import greenfoot.*;  // (World, Actor, GreenfootImage, Greenfoot and MouseInfo)

/**
 * Write a description of class TwoPlayer_Button here.
 * 
 * @author (your name) 
 * @version (a version number or a date)
 */
public class TwoPlayer_Button extends Actor
{    
    /**
     * Act - do whatever the TwoPlayer_Button wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        startGame2P();
    }    

    /**
     * Start 2 player world.
     */
    public void startGame2P()
    {
        if (Greenfoot.mouseClicked(this))
        {
            Greenfoot.setWorld(new TwoPlayer());
        }
    }    
}    

import greenfoot.*;

public class TwoPlayer extends Main_Menu
{
    private TwoPlayer_Button twoPlayer_Button;
    private int countdown;
    private Actor[] survivors;
    private Walls Walls;
    public TwoPlayer()
    {
        
    }

    public void act()
    {
        prepare();
        canAddWalls();//Walls kept spawning 5 million times
        walls(); 
        infect();
    }
    
    private void walls()
    {
        if (canAddWalls()) addObject(new Walls(), 683, 384);
    }
    public boolean canAddWalls()
    {
        // in the world class, use this line
        return getObjects(Walls.class).size() < 1;
    }

    private void prepare()
    {
        if (survivors == null)
        {
            survivors = new Actor[] { new SurvivorWASD(), new SurvivorARROW() };
            addObject(survivors[0], 393, 372);
            addObject(survivors[1], 574, 536);
            canAddWalls();
        }
    }

    private void infect()
    {
        if (Greenfoot.mouseClicked(twoPlayer_Button)) // countdown starting
        {
            countdown = 300;
        }
        if (countdown == 0) return; // countdown already completed
        countdown--;
        if (countdown == 0) // countdown just completed
        {
            Actor[] viruses = new Actor[] { new VirusWASD(), new VirusARROW() }; // must be same order as 'survivors'
            int chosen = Greenfoot.getRandomNumber(2);
            addObject(viruses[chosen], survivors[chosen].getX(), survivors[chosen].getY());
            removeObject(survivors[chosen]);
            //removeObject(twoPlayer_Button); // optional
        }
    }
}