Ending a Game

public class Game  extends World
{
    private boolean isGameOver;
    private boolean isGameRunning;
    
    /**
     * Constructor for objects of class Game.
     * 
     */
    public Game()
    {    
        // Create a new world with 600x400 cells with a cell size of 1x1 pixels.
        super(600, 400, 1);
        
        isGameOver = false;
        isGameRunning = false;
    }
    
    public void act()
    {
        if ( ! isGameOver ) {
            int numEnemies = getObjects( Enemy.class ).size();
        
            if ( !isGameRunning && numEnemies > 0 ) {
                isGameRunning = true;
            } else if ( isGameRunning && numEnemies == 0 ) {
                isGameOver = true;
                onEndGame();
            }
        }
    }
    
    public void onEndGame()
    {
        System.out.println( "game has ended" );
    }
}