Add tab-completion API. Fixes BUKKIT-2181. Adds BUKKIT-2602

CommandMap contains a method that will auto-complete commands
appropriately. Before the first space, it searches for commands of which
the sender has permission. After the first space, it delegates to the
individual command.

Vanilla commands contain implementations to mimic vanilla
implementation. Exception would be give, that allows for name matching;
a feature we already allowed as part of the command is now supported for
auto-complete as well.

Plugin commands can get a tab completer set to delegate the completion
for. If no tab completer is set, it can check the executor to see if it
implements the tab completion interface. It will also attempt to chain
calls if null gets returned from these interfaces. Plugins also
implement the new TabCompleter interface, to add ease-of-use for plugin
developers, similar to the onCommand() method.

The default command implementation simply searches for player names.

To help facilitate command completion, a utility class was added with
two functions. One checks two strings, to see if the specified string
starts with (ignoring case) the second. The other method uses the first
to selectively copy elements from one collection to another.

By: Score_Under <seejay.11@gmail.com>

paper-api/src/main/java/org/bukkit/command/Command.java

@@ -1,12 +1,19 @@
 package org.bukkit.command;
 
 import java.util.ArrayList;
+import java.util.Collections;
 import java.util.List;
 import java.util.Set;
+
+import org.apache.commons.lang.Validate;
 import org.bukkit.Bukkit;
 import org.bukkit.ChatColor;
 import org.bukkit.Server;
+import org.bukkit.entity.Player;
 import org.bukkit.permissions.Permissible;
+import org.bukkit.util.StringUtil;
+
+import com.google.common.collect.ImmutableList;
 
 /**
  * Represents a Command, which executes various tasks upon user input
@@ -48,21 +55,48 @@ public abstract class Command {
     public abstract boolean execute(CommandSender sender, String commandLabel, String[] args);
 
     /**
-     * Executed on tab completion for this command, returning a list of options
-     * the player can tab through.
-     * <p />
-     * By returning null, you tell Bukkit to generate a list of players to send
-     * to the sender.
-     * By returning an empty list, no options will be sent.
-     *
-     * @param sender Source object which is executing this command
-     * @param args All arguments passed to the command, split via ' '
-     * @return null to generate a Player list, otherwise a list of options
+     * @deprecated This method is not supported and returns null
      */
+    @Deprecated
     public List<String> tabComplete(CommandSender sender, String[] args) {
         return null;
     }
 
+    /**
+     * Executed on tab completion for this command, returning a list of options
+     * the player can tab through.
+     *
+     * @param sender Source object which is executing this command
+     * @param alias the alias being used
+     * @param args All arguments passed to the command, split via ' '
+     * @return a list of tab-completions for the specified arguments. This will never be null. List may be immutable.
+     * @throws IllegalArgumentException if sender, alias, or args is null
+     */
+    public List<String> tabComplete(CommandSender sender, String alias, String[] args) throws IllegalArgumentException {
+        Validate.notNull(sender, "Sender cannot be null");
+        Validate.notNull(args, "Arguments cannot be null");
+        Validate.notNull(alias, "Alias cannot be null");
+
+        if (!(sender instanceof Player) || args.length == 0) {
+            return ImmutableList.of();
+        }
+
+        String lastWord = args[args.length - 1];
+
+        Player senderPlayer = (Player) sender;
+
+        ArrayList<String> matchedPlayers = new ArrayList<String>();
+        for (Player player : sender.getServer().getOnlinePlayers()) {
+            String name = player.getName();
+            if (senderPlayer.canSee(player) && StringUtil.startsWithIgnoreCase(name, lastWord)) {
+                matchedPlayers.add(name);
+            }
+        }
+
+        Collections.sort(matchedPlayers, String.CASE_INSENSITIVE_ORDER);
+        return matchedPlayers;
+    }
+
     /**
      * Returns the name of this command
      *