault
     public Waypoint(PlayerProfile profile, String id, Location l, String name) {
         Validate.notNull(profile, "Profile must never be null!");
         Validate.notNull(id, "id must never be null!");
@@ -43,18 +47,22 @@ public class Waypoint {
         this.name = name;
     }
 
+    @Nonnull
     public PlayerProfile getOwner() {
         return profile;
     }
 
+    @Nonnull
     public String getId() {
         return id;
     }
 
+    @Nonnull
     public String getName() {
         return name;
     }
 
+    @Nonnull
     public Location getLocation() {
         return location;
     }
@@ -63,6 +71,7 @@ public class Waypoint {
         return name.startsWith("player:death ");
     }
 
+    @Nonnull
     public ItemStack getIcon() {
         return SlimefunPlugin.getGPSNetwork().getIcon(name, location.getWorld().getEnvironment());
     }
@@ -79,7 +88,6 @@ public class Waypoint {
         }
 
         Waypoint waypoint = (Waypoint) obj;
-
         return profile.getUUID().equals(waypoint.getOwner().getUUID()) && id.equals(waypoint.getId()) && location.equals(waypoint.getLocation()) && name.equals(waypoint.getName());
     }
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/HashedArmorpiece.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/HashedArmorpiece.java
index a3bbe0185..826da2058 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/HashedArmorpiece.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/HashedArmorpiece.java
@@ -2,6 +2,9 @@ package io.github.thebusybiscuit.slimefun4.api.items;
 
 import java.util.Optional;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 import org.bukkit.Material;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
@@ -48,7 +51,7 @@ public final class HashedArmorpiece {
      * @param item
      *            The {@link SlimefunItem} corresponding to the provided {@link ItemStack}, may be null
      */
-    public void update(ItemStack stack, SlimefunItem item) {
+    public void update(@Nullable ItemStack stack, @Nullable SlimefunItem item) {
         if (stack == null || stack.getType() == Material.AIR) {
             this.hash = 0;
         }
@@ -76,7 +79,7 @@ public final class HashedArmorpiece {
      *            The {@link ItemStack} to compare
      * @return Whether the {@link HashedArmorpiece} and the given {@link ItemStack} mismatch
      */
-    public boolean hasDiverged(ItemStack stack) {
+    public boolean hasDiverged(@Nullable ItemStack stack) {
         if (stack == null || stack.getType() == Material.AIR) {
             return hash != 0;
         }
@@ -95,13 +98,14 @@ public final class HashedArmorpiece {
      * 
      * @return An {@link Optional} describing the result
      */
+    @Nonnull
     public Optional<SlimefunArmorPiece> getItem() {
         return item;
     }
 
     @Override
     public String toString() {
-        return "HashedArmorpiece {hash=" + hash + ",item=" + item.map(SlimefunItem::getID).orElse(null) + '}';
+        return "HashedArmorpiece {hash=" + hash + ",item=" + item.map(SlimefunItem::getID).orElse("null") + '}';
     }
 
 }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/ItemSetting.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/ItemSetting.java
index a406199c6..ce98a6a35 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/ItemSetting.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/items/ItemSetting.java
@@ -2,6 +2,9 @@ package io.github.thebusybiscuit.slimefun4.api.items;
 
 import java.util.logging.Level;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.apache.commons.lang.Validate;
 
 import io.github.thebusybiscuit.cscorelib2.config.Config;
@@ -33,6 +36,7 @@ public class ItemSetting<T> {
      * @param defaultValue
      *            The default value for this {@link ItemSetting}
      */
+    @ParametersAreNonnullByDefault
     public ItemSetting(String key, T defaultValue) {
         Validate.notNull(key, "The key of an ItemSetting is not allowed to be null!");
         Validate.notNull(defaultValue, "The default value of an ItemSetting is not allowed to be null!");
@@ -62,7 +66,7 @@ public class ItemSetting<T> {
      * @param newValue
      *            The new value for this {@link ItemSetting}
      */
-    public void update(T newValue) {
+    public void update(@Nonnull T newValue) {
         if (validateInput(newValue)) {
             this.value = newValue;
         }
@@ -78,6 +82,7 @@ public class ItemSetting<T> {
      * 
      * @return The key under which this setting is stored (relative to the {@link SlimefunItem})
      */
+    @Nonnull
     public String getKey() {
         return key;
     }
@@ -87,6 +92,7 @@ public class ItemSetting<T> {
      * 
      * @return The current value
      */
+    @Nonnull
     public T getValue() {
         Validate.notNull(value, "An ItemSetting was invoked but was not initialized yet.");
 
@@ -98,6 +104,7 @@ public class ItemSetting<T> {
      * 
      * @return The default value
      */
+    @Nonnull
     public T getDefaultValue() {
         return defaultValue;
     }
@@ -109,7 +116,7 @@ public class ItemSetting<T> {
      *            The class of data type you want to compare
      * @return Whether this {@link ItemSetting} stores the given type
      */
-    public boolean isType(Class<?> c) {
+    public boolean isType(@Nonnull Class<?> c) {
         return c.isInstance(defaultValue);
     }
 
@@ -121,7 +128,7 @@ public class ItemSetting<T> {
      *            The {@link SlimefunItem} who called this method
      */
     @SuppressWarnings("unchecked")
-    public void load(SlimefunItem item) {
+    public void load(@Nonnull SlimefunItem item) {
         SlimefunPlugin.getItemCfg().setDefaultValue(item.getID() + '.' + getKey(), getDefaultValue());
         Object configuredValue = SlimefunPlugin.getItemCfg().getValue(item.getID() + '.' + getKey());
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerBackpack.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerBackpack.java
index c7a5f5cf2..581198d6e 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerBackpack.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerBackpack.java
@@ -2,6 +2,8 @@ package io.github.thebusybiscuit.slimefun4.api.player;
 
 import java.io.File;
 
+import javax.annotation.Nonnull;
+
 import org.bukkit.Bukkit;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.Inventory;
@@ -41,7 +43,7 @@ public class PlayerBackpack {
      * @param id
      *            The id of this Backpack
      */
-    public PlayerBackpack(PlayerProfile profile, int id) {
+    public PlayerBackpack(@Nonnull PlayerProfile profile, int id) {
         this(profile, id, profile.getConfig().getInt("backpacks." + id + ".size"));
 
         for (int i = 0; i < size; i++) {
@@ -59,7 +61,7 @@ public class PlayerBackpack {
      * @param size
      *            The size of this Backpack
      */
-    public PlayerBackpack(PlayerProfile profile, int id, int size) {
+    public PlayerBackpack(@Nonnull PlayerProfile profile, int id, int size) {
         if (size < 9 || size > 54 || size % 9 != 0) {
             throw new IllegalArgumentException("Invalid size! Size must be one of: [9, 18, 27, 36, 45, 54]");
         }
@@ -89,6 +91,7 @@ public class PlayerBackpack {
      * 
      * @return The owning {@link PlayerProfile}
      */
+    @Nonnull
     public PlayerProfile getOwner() {
         return profile;
     }
@@ -107,6 +110,7 @@ public class PlayerBackpack {
      * 
      * @return The {@link Inventory} of this {@link PlayerBackpack}
      */
+    @Nonnull
     public Inventory getInventory() {
         return inventory;
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerProfile.java b/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerProfile.java
index 0184fb95c..ac30d55ff 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerProfile.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/api/player/PlayerProfile.java
@@ -15,6 +15,9 @@ import java.util.function.Consumer;
 import java.util.logging.Level;
 import java.util.stream.IntStream;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 import org.apache.commons.lang.Validate;
 import org.bukkit.Bukkit;
 import org.bukkit.ChatColor;
@@ -72,7 +75,7 @@ public final class PlayerProfile {
 
     private final HashedArmorpiece[] armor = { new HashedArmorpiece(), new HashedArmorpiece(), new HashedArmorpiece(), new HashedArmorpiece() };
 
-    private PlayerProfile(OfflinePlayer p) {
+    private PlayerProfile(@Nonnull OfflinePlayer p) {
         this.uuid = p.getUniqueId();
         this.name = p.getName();
 
@@ -105,6 +108,7 @@ public final class PlayerProfile {
      * 
      * @return The cached armor for this {@link Player}
      */
+    @Nonnull
     public HashedArmorpiece[] getArmor() {
         return armor;
     }
@@ -115,6 +119,7 @@ public final class PlayerProfile {
      * 
      * @return The {@link Config} associated with this {@link PlayerProfile}
      */
+    @Nonnull
     public Config getConfig() {
         return configFile;
     }
@@ -124,6 +129,7 @@ public final class PlayerProfile {
      * 
      * @return The {@link UUID} of our {@link PlayerProfile}
      */
+    @Nonnull
     public UUID getUUID() {
         return uuid;
     }
@@ -169,7 +175,7 @@ public final class PlayerProfile {
      * @param unlock
      *            Whether the {@link Research} should be unlocked or locked
      */
-    public void setResearched(Research research, boolean unlock) {
+    public void setResearched(@Nonnull Research research, boolean unlock) {
         Validate.notNull(research, "Research must not be null!");
         dirty = true;
 
@@ -190,7 +196,7 @@ public final class PlayerProfile {
      *            The {@link Research} that is being queried
      * @return Whether this {@link Research} has been unlocked
      */
-    public boolean hasUnlocked(Research research) {
+    public boolean hasUnlocked(@Nullable Research research) {
         if (research == null) {
             // No Research, no restriction
             return true;
@@ -204,6 +210,7 @@ public final class PlayerProfile {
      * 
      * @return A {@code Hashset<Research>} of all Researches this {@link Player} has unlocked
      */
+    @Nonnull
     public Set<Research> getResearches() {
         return ImmutableSet.copyOf(researches);
     }
@@ -214,6 +221,7 @@ public final class PlayerProfile {
      * 
      * @return A {@link List} containing every {@link Waypoint}
      */
+    @Nonnull
     public List<Waypoint> getWaypoints() {
         return ImmutableList.copyOf(waypoints);
     }
@@ -225,7 +233,7 @@ public final class PlayerProfile {
      * @param waypoint
      *            The {@link Waypoint} to add
      */
-    public void addWaypoint(Waypoint waypoint) {
+    public void addWaypoint(@Nonnull Waypoint waypoint) {
         Validate.notNull(waypoint, "Cannot add a 'null' waypoint!");
 
         for (Waypoint wp : waypoints) {
@@ -250,7 +258,7 @@ public final class PlayerProfile {
      * @param waypoint
      *            The {@link Waypoint} to remove
      */
-    public void removeWaypoint(Waypoint waypoint) {
+    public void removeWaypoint(@Nonnull Waypoint waypoint) {
         Validate.notNull(waypoint, "Cannot remove a 'null' waypoint!");
 
         if (waypoints.remove(waypoint)) {
@@ -274,6 +282,7 @@ public final class PlayerProfile {
         dirty = true;
     }
 
+    @Nonnull
     public PlayerBackpack createBackpack(int size) {
         IntStream stream = IntStream.iterate(0, i -> i + 1).filter(i -> !configFile.contains("backpacks." + i + ".size"));
         int id = stream.findFirst().getAsInt();
@@ -284,6 +293,7 @@ public final class PlayerProfile {
         return backpack;
     }
 
+    @Nonnull
     public Optional<PlayerBackpack> getBackpack(int id) {
         if (id < 0) {
             throw new IllegalArgumentException("Backpacks cannot have negative ids!");
@@ -303,6 +313,7 @@ public final class PlayerProfile {
         return Optional.empty();
     }
 
+    @Nonnull
     public String getTitle() {
         List<String> titles = SlimefunPlugin.getRegistry().getResearchRanks();
 
@@ -312,7 +323,7 @@ public final class PlayerProfile {
         return titles.get(index);
     }
 
-    public void sendStats(CommandSender sender) {
+    public void sendStats(@Nonnull CommandSender sender) {
         Set<Research> unlockedResearches = getResearches();
         int levels = unlockedResearches.stream().mapToInt(Research::getCost).sum();
         int allResearches = SlimefunPlugin.getRegistry().getResearches().size();
@@ -333,6 +344,7 @@ public final class PlayerProfile {
      * 
      * @return The {@link Player} of this {@link PlayerProfile} or null
      */
+    @Nullable
     public Player getPlayer() {
         return Bukkit.getPlayer(getUUID());
     }
@@ -343,11 +355,12 @@ public final class PlayerProfile {
      * 
      * @return The {@link GuideHistory} of this {@link Player}
      */
+    @Nonnull
     public GuideHistory getGuideHistory() {
         return guideHistory;
     }
 
-    public static boolean fromUUID(UUID uuid, Consumer<PlayerProfile> callback) {
+    public static boolean fromUUID(@Nonnull UUID uuid, @Nonnull Consumer<PlayerProfile> callback) {
         return get(Bukkit.getOfflinePlayer(uuid), callback);
     }
 
@@ -361,7 +374,7 @@ public final class PlayerProfile {
      * 
      * @return If the {@link OfflinePlayer} was cached or not.
      */
-    public static boolean get(OfflinePlayer p, Consumer<PlayerProfile> callback) {
+    public static boolean get(@Nonnull OfflinePlayer p, @Nonnull Consumer<PlayerProfile> callback) {
         Validate.notNull(p, "Cannot get a PlayerProfile for: null!");
 
         UUID uuid = p.getUniqueId();
@@ -390,7 +403,7 @@ public final class PlayerProfile {
      * 
      * @return Whether the {@link PlayerProfile} was already loaded
      */
-    public static boolean request(OfflinePlayer p) {
+    public static boolean request(@Nonnull OfflinePlayer p) {
         if (!SlimefunPlugin.getRegistry().getPlayerProfiles().containsKey(p.getUniqueId())) {
             // Should probably prevent multiple requests for the same profile in the future
             Bukkit.getScheduler().runTaskAsynchronously(SlimefunPlugin.instance(), () -> {
@@ -414,15 +427,17 @@ public final class PlayerProfile {
      * 
      * @return An {@link Optional} describing the result
      */
-    public static Optional<PlayerProfile> find(OfflinePlayer p) {
+    @Nonnull
+    public static Optional<PlayerProfile> find(@Nonnull OfflinePlayer p) {
         return Optional.ofNullable(SlimefunPlugin.getRegistry().getPlayerProfiles().get(p.getUniqueId()));
     }
 
+    @Nonnull
     public static Iterator<PlayerProfile> iterator() {
         return SlimefunPlugin.getRegistry().getPlayerProfiles().values().iterator();
     }
 
-    public static void getBackpack(ItemStack item, Consumer<PlayerBackpack> callback) {
+    public static void getBackpack(@Nullable ItemStack item, @Nonnull Consumer<PlayerBackpack> callback) {
         if (item == null || !item.hasItemMeta() || !item.getItemMeta().hasLore()) {
             return;
         }
@@ -451,9 +466,8 @@ public final class PlayerProfile {
         }
     }
 
-    public boolean hasFullProtectionAgainst(ProtectionType type) {
+    public boolean hasFullProtectionAgainst(@Nonnull ProtectionType type) {
         int armorCount = 0;
-
         NamespacedKey setId = null;
 
         for (HashedArmorpiece armorpiece : armor) {
@@ -461,10 +475,8 @@ public final class PlayerProfile {
 
             if (!armorPiece.isPresent()) {
                 setId = null;
-                continue;
             }
-
-            if (armorPiece.get() instanceof ProtectiveArmor) {
+            else if (armorPiece.get() instanceof ProtectiveArmor) {
                 ProtectiveArmor protectedArmor = (ProtectiveArmor) armorPiece.get();
 
                 if (setId == null && protectedArmor.isFullSetRequired()) {
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/FlexCategory.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/FlexCategory.java
index c8e178e7a..8f6494a84 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/FlexCategory.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/FlexCategory.java
@@ -2,6 +2,9 @@ package io.github.thebusybiscuit.slimefun4.core.categories;
 
 import java.util.List;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.NamespacedKey;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
@@ -23,10 +26,12 @@ import me.mrCookieSlime.Slimefun.Objects.SlimefunItem.SlimefunItem;
  */
 public abstract class FlexCategory extends Category {
 
+    @ParametersAreNonnullByDefault
     public FlexCategory(NamespacedKey key, ItemStack item) {
         this(key, item, 3);
     }
 
+    @ParametersAreNonnullByDefault
     public FlexCategory(NamespacedKey key, ItemStack item, int tier) {
         super(key, item, tier);
     }
@@ -44,6 +49,7 @@ public abstract class FlexCategory extends Category {
      * 
      * @return Whether to display this {@link FlexCategory}
      */
+    @ParametersAreNonnullByDefault
     public abstract boolean isVisible(Player p, PlayerProfile profile, SlimefunGuideLayout layout);
 
     /**
@@ -61,7 +67,7 @@ public abstract class FlexCategory extends Category {
     public abstract void open(Player p, PlayerProfile profile, SlimefunGuideLayout layout);
 
     @Override
-    public final boolean isHidden(Player p) {
+    public final boolean isHidden(@Nonnull Player p) {
         // We can stop this method right here.
         // We provide a custom method with more parameters for this. See isVisible(...)
         return false;
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/LockedCategory.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/LockedCategory.java
index 61f86fc15..97a529e8b 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/LockedCategory.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/LockedCategory.java
@@ -6,6 +6,9 @@ import java.util.List;
 import java.util.Set;
 import java.util.logging.Level;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.apache.commons.lang.Validate;
 import org.bukkit.NamespacedKey;
 import org.bukkit.entity.Player;
@@ -46,6 +49,7 @@ public class LockedCategory extends Category {
      *            The parent categories for this category
      * 
      */
+    @ParametersAreNonnullByDefault
     public LockedCategory(NamespacedKey key, ItemStack item, NamespacedKey... parents) {
         this(key, item, 3, parents);
     }
@@ -63,6 +67,7 @@ public class LockedCategory extends Category {
      *            The parent categories for this category
      * 
      */
+    @ParametersAreNonnullByDefault
     public LockedCategory(NamespacedKey key, ItemStack item, int tier, NamespacedKey... parents) {
         super(key, item, tier);
         Validate.noNullElements(parents, "A LockedCategory must not have any 'null' parents!");
@@ -101,6 +106,7 @@ public class LockedCategory extends Category {
      * @see #addParent(Category)
      * @see #removeParent(Category)
      */
+    @Nonnull
     public Set<Category> getParents() {
         return parents;
     }
@@ -131,7 +137,7 @@ public class LockedCategory extends Category {
      * @see #getParents()
      * @see #addParent(Category)
      */
-    public void removeParent(Category category) {
+    public void removeParent(@Nonnull Category category) {
         parents.remove(category);
     }
 
@@ -144,7 +150,7 @@ public class LockedCategory extends Category {
      *            The {@link PlayerProfile} that belongs to the given {@link Player}
      * @return Whether the {@link Player} has fully completed all parent categories, otherwise false
      */
-    public boolean hasUnlocked(Player p, PlayerProfile profile) {
+    public boolean hasUnlocked(@Nonnull Player p, @Nonnull PlayerProfile profile) {
         for (Category category : parents) {
             for (SlimefunItem item : category.getItems()) {
                 // Should probably be replaced with Slimefun.hasUnlocked(...)
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/SeasonalCategory.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/SeasonalCategory.java
index dcb3cc516..538b0aaed 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/SeasonalCategory.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/categories/SeasonalCategory.java
@@ -3,6 +3,10 @@ package io.github.thebusybiscuit.slimefun4.core.categories;
 import java.time.LocalDate;
 import java.time.Month;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
+import org.apache.commons.lang.Validate;
 import org.bukkit.NamespacedKey;
 import org.bukkit.entity.Player;
 import org.bukkit.inventory.ItemStack;
@@ -34,8 +38,10 @@ public class SeasonalCategory extends Category {
      * @param item
      *            The display item for this category
      */
+    @ParametersAreNonnullByDefault
     public SeasonalCategory(NamespacedKey key, Month month, int tier, ItemStack item) {
         super(key, item, tier);
+        Validate.notNull(month, "The Month cannot be null");
 
         this.month = month;
     }
@@ -45,12 +51,13 @@ public class SeasonalCategory extends Category {
      * 
      * @return the {@link Month} in which this {@link SeasonalCategory} appears
      */
+    @Nonnull
     public Month getMonth() {
         return month;
     }
 
     @Override
-    public boolean isHidden(Player p) {
+    public boolean isHidden(@Nonnull Player p) {
         // Hide this Category if the month differs
         if (month != LocalDate.now().getMonth()) {
             return true;
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/commands/SlimefunTabCompleter.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/commands/SlimefunTabCompleter.java
index 13cc27ebf..dcbce8ff5 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/commands/SlimefunTabCompleter.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/commands/SlimefunTabCompleter.java
@@ -8,6 +8,7 @@ import java.util.List;
 import java.util.Locale;
 
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 
 import org.bukkit.command.Command;
 import org.bukkit.command.CommandSender;
@@ -27,6 +28,7 @@ class SlimefunTabCompleter implements TabCompleter {
         this.command = command;
     }
 
+    @Nullable
     @Override
     public List<String> onTabComplete(CommandSender sender, Command cmd, String label, String[] args) {
         if (args.length == 1) {
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/ContributionsConnector.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/ContributionsConnector.java
index 7aea16847..c28756c7c 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/ContributionsConnector.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/ContributionsConnector.java
@@ -6,6 +6,9 @@ import java.util.List;
 import java.util.Map;
 import java.util.logging.Level;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import kong.unirest.JsonNode;
 import kong.unirest.json.JSONArray;
 import kong.unirest.json.JSONObject;
@@ -38,6 +41,7 @@ class ContributionsConnector extends GitHubConnector {
 
     private boolean finished = false;
 
+    @ParametersAreNonnullByDefault
     ContributionsConnector(GitHubService github, String prefix, int page, String repository, String role) {
         super(github, repository);
 
@@ -58,6 +62,7 @@ class ContributionsConnector extends GitHubConnector {
     @Override
     public void onSuccess(JsonNode element) {
         finished = true;
+
         if (element.isArray()) {
             computeContributors(element.getArray());
         }
@@ -81,7 +86,7 @@ class ContributionsConnector extends GitHubConnector {
         return "/contributors?per_page=100&page=" + page;
     }
 
-    private void computeContributors(JSONArray array) {
+    private void computeContributors(@Nonnull JSONArray array) {
         for (int i = 0; i < array.length(); i++) {
             JSONObject object = array.getJSONObject(i);
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/Contributor.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/Contributor.java
index 371a223e9..d48bbf8e5 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/Contributor.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/Contributor.java
@@ -42,7 +42,7 @@ public class Contributor {
 
     public Contributor(@Nonnull String username, @Nonnull String profile) {
         Validate.notNull(username, "Username must never be null!");
-        Validate.notNull(profile, "The profile link must never be null!");
+        Validate.notNull(profile, "The profile cannot be null!");
 
         githubUsername = profile.substring(profile.lastIndexOf('/') + 1);
         minecraftUsername = username;
@@ -70,6 +70,7 @@ public class Contributor {
      *
      * @return the name of this contributor
      */
+    @Nonnull
     public String getName() {
         return githubUsername;
     }
@@ -80,6 +81,7 @@ public class Contributor {
      *
      * @return The MC username of this contributor.
      */
+    @Nonnull
     public String getMinecraftName() {
         return minecraftUsername;
     }
@@ -89,10 +91,12 @@ public class Contributor {
      *
      * @return The GitHub profile of this {@link Contributor}
      */
+    @Nullable
     public String getProfile() {
         return profileLink;
     }
 
+    @Nonnull
     public List<Map.Entry<String, Integer>> getContributions() {
         List<Map.Entry<String, Integer>> list = new ArrayList<>(contributions.entrySet());
         list.sort(Comparator.comparingInt(entry -> -entry.getValue()));
@@ -107,7 +111,7 @@ public class Contributor {
      *            The role for which to count the contributions.
      * @return The amount of contributions this {@link Contributor} submitted as the given role
      */
-    public int getContributions(String role) {
+    public int getContributions(@Nonnull String role) {
         return contributions.getOrDefault(role, 0);
     }
 
@@ -127,6 +131,7 @@ public class Contributor {
      * 
      * @return The {@link UUID} of this {@link Contributor}
      */
+    @Nonnull
     public Optional<UUID> getUniqueId() {
         return uuid;
     }
@@ -138,6 +143,7 @@ public class Contributor {
      * 
      * @return A Base64-Head Texture
      */
+    @Nonnull
     public String getTexture() {
         if (!headTexture.isComputed() || !headTexture.isPresent()) {
             GitHubService github = SlimefunPlugin.getGitHubService();
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubConnector.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubConnector.java
index 0120fe455..2b563500e 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubConnector.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubConnector.java
@@ -10,6 +10,7 @@ import java.nio.charset.StandardCharsets;
 import java.util.logging.Level;
 
 import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
 
 import kong.unirest.HttpResponse;
 import kong.unirest.JsonNode;
@@ -26,6 +27,7 @@ abstract class GitHubConnector {
     protected String repository;
     protected final GitHubService github;
 
+    @ParametersAreNonnullByDefault
     public GitHubConnector(GitHubService github, String repository) {
         this.github = github;
         this.repository = repository;
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubIssuesTracker.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubIssuesTracker.java
index c572e20af..4266e3c32 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubIssuesTracker.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubIssuesTracker.java
@@ -2,6 +2,8 @@ package io.github.thebusybiscuit.slimefun4.core.services.github;
 
 import java.util.logging.Level;
 
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import kong.unirest.JsonNode;
 import kong.unirest.json.JSONArray;
 import kong.unirest.json.JSONObject;
@@ -11,6 +13,7 @@ class GitHubIssuesTracker extends GitHubConnector {
 
     private final IssuesTrackerConsumer callback;
 
+    @ParametersAreNonnullByDefault
     GitHubIssuesTracker(GitHubService github, String repository, IssuesTrackerConsumer callback) {
         super(github, repository);
         this.callback = callback;
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubService.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubService.java
index 6853ee218..e2d85b40c 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubService.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/github/GitHubService.java
@@ -10,6 +10,7 @@ import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ConcurrentMap;
 
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 
 import io.github.thebusybiscuit.cscorelib2.config.Config;
 import io.github.thebusybiscuit.slimefun4.core.services.localization.Translators;
@@ -77,6 +78,7 @@ public class GitHubService {
         contributors.put(name, contributor);
     }
 
+    @Nonnull
     public Contributor addContributor(@Nonnull String minecraftName, @Nonnull String profileURL, @Nonnull String role, int commits) {
         String username = profileURL.substring(profileURL.lastIndexOf('/') + 1);
 
@@ -128,6 +130,7 @@ public class GitHubService {
         });
     }
 
+    @Nonnull
     protected Set<GitHubConnector> getConnectors() {
         return connectors;
     }
@@ -141,6 +144,7 @@ public class GitHubService {
      * 
      * @return A {@link ConcurrentMap} containing all {@link Contributor Contributors}
      */
+    @Nonnull
     public ConcurrentMap<String, Contributor> getContributors() {
         return contributors;
     }
@@ -177,6 +181,7 @@ public class GitHubService {
      * 
      * @return The id of our GitHub Repository
      */
+    @Nonnull
     public String getRepository() {
         return repository;
     }
@@ -195,6 +200,7 @@ public class GitHubService {
      * 
      * @return A {@link LocalDateTime} object representing the date and time of the latest commit
      */
+    @Nonnull
     public LocalDateTime getLastUpdate() {
         return lastUpdate;
     }
@@ -222,6 +228,7 @@ public class GitHubService {
         texturesCache.save();
     }
 
+    @Nullable
     protected String getCachedTexture(@Nonnull String name) {
         return texturesCache.getString(name);
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Language.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Language.java
index a37bb397b..d7a937ba4 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Language.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Language.java
@@ -2,6 +2,9 @@ package io.github.thebusybiscuit.slimefun4.core.services.localization;
 
 import java.util.Locale;
 
+import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
+
 import org.apache.commons.lang.Validate;
 import org.bukkit.Server;
 import org.bukkit.configuration.file.FileConfiguration;
@@ -42,7 +45,7 @@ public final class Language {
      * @param hash
      *            The hash of the skull texture to use
      */
-    public Language(String id, String hash) {
+    public Language(@Nonnull String id, @Nonnull String hash) {
         Validate.notNull(id, "A Language must have an id that is not null!");
         Validate.notNull(hash, "A Language must have a texture that is not null!");
 
@@ -57,6 +60,7 @@ public final class Language {
      * 
      * @return The identifier of this {@link Language}
      */
+    @Nonnull
     public String getId() {
         return id;
     }
@@ -81,51 +85,56 @@ public final class Language {
         }
     }
 
+    @Nullable
     FileConfiguration getMessagesFile() {
         return messages;
     }
 
+    @Nullable
     FileConfiguration getResearchesFile() {
         return researches;
     }
 
+    @Nullable
     FileConfiguration getResourcesFile() {
         return resources;
     }
 
+    @Nullable
     FileConfiguration getCategoriesFile() {
         return categories;
     }
 
+    @Nullable
     FileConfiguration getRecipeTypesFile() {
         return recipeTypes;
     }
 
-    public void setMessagesFile(FileConfiguration config) {
+    public void setMessagesFile(@Nonnull FileConfiguration config) {
         Validate.notNull(config);
 
         this.messages = config;
     }
 
-    public void setResearchesFile(FileConfiguration config) {
+    public void setResearchesFile(@Nonnull FileConfiguration config) {
         Validate.notNull(config);
 
         this.researches = config;
     }
 
-    public void setResourcesFile(FileConfiguration config) {
+    public void setResourcesFile(@Nonnull FileConfiguration config) {
         Validate.notNull(config);
 
         this.resources = config;
     }
 
-    public void setCategoriesFile(FileConfiguration config) {
+    public void setCategoriesFile(@Nonnull FileConfiguration config) {
         Validate.notNull(config);
 
         this.categories = config;
     }
 
-    public void setRecipeTypesFile(FileConfiguration config) {
+    public void setRecipeTypesFile(@Nonnull FileConfiguration config) {
         Validate.notNull(config);
 
         this.recipeTypes = config;
@@ -137,6 +146,7 @@ public final class Language {
      * 
      * @return The {@link ItemStack} used to display this {@link Language}
      */
+    @Nonnull
     public ItemStack getItem() {
         return item;
     }
@@ -149,7 +159,8 @@ public final class Language {
      *            The {@link Player} to localize the name for
      * @return The localized name of this {@link Language}
      */
-    public String getName(Player p) {
+    @Nonnull
+    public String getName(@Nonnull Player p) {
         String name = SlimefunPlugin.getLocalization().getMessage(p, "languages." + id);
         return name != null ? name : toString();
     }
@@ -169,6 +180,7 @@ public final class Language {
         return "Language {id= " + id + ", default=" + isDefault() + " }";
     }
 
+    @Nonnull
     public FileConfiguration[] getFiles() {
         return new FileConfiguration[] { getMessagesFile(), getCategoriesFile(), getResearchesFile(), getResourcesFile() };
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SlimefunLocalization.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SlimefunLocalization.java
index bad6b2579..1f765a0fd 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SlimefunLocalization.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SlimefunLocalization.java
@@ -6,6 +6,7 @@ import java.util.List;
 import java.util.function.UnaryOperator;
 
 import javax.annotation.Nonnull;
+import javax.annotation.Nullable;
 
 import org.bukkit.ChatColor;
 import org.bukkit.Keyed;
@@ -47,8 +48,10 @@ public abstract class SlimefunLocalization extends Localization implements Keyed
      * 
      * @param id
      *            The language code
+     * 
      * @return A {@link Language} with the given id or null
      */
+    @Nullable
     public abstract Language getLanguage(@Nonnull String id);
 
     /**
@@ -56,6 +59,7 @@ public abstract class SlimefunLocalization extends Localization implements Keyed
      * 
      * @param p
      *            The {@link Player} to query
+     * 
      * @return The {@link Language} that was selected by the given {@link Player}
      */
     public abstract Language getLanguage(@Nonnull Player p);
@@ -75,6 +79,7 @@ public abstract class SlimefunLocalization extends Localization implements Keyed
      * 
      * @return A {@link Collection} that contains every installed {@link Language}
      */
+    @Nonnull
     public abstract Collection<Language> getLanguages();
 
     protected abstract void addLanguage(@Nonnull String id, @Nonnull String texture);
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SupportedLanguage.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SupportedLanguage.java
index 8630567ec..704ef7125 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SupportedLanguage.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/SupportedLanguage.java
@@ -1,5 +1,8 @@
 package io.github.thebusybiscuit.slimefun4.core.services.localization;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.inventory.ItemStack;
 
 /**
@@ -59,12 +62,14 @@ enum SupportedLanguage {
     private final boolean releaseReady;
     private final String textureHash;
 
+    @ParametersAreNonnullByDefault
     SupportedLanguage(String id, boolean releaseReady, String textureHash) {
         this.id = id;
         this.releaseReady = releaseReady;
         this.textureHash = textureHash;
     }
 
+    @Nonnull
     public String getLanguageId() {
         return id;
     }
@@ -73,6 +78,7 @@ enum SupportedLanguage {
         return releaseReady;
     }
 
+    @Nonnull
     public String getTexture() {
         return textureHash;
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Translators.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Translators.java
index 27006d207..cd03ff1d4 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Translators.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/localization/Translators.java
@@ -1,5 +1,8 @@
 package io.github.thebusybiscuit.slimefun4.core.services.localization;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import io.github.thebusybiscuit.slimefun4.core.services.github.Contributor;
 import io.github.thebusybiscuit.slimefun4.core.services.github.GitHubService;
 
@@ -17,7 +20,7 @@ public class Translators {
     private final GitHubService github;
 
     // We maybe should switch to a json file in our resources folder at some point.
-    public Translators(GitHubService github) {
+    public Translators(@Nonnull GitHubService github) {
         this.github = github;
 
         // Translators - German
@@ -164,10 +167,12 @@ public class Translators {
         addTranslator("FaolanMalcadh", SupportedLanguage.PORTUGUESE_BRAZIL, true);
     }
 
+    @ParametersAreNonnullByDefault
     private void addTranslator(String name, SupportedLanguage lang, boolean lock) {
         addTranslator(name, name, lang, lock);
     }
 
+    @ParametersAreNonnullByDefault
     private void addTranslator(String username, String minecraftName, SupportedLanguage lang, boolean lock) {
         Contributor contributor = github.addContributor(minecraftName, "https://github.com/" + username, "translator," + lang.getLanguageId(), 0);
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/ClearLagHook.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/ClearLagHook.java
index 49b008dbc..3722fcfd6 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/ClearLagHook.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/ClearLagHook.java
@@ -2,6 +2,8 @@ package io.github.thebusybiscuit.slimefun4.core.services.plugins;
 
 import java.util.Iterator;
 
+import javax.annotation.Nonnull;
+
 import org.bukkit.entity.Entity;
 import org.bukkit.entity.Item;
 import org.bukkit.event.EventHandler;
@@ -13,7 +15,7 @@ import me.minebuilders.clearlag.events.EntityRemoveEvent;
 
 class ClearLagHook implements Listener {
 
-    ClearLagHook(SlimefunPlugin plugin) {
+    ClearLagHook(@Nonnull SlimefunPlugin plugin) {
         plugin.getServer().getPluginManager().registerEvents(this, plugin);
     }
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/EmeraldEnchantsCategory.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/EmeraldEnchantsCategory.java
index dfab32455..ecd2fd7b0 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/EmeraldEnchantsCategory.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/EmeraldEnchantsCategory.java
@@ -1,5 +1,7 @@
 package io.github.thebusybiscuit.slimefun4.core.services.plugins;
 
+import javax.annotation.Nonnull;
+
 import org.bukkit.Material;
 import org.bukkit.NamespacedKey;
 import org.bukkit.entity.Player;
@@ -12,7 +14,7 @@ import me.mrCookieSlime.EmeraldEnchants.EnchantmentGuide;
 
 class EmeraldEnchantsCategory extends FlexCategory {
 
-    public EmeraldEnchantsCategory(NamespacedKey key) {
+    public EmeraldEnchantsCategory(@Nonnull NamespacedKey key) {
         super(key, new CustomItem(Material.ENCHANTED_BOOK, "&2EmeraldEnchants &a(Enchantment Guide)"), 2);
     }
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/PlaceholderAPIHook.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/PlaceholderAPIHook.java
index 425fd9e8a..5201b10e9 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/PlaceholderAPIHook.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/plugins/PlaceholderAPIHook.java
@@ -4,6 +4,8 @@ import java.util.Optional;
 import java.util.Set;
 import java.util.stream.Stream;
 
+import javax.annotation.Nonnull;
+
 import org.bukkit.OfflinePlayer;
 import org.bukkit.entity.Player;
 
@@ -17,7 +19,7 @@ class PlaceholderAPIHook extends PlaceholderExpansion {
     private final String version;
     private final String author;
 
-    public PlaceholderAPIHook(SlimefunPlugin plugin) {
+    public PlaceholderAPIHook(@Nonnull SlimefunPlugin plugin) {
         this.version = plugin.getDescription().getVersion();
         this.author = plugin.getDescription().getAuthors().toString();
     }
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/profiler/PerformanceSummary.java b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/profiler/PerformanceSummary.java
index 2d00bf100..d46e5752a 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/profiler/PerformanceSummary.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/core/services/profiler/PerformanceSummary.java
@@ -9,6 +9,7 @@ import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
 
 import org.bukkit.command.CommandSender;
 import org.bukkit.entity.Player;
@@ -90,6 +91,7 @@ class PerformanceSummary {
         });
     }
 
+    @ParametersAreNonnullByDefault
     private void summarizeTimings(int count, String name, CommandSender sender, Map<String, Long> map, Function<Map.Entry<String, Long>, String> formatter) {
         Stream<Map.Entry<String, Long>> stream = map.entrySet().stream();
         List<Entry<String, Long>> results = stream.sorted(Map.Entry.comparingByValue(Comparator.reverseOrder())).collect(Collectors.toList());
@@ -105,6 +107,8 @@ class PerformanceSummary {
         }
     }
 
+    @Nonnull
+    @ParametersAreNonnullByDefault
     private TextComponent summarizeAsTextComponent(int count, String prefix, List<Map.Entry<String, Long>> results, Function<Entry<String, Long>, String> formatter) {
         TextComponent component = new TextComponent(prefix);
         component.setColor(ChatColor.YELLOW);
@@ -140,6 +144,8 @@ class PerformanceSummary {
         return component;
     }
 
+    @Nonnull
+    @ParametersAreNonnullByDefault
     private String summarizeAsString(int count, String prefix, List<Entry<String, Long>> results, Function<Entry<String, Long>, String> formatter) {
         int shownEntries = 0;
         int hiddenEntries = 0;
@@ -169,6 +175,7 @@ class PerformanceSummary {
         return builder.toString();
     }
 
+    @Nonnull
     private String getPerformanceRating() {
         StringBuilder builder = new StringBuilder();
         builder.append(NumberUtils.getColorFromPercentage(100 - Math.min(percentage, 100)));
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/gps/ElevatorPlate.java b/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/gps/ElevatorPlate.java
index e177db8e2..e3eb557ca 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/gps/ElevatorPlate.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/gps/ElevatorPlate.java
@@ -6,6 +6,9 @@ import java.util.List;
 import java.util.Set;
 import java.util.UUID;
 
+import javax.annotation.Nonnull;
+import javax.annotation.ParametersAreNonnullByDefault;
+
 import org.bukkit.ChatColor;
 import org.bukkit.Location;
 import org.bukkit.Material;
@@ -57,6 +60,7 @@ public class ElevatorPlate extends SimpleSlimefunItem<BlockUseHandler> {
         };
     }
 
+    @Nonnull
     public Set<UUID> getUsers() {
         return users;
     }
@@ -72,7 +76,8 @@ public class ElevatorPlate extends SimpleSlimefunItem<BlockUseHandler> {
         };
     }
 
-    public List<Block> getFloors(Block b) {
+    @Nonnull
+    public List<Block> getFloors(@Nonnull Block b) {
         List<Block> floors = new LinkedList<>();
 
         for (int y = b.getWorld().getMaxHeight(); y > 0; y--) {
@@ -91,19 +96,26 @@ public class ElevatorPlate extends SimpleSlimefunItem<BlockUseHandler> {
         return floors;
     }
 
-    public void open(Player p, Block b) {
+    @ParametersAreNonnullByDefault
+    public void openInterface(Player p, Block b) {
         if (users.remove(p.getUniqueId())) {
             return;
         }
 
-        CustomBookInterface book = new CustomBookInterface(SlimefunPlugin.instance());
-        ChatComponent page = null;
-
         List<Block> floors = getFloors(b);
 
         if (floors.size() < 2) {
             SlimefunPlugin.getLocalization().sendMessage(p, "machines.ELEVATOR.no-destinations", true);
         }
+        else {
+            openFloorSelector(b, floors, p);
+        }
+    }
+
+    @ParametersAreNonnullByDefault
+    private void openFloorSelector(Block b, List<Block> floors, Player p) {
+        CustomBookInterface book = new CustomBookInterface(SlimefunPlugin.instance());
+        ChatComponent page = null;
 
         for (int i = 0; i < floors.size(); i++) {
             if (i % 10 == 0) {
@@ -125,23 +137,7 @@ public class ElevatorPlate extends SimpleSlimefunItem<BlockUseHandler> {
             else {
                 line = new ChatComponent("\n" + ChatColor.GRAY + (floors.size() - i) + ". " + ChatColor.BLACK + floor);
                 line.setHoverEvent(new HoverEvent(ChatColors.color(SlimefunPlugin.getLocalization().getMessage(p, "machines.ELEVATOR.click-to-teleport")), "", ChatColor.WHITE + floor, ""));
-                line.setClickEvent(new ClickEvent(new NamespacedKey(SlimefunPlugin.instance(), DATA_KEY + i), player -> Slimefun.runSync(() -> {
-                    users.add(player.getUniqueId());
-
-                    float yaw = player.getEyeLocation().getYaw() + 180;
-
-                    if (yaw > 180) {
-                        yaw = -180 + (yaw - 180);
-                    }
-
-                    Location destination = new Location(player.getWorld(), block.getX() + 0.5, block.getY() + 0.4, block.getZ() + 0.5, yaw, player.getEyeLocation().getPitch());
-
-                    PaperLib.teleportAsync(player, destination).thenAccept(teleported -> {
-                        if (teleported.booleanValue()) {
-                            player.sendTitle(ChatColor.WHITE + ChatColors.color(floor), null, 20, 60, 20);
-                        }
-                    });
-                })));
+                line.setClickEvent(new ClickEvent(new NamespacedKey(SlimefunPlugin.instance(), DATA_KEY + i), player -> teleport(player, floor, block)));
             }
 
             page.append(line);
@@ -154,6 +150,28 @@ public class ElevatorPlate extends SimpleSlimefunItem<BlockUseHandler> {
         book.open(p);
     }
 
+    @ParametersAreNonnullByDefault
+    private void teleport(Player player, String floorName, Block target) {
+        Slimefun.runSync(() -> {
+            users.add(player.getUniqueId());
+
+            float yaw = player.getEyeLocation().getYaw() + 180;
+
+            if (yaw > 180) {
+                yaw = -180 + (yaw - 180);
+            }
+
+            Location destination = new Location(player.getWorld(), target.getX() + 0.5, target.getY() + 0.4, target.getZ() + 0.5, yaw, player.getEyeLocation().getPitch());
+
+            PaperLib.teleportAsync(player, destination).thenAccept(teleported -> {
+                if (teleported.booleanValue()) {
+                    player.sendTitle(ChatColor.WHITE + ChatColors.color(floorName), null, 20, 60, 20);
+                }
+            });
+        });
+    }
+
+    @ParametersAreNonnullByDefault
     public void openEditor(Player p, Block b) {
         ChestMenu menu = new ChestMenu("Elevator Settings");
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/multiblocks/miner/ActiveMiner.java b/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/multiblocks/miner/ActiveMiner.java
index 36f42092b..4ed3f3d72 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/multiblocks/miner/ActiveMiner.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/items/multiblocks/miner/ActiveMiner.java
@@ -294,28 +294,33 @@ class ActiveMiner implements Runnable {
 
             if (state instanceof Chest) {
                 Inventory inv = ((Chest) state).getBlockInventory();
-
-                for (int i = 0; i < inv.getSize(); i++) {
-                    for (MachineFuel fuelType : miner.fuelTypes) {
-                        ItemStack item = inv.getContents()[i];
-
-                        if (fuelType.test(item)) {
-                            ItemUtils.consumeItem(item, false);
-
-                            if (miner instanceof AdvancedIndustrialMiner) {
-                                inv.addItem(new ItemStack(Material.BUCKET));
-                            }
-
-                            return fuelType.getTicks();
-                        }
-                    }
-                }
+                return consumeFuel(inv);
             }
         }
 
         return 0;
     }
 
+    private int consumeFuel(Inventory inv) {
+        for (int i = 0; i < inv.getSize(); i++) {
+            for (MachineFuel fuelType : miner.fuelTypes) {
+                ItemStack item = inv.getContents()[i];
+
+                if (fuelType.test(item)) {
+                    ItemUtils.consumeItem(item, false);
+
+                    if (miner instanceof AdvancedIndustrialMiner) {
+                        inv.addItem(new ItemStack(Material.BUCKET));
+                    }
+
+                    return fuelType.getTicks();
+                }
+            }
+        }
+        
+        return 0;
+    }
+
     private void setPistonState(Block block, boolean extended) {
         if (!running) {
             return;
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/listeners/TeleporterListener.java b/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/listeners/TeleporterListener.java
index c053b1a44..417a42bca 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/listeners/TeleporterListener.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/implementation/listeners/TeleporterListener.java
@@ -49,7 +49,8 @@ public class TeleporterListener implements Listener {
             }
         }
         else if (id.equals(SlimefunItems.ELEVATOR_PLATE.getItemId())) {
-            ((ElevatorPlate) SlimefunItems.ELEVATOR_PLATE.getItem()).open(e.getPlayer(), e.getClickedBlock());
+            ElevatorPlate elevator = ((ElevatorPlate) SlimefunItems.ELEVATOR_PLATE.getItem());
+            elevator.openInterface(e.getPlayer(), e.getClickedBlock());
         }
     }
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/ItemStackWrapper.java b/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/ItemStackWrapper.java
index 6df8146d0..ef0dbbd3e 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/ItemStackWrapper.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/ItemStackWrapper.java
@@ -3,6 +3,8 @@ package io.github.thebusybiscuit.slimefun4.utils.itemstack;
 import java.util.ArrayList;
 import java.util.List;
 
+import javax.annotation.Nonnull;
+
 import org.apache.commons.lang.Validate;
 import org.bukkit.Material;
 import org.bukkit.enchantments.Enchantment;
@@ -28,7 +30,7 @@ public final class ItemStackWrapper extends ItemStack {
     private final int amount;
     private final boolean hasItemMeta;
 
-    public ItemStackWrapper(ItemStack item) {
+    public ItemStackWrapper(@Nonnull ItemStack item) {
         super(item.getType());
         amount = item.getAmount();
         hasItemMeta = item.hasItemMeta();
@@ -108,7 +110,8 @@ public final class ItemStackWrapper extends ItemStack {
      * 
      * @return An {@link ItemStackWrapper} array
      */
-    public static ItemStackWrapper[] wrapArray(ItemStack[] items) {
+    @Nonnull
+    public static ItemStackWrapper[] wrapArray(@Nonnull ItemStack[] items) {
         Validate.notNull(items, "The array must not be null!");
         ItemStackWrapper[] array = new ItemStackWrapper[items.length];
 
@@ -129,7 +132,8 @@ public final class ItemStackWrapper extends ItemStack {
      * 
      * @return An {@link ItemStackWrapper} array
      */
-    public static List<ItemStackWrapper> wrapList(List<ItemStack> items) {
+    @Nonnull
+    public static List<ItemStackWrapper> wrapList(@Nonnull List<ItemStack> items) {
         Validate.notNull(items, "The list must not be null!");
         List<ItemStackWrapper> list = new ArrayList<>(items.size());
 
diff --git a/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/SlimefunGuideItem.java b/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/SlimefunGuideItem.java
index e3ffc62e9..78bff2c33 100644
--- a/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/SlimefunGuideItem.java
+++ b/src/main/java/io/github/thebusybiscuit/slimefun4/utils/itemstack/SlimefunGuideItem.java
@@ -3,6 +3,8 @@ package io.github.thebusybiscuit.slimefun4.utils.itemstack;
 import java.util.LinkedList;
 import java.util.List;
 
+import javax.annotation.Nonnull;
+
 import org.bukkit.Material;
 import org.bukkit.inventory.ItemStack;
 import org.bukkit.inventory.meta.ItemMeta;
@@ -24,15 +26,13 @@ import io.github.thebusybiscuit.slimefun4.implementation.guide.CheatSheetSlimefu
  */
 public class SlimefunGuideItem extends ItemStack {
 
-    public SlimefunGuideItem(SlimefunGuideImplementation implementation, String name) {
+    public SlimefunGuideItem(@Nonnull SlimefunGuideImplementation implementation, @Nonnull String name) {
         super(Material.ENCHANTED_BOOK);
 
         ItemMeta meta = getItemMeta();
-
         meta.setDisplayName(ChatColors.color(name));
 
         List<String> lore = new LinkedList<>();
-
         lore.add(implementation instanceof CheatSheetSlimefunGuide ? "&4&lOnly openable by Admins" : "");
         lore.add(ChatColors.color("&eRight Click &8\u21E8 &7Browse Items"));
         lore.add(ChatColors.color("&eShift + Right Click &8\u21E8 &7Open Settings / Credits"));