Implements the group list command

src/main/java/net/knarcraft/dropper/arena/DropperArenaHandler.java

@@ -23,6 +23,15 @@ public class DropperArenaHandler {
     private Map<UUID, DropperArenaGroup> arenaGroups = new HashMap<>();
     private Map<String, UUID> arenaNameLookup = new HashMap<>();
 
+    /**
+     * Gets a copy of all dropper groups
+     *
+     * @return <p>All dropper groups</p>
+     */
+    public Set<DropperArenaGroup> getAllGroups() {
+        return new HashSet<>(arenaGroups.values());
+    }
+
     /**
      * Gets the group the given arena belongs to
      *

src/main/java/net/knarcraft/dropper/command/GroupListCommand.java

@@ -1,19 +1,93 @@
 package net.knarcraft.dropper.command;
 
+import net.knarcraft.dropper.Dropper;
+import net.knarcraft.dropper.arena.DropperArena;
+import net.knarcraft.dropper.arena.DropperArenaGroup;
+import net.knarcraft.dropper.arena.DropperArenaHandler;
 import org.bukkit.command.Command;
-import org.bukkit.command.CommandExecutor;
 import org.bukkit.command.CommandSender;
+import org.bukkit.command.TabExecutor;
 import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
 
-public class GroupListCommand implements CommandExecutor {
+import java.util.ArrayList;
+import java.util.List;
+import java.util.UUID;
+
+/**
+ * The command for listing groups and the stages within
+ */
+public class GroupListCommand implements TabExecutor {
 
     @Override
     public boolean onCommand(@NotNull CommandSender commandSender, @NotNull Command command, @NotNull String s,
                              @NotNull String[] arguments) {
-        // TODO: Display all groups if no argument is specified.
-        // TODO: If a group is set and it exists, list all arenas in the correct order, and numbered (the order denotes
-        //  the order players need to complete the arenas in)
-        return false;
+        DropperArenaHandler arenaHandler = Dropper.getInstance().getArenaHandler();
+        if (arguments.length == 0) {
+            displayExistingGroups(arenaHandler, commandSender);
+            return true;
+        } else if (arguments.length == 1) {
+            return displayOrderedArenaNames(arenaHandler, commandSender, arguments[0]);
+        } else {
+            return false;
+        }
+    }
+
+    /**
+     * Displays all currently existing dropper arena groups
+     *
+     * @param arenaHandler <p>The arena handler to get groups from</p>
+     * @param sender       <p>The command sender to display the groups to</p>
+     */
+    private void displayExistingGroups(@NotNull DropperArenaHandler arenaHandler, @NotNull CommandSender sender) {
+        StringBuilder builder = new StringBuilder("Dropper arena groups:").append("\n");
+        arenaHandler.getAllGroups().stream().sorted().forEachOrdered((group) ->
+                builder.append(group.getGroupName()).append("\n"));
+        sender.sendMessage(builder.toString());
+    }
+
+    /**
+     * Displays the ordered stages in a specified group to the specified command sender
+     *
+     * @param arenaHandler <p>The arena handler to get groups from</p>
+     * @param sender       <p>The command sender to display the stages to</p>
+     * @param groupName    <p>The name of the group to display stages for</p>
+     * @return <p>True if the stages were successfully displayed</p>
+     */
+    private boolean displayOrderedArenaNames(@NotNull DropperArenaHandler arenaHandler, @NotNull CommandSender sender,
+                                             @NotNull String groupName) {
+        DropperArenaGroup arenaGroup = arenaHandler.getGroup(groupName);
+        if (arenaGroup == null) {
+            sender.sendMessage("Unable to find the specified group!");
+            return false;
+        }
+
+        // Send a list of all stages (arenas in the group)
+        StringBuilder builder = new StringBuilder(groupName).append("'s stages:").append("\n");
+        int counter = 1;
+        for (UUID arenaId : arenaGroup.getArenas()) {
+            DropperArena arena = arenaHandler.getArena(arenaId);
+            if (arena != null) {
+                builder.append(counter++).append(". ").append(arena.getArenaName()).append("\n");
+            }
+        }
+        sender.sendMessage(builder.toString());
+        return true;
+    }
+
+    @Nullable
+    @Override
+    public List<String> onTabComplete(@NotNull CommandSender commandSender, @NotNull Command command, @NotNull String s,
+                                      @NotNull String[] arguments) {
+        if (arguments.length == 1) {
+            List<String> groupNames = new ArrayList<>();
+            for (DropperArenaGroup group : Dropper.getInstance().getArenaHandler().getAllGroups()) {
+                groupNames.add(group.getGroupName());
+            }
+            return groupNames;
+        } else {
+            return new ArrayList<>();
+        }
     }
 
 }

src/main/java/net/knarcraft/dropper/command/GroupSetCommand.java

@@ -67,6 +67,9 @@ public class GroupSetCommand implements TabExecutor {
             List<String> possibleValues = new ArrayList<>();
             possibleValues.add("none");
             possibleValues.add("GroupName");
+            for (DropperArenaGroup group : Dropper.getInstance().getArenaHandler().getAllGroups()) {
+                possibleValues.add(group.getGroupName());
+            }
             return possibleValues;
         } else {
             return new ArrayList<>();

src/main/java/net/knarcraft/dropper/command/GroupSwapCommand.java

@@ -17,4 +17,7 @@ public class GroupSwapCommand implements CommandExecutor {
         return false;
     }
 
+    // Tab-completion TODO: Only give arenas with a group for the first value. Only give arenas within the same group
+    //  for the second value
+
 }