Implement suggested improvements.

* Use singleton pattern for DefaultContentFilter with getInstance method.
* Have a default SendHandler.
* Don't need streams for small dataset.
* Private WorldListContentParser class to improve readability.

src/main/java/com/onarandombox/MultiverseCore/commands/GamerulesCommand.java

@@ -22,7 +22,6 @@ import org.bukkit.command.CommandSender;
 import org.bukkit.entity.Player;
 import org.bukkit.permissions.PermissionDefault;
 
-import java.util.Arrays;
 import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
@@ -51,7 +50,7 @@ public class GamerulesCommand extends MultiverseCommand {
         // We NEED a world from the command line
         Player p;
         World world;
-        ContentFilter filter = DefaultContentFilter.INSTANCE;
+        ContentFilter filter = DefaultContentFilter.getInstance();
 
         if (sender instanceof Player) {
             p = (Player) sender;
@@ -103,10 +102,10 @@ public class GamerulesCommand extends MultiverseCommand {
 
     private Map<String, Object> getGameRuleMap(World world) {
         Map<String, Object> gameRuleMap = new HashMap<>();
-        Arrays.stream(GameRule.values()).forEach(rule -> {
+        for (GameRule<?> rule : GameRule.values()) {
             Object value = world.getGameRuleValue(rule);
             gameRuleMap.put(rule.getName(), value);
-        });
+        }
         return gameRuleMap;
     }
 }

src/main/java/com/onarandombox/MultiverseCore/commands/ListCommand.java

@@ -20,6 +20,7 @@ import org.bukkit.World;
 import org.bukkit.command.CommandSender;
 import org.bukkit.entity.Player;
 import org.bukkit.permissions.PermissionDefault;
+import org.jetbrains.annotations.NotNull;
 
 import java.util.List;
 
@@ -41,7 +42,7 @@ public class ListCommand extends MultiverseCommand {
 
     @Override
     public void runCommand(CommandSender sender, List<String> args) {
-        ContentFilter filter = DefaultContentFilter.INSTANCE;
+        ContentFilter filter = DefaultContentFilter.getInstance();
         int page = 1;
 
         // Either page or filter.
@@ -72,27 +73,28 @@ public class ListCommand extends MultiverseCommand {
                 .send(sender);
     }
 
-    private ContentParser newWorldListContentParser() {
+    private class WorldListContentParser implements ContentParser {
-        return (sender, content) -> {
+
+        @Override
+        public void parse(@NotNull CommandSender sender, @NotNull List<String> content) {
             Player player = (sender instanceof Player) ? (Player) sender : null;
 
-            this.plugin.getMVWorldManager().getMVWorlds().stream()
+            plugin.getMVWorldManager().getMVWorlds().stream()
                     .filter(world -> player == null || plugin.getMVPerms().canEnterWorld(player, world))
                     .filter(world -> canSeeWorld(player, world))
                     .map(world -> hiddenText(world) + world.getColoredWorldString() + " - " + parseColouredEnvironment(world.getEnvironment()))
                     .forEach(content::add);
 
-            this.plugin.getMVWorldManager().getUnloadedWorlds().stream()
+            plugin.getMVWorldManager().getUnloadedWorlds().stream()
                     .filter(world -> plugin.getMVPerms().hasPermission(sender, "multiverse.access." + world, true))
                     .map(world -> ChatColor.GRAY + world + " - UNLOADED")
                     .forEach(content::add);
-        };
         }
 
         private boolean canSeeWorld(Player player, MultiverseWorld world) {
             return !world.isHidden()
                     || player == null
-                || this.plugin.getMVPerms().hasPermission(player, "multiverse.core.modify", true);
+                    || plugin.getMVPerms().hasPermission(player, "multiverse.core.modify", true);
         }
 
         private String hiddenText(MultiverseWorld world) {
@@ -115,3 +117,4 @@ public class ListCommand extends MultiverseCommand {
             return color + env.toString();
         }
     }
+}

src/main/java/com/onarandombox/MultiverseCore/display/ContentDisplay.java

@@ -1,5 +1,6 @@
 package com.onarandombox.MultiverseCore.display;
 
+import com.onarandombox.MultiverseCore.display.handlers.DefaultSendHandler;
 import com.onarandombox.MultiverseCore.display.handlers.SendHandler;
 import com.onarandombox.MultiverseCore.display.parsers.ContentParser;
 import org.bukkit.command.CommandSender;
@@ -25,7 +26,7 @@ public class ContentDisplay {
     }
 
     private final List<ContentParser> contentParsers = new ArrayList<>();
-    private SendHandler sendHandler;
+    private SendHandler sendHandler = DefaultSendHandler.getInstance();
 
     public ContentDisplay() {
     }

src/main/java/com/onarandombox/MultiverseCore/display/filters/DefaultContentFilter.java

@@ -5,9 +5,16 @@ package com.onarandombox.MultiverseCore.display.filters;
  */
 public class DefaultContentFilter implements ContentFilter {
 
-    public static DefaultContentFilter INSTANCE = new DefaultContentFilter();
+    public static DefaultContentFilter instance;
 
-    public DefaultContentFilter() {
+    public static DefaultContentFilter getInstance() {
+        if (instance == null) {
+            instance = new DefaultContentFilter();
+        }
+        return instance;
+    }
+
+    private DefaultContentFilter() {
     }
 
     /**

src/main/java/com/onarandombox/MultiverseCore/display/handlers/BaseSendHandler.java

@@ -18,7 +18,7 @@ import java.util.stream.Collectors;
 public abstract class BaseSendHandler<T extends BaseSendHandler<?>> implements SendHandler {
 
     protected String header = "";
-    protected ContentFilter filter = DefaultContentFilter.INSTANCE;
+    protected ContentFilter filter = DefaultContentFilter.getInstance();
 
     /**
      * {@inheritDoc}

src/main/java/com/onarandombox/MultiverseCore/display/handlers/DefaultSendHandler.java

@@ -0,0 +1,29 @@
+package com.onarandombox.MultiverseCore.display.handlers;
+
+import org.bukkit.command.CommandSender;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.List;
+
+public class DefaultSendHandler implements SendHandler {
+
+    private static DefaultSendHandler instance;
+
+    public static DefaultSendHandler getInstance() {
+        if (instance == null) {
+            instance = new DefaultSendHandler();
+        }
+        return instance;
+    }
+
+    private DefaultSendHandler() {
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void send(@NotNull CommandSender sender, @NotNull List<String> content) {
+        sender.sendMessage(content.toArray(new String[0]));
+    }
+}