Chat system

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Scanner;
import java.util.TreeSet;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.ListIterator;
import java.util.Map;
import java.util.TreeMap;
import java.util.TreeSet;
class NoSuchRoomException extends Exception
{
	public NoSuchRoomException(String roomName){
		super(roomName);
	}
}
class NoSuchUserException extends Exception
{
	public NoSuchUserException(String userName)
	{
		super(userName);
	}
}

class ChatRoom
{
	String name;
	TreeSet<String> al;
	public ChatRoom(String name)
	{
		this.name = name;
		this.al = new TreeSet<>();
	}
	public void addUser(String username)
	{
		al.add(username);
	}
	public void removeUser(String username)
	{
		while(al.contains(username))
		{
			al.remove(username);
		}
	}
	public String toString()
	{
		StringBuilder sb = new StringBuilder();
		sb.append(name + "\n");

		if(al.isEmpty())
			sb.append("EMPTY\n");
		al.forEach(t -> sb.append(t + "\n"));
		return sb.toString();
	}
	public boolean hasUser(String username)
	{
		return al.contains(username);
	}

	public int numUsers()
	{
		return al.size();
	}
}

class ChatSystem
{
	TreeMap<String, ChatRoom> tm;
	HashSet<String> users;
	public ChatSystem()
	{
		this.tm = new TreeMap<>();
		users = new HashSet<>();
	}
	public void addRoom(String roomName)
	{
		tm.put(roomName, new ChatRoom(roomName));
	}
	public void removeRoom(String roomName)
	{
		tm.remove(roomName);
	}

	public ChatRoom getRoom(String roomname) throws NoSuchRoomException
	{
		if(!tm.containsKey(roomname))
			throw new NoSuchRoomException(roomname);

		return tm.get(roomname);

	}
	public void register(String userName) throws NoSuchRoomException, NoSuchUserException
	{
		users.add(userName);
		ChatRoom room = tm.values().stream().min((left,right) -> {
					if(left.numUsers()==right.numUsers())
						return left.name.compareTo(right.name);
					else
						return left.numUsers()- right.numUsers();
				}).orElse(null);
		if(room == null)
			return;
		joinRoom(userName, room.name);
	}
	public void registerAndJoin(String userName, String roomName) throws NoSuchRoomException, NoSuchUserException
	{
		users.add(userName);
		joinRoom(userName, roomName);
	}

	public void joinRoom(String userName, String roomName) throws NoSuchRoomException, NoSuchUserException
	{
		check(roomName, userName);
		tm.computeIfPresent(roomName, (rname, room) -> {
			room.addUser(userName);
			return room;
		});
	}
	public void leaveRoom(String userName, String roomName) throws NoSuchRoomException, NoSuchUserException
	{
		check(roomName, userName);
		tm.computeIfPresent(roomName, (rname, room) ->
		{
			room.removeUser(userName);
			return room;
		});
	}
	public void check(String roomName, String userName) throws NoSuchRoomException, NoSuchUserException
	{
		if(!tm.containsKey(roomName))
			throw new NoSuchRoomException(roomName);
		if(!users.contains(userName))
			throw new NoSuchUserException(userName);

	}

	public void followFriend(String userName, String friend_username) throws NoSuchUserException
	{
		if(!users.contains(userName))
			throw new NoSuchUserException(userName);

		tm.values()
		.forEach(t -> {
			if(t.hasUser(friend_username))
				t.addUser(userName);
		});

	}

}

public class ChatSystemTest {

	public static void main(String[] args) throws IllegalArgumentException, IllegalAccessException, InvocationTargetException, NoSuchRoomException {
		Scanner jin = new Scanner(System.in);
		int k = jin.nextInt();
		if ( k == 0 ) {
			ChatRoom cr = new ChatRoom(jin.next());
			int n = jin.nextInt();
			for ( int i = 0 ; i < n ; ++i ) {
				k = jin.nextInt();
				if ( k == 0 ) cr.addUser(jin.next());
				if ( k == 1 ) cr.removeUser(jin.next());
				if ( k == 2 ) System.out.println(cr.hasUser(jin.next()));
			}
			System.out.println("");
			System.out.println(cr.toString());
			n = jin.nextInt();
			if ( n == 0 ) return;
			ChatRoom cr2 = new ChatRoom(jin.next());
			for ( int i = 0 ; i < n ; ++i ) {
				k = jin.nextInt();
				if ( k == 0 ) cr2.addUser(jin.next());
				if ( k == 1 ) cr2.removeUser(jin.next());
				if ( k == 2 ) cr2.hasUser(jin.next());
			}
            System.out.println(cr2.toString());
		}
       if ( k == 1 ) {
			ChatSystem cs = new ChatSystem();
			Method mts[] = cs.getClass().getMethods();
			while ( true ) {
				String cmd = jin.next();
				if ( cmd.equals("stop") ) break;
				if ( cmd.equals("print") ) {
					System.out.println(cs.getRoom(jin.next())+"\n");continue;
				}
				for ( Method m : mts ) {
					if ( m.getName().equals(cmd) ) {
						String params[] = new String[m.getParameterTypes().length];
						for ( int i = 0 ; i < params.length ; ++i ) params[i] = jin.next();
						m.invoke(cs,params);
					}
				}
			}
		}
	}

}