Make various things final

src/main/java/me/clip/placeholderapi/PlaceholderListener.java

@@ -40,7 +40,7 @@ import org.bukkit.event.server.PluginDisableEvent;
 
 public class PlaceholderListener implements Listener {
 
-  private PlaceholderAPIPlugin plugin;
+  private final PlaceholderAPIPlugin plugin;
 
   public PlaceholderListener(PlaceholderAPIPlugin instance) {
     plugin = instance;

src/main/java/me/clip/placeholderapi/ServerLoadEventListener.java

@@ -9,7 +9,7 @@ import org.bukkit.event.server.ServerLoadEvent;
 
 public class ServerLoadEventListener implements Listener {
 
-    private PlaceholderAPIPlugin plugin;
+    private final PlaceholderAPIPlugin plugin;
 
     public ServerLoadEventListener(PlaceholderAPIPlugin instance) {
         plugin = instance;

src/main/java/me/clip/placeholderapi/commands/PlaceholderAPICommands.java

@@ -36,8 +36,8 @@ import org.bukkit.entity.Player;
 
 public class PlaceholderAPICommands implements CommandExecutor {
 
-  private PlaceholderAPIPlugin plugin;
-  private CommandExecutor eCloud;
+  private final PlaceholderAPIPlugin plugin;
+  private final CommandExecutor eCloud;
 
   public PlaceholderAPICommands(PlaceholderAPIPlugin i) {
     plugin = i;

src/main/java/me/clip/placeholderapi/configuration/PlaceholderAPIConfig.java

@@ -24,7 +24,7 @@ import me.clip.placeholderapi.PlaceholderAPIPlugin;
 
 public class PlaceholderAPIConfig {
 
-  private PlaceholderAPIPlugin plugin;
+  private final PlaceholderAPIPlugin plugin;
 
   public PlaceholderAPIConfig(PlaceholderAPIPlugin i) {
     plugin = i;

src/main/java/me/clip/placeholderapi/events/ExpansionRegisterEvent.java

@@ -27,7 +27,7 @@ import org.bukkit.event.HandlerList;
 public class ExpansionRegisterEvent extends Event {
 
   private static final HandlerList HANDLERS = new HandlerList();
-  private PlaceholderExpansion expansion;
+  private final PlaceholderExpansion expansion;
 
   public ExpansionRegisterEvent(PlaceholderExpansion expansion) {
     this.expansion = expansion;

src/main/java/me/clip/placeholderapi/events/ExpansionUnregisterEvent.java

@@ -27,7 +27,7 @@ import org.bukkit.event.HandlerList;
 public class ExpansionUnregisterEvent extends Event {
 
   private static final HandlerList HANDLERS = new HandlerList();
-  private PlaceholderExpansion expansion;
+  private final PlaceholderExpansion expansion;
 
   public ExpansionUnregisterEvent(PlaceholderExpansion expansion) {
     this.expansion = expansion;

src/main/java/me/clip/placeholderapi/events/PlaceholderHookUnloadEvent.java

@@ -28,8 +28,8 @@ import org.bukkit.event.HandlerList;
 public class PlaceholderHookUnloadEvent extends Event {
 
   private static final HandlerList HANDLERS = new HandlerList();
-  private String plugin;
-  private PlaceholderHook hook;
+  private final String plugin;
+  private final PlaceholderHook hook;
 
   public PlaceholderHookUnloadEvent(String plugin, PlaceholderHook placeholderHook) {
     this.plugin = plugin;

src/main/java/me/clip/placeholderapi/expansion/ExpansionManager.java

@@ -36,7 +36,7 @@ import java.util.Map;
 import java.util.Map.Entry;
 
 public final class ExpansionManager {
-  private PlaceholderAPIPlugin plugin;
+  private final PlaceholderAPIPlugin plugin;
 
   public ExpansionManager(PlaceholderAPIPlugin instance) {
     plugin = instance;

src/main/java/me/clip/placeholderapi/expansion/NMSVersion.java

@@ -39,7 +39,7 @@ public enum NMSVersion {
   SPIGOT_1_13_R2("v1_13_R2"),
   SPIGOT_1_14_R1("v1_14_R1");
 
-  private String version;
+  private final String version;
 
   NMSVersion(String version) {
     this.version = version;

src/main/java/me/clip/placeholderapi/expansion/Version.java

@@ -22,9 +22,8 @@ package me.clip.placeholderapi.expansion;
 
 public final class Version {
 
-  private boolean isSpigot;
-
-  private String version;
+  private final boolean isSpigot;
+  private final String version;
 
   public Version(String version, boolean isSpigot) {
     this.version = version;

src/main/java/me/clip/placeholderapi/external/EZPlaceholderHook.java

@@ -28,9 +28,8 @@ import org.bukkit.plugin.Plugin;
 @Deprecated
 public abstract class EZPlaceholderHook extends PlaceholderHook {
 
-  private String identifier;
-
-  private String plugin;
+  private final String identifier;
+  private final String plugin;
 
   public EZPlaceholderHook(Plugin plugin, String identifier) {
     Validate.notNull(plugin, "Plugin can not be null!");

src/main/java/me/clip/placeholderapi/updatechecker/UpdateChecker.java

@@ -15,8 +15,9 @@ import org.bukkit.event.player.PlayerJoinEvent;
 public class UpdateChecker implements Listener {
 
   private final int RESOURCE_ID = 6245;
-  private PlaceholderAPIPlugin plugin;
-  private String spigotVersion, pluginVersion;
+  private final PlaceholderAPIPlugin plugin;
+  private String spigotVersion;
+  private final String pluginVersion;
   private boolean updateAvailable;
 
   public UpdateChecker(PlaceholderAPIPlugin i) {