Out of curiosity, please supply your World subclass code after saving the world.

import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) /** * Write a description of class MyWorld here. * * @author (your name) * @version (a version number or a date) */ public class MyWorld extends World { /** * Constructor for objects of class MyWorld. * */ public MyWorld() { // Create a new world with 600x400 cells with a cell size of 1x1 pixels. super(600, 400, 1); Platform platform = new Platform(200, 30); addObject(platform, 300, 300); prepare(); } /** * Prepare the world for the start of the program. * That is: create the initial objects and add them to the world. */ /** * Prepare the world for the start of the program. * That is: create the initial objects and add them to the world. */ private void prepare() { Jumper jumper = new Jumper(); addObject(jumper,314,153); } }

Okay -- let's look at the Jumper class.

import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) /** * Write a description of class Jumoer here. * * @author (your name) * @version (a version number or a date) */ public class Jumper extends Actor { private final int GRAVITY = 1; private int velocity; public Jumper() { velocity = 0; } public void act() { fall(); if (Greenfoot.isKeyDown("space") && isOnSolidGround()) jump(); move(); } public void fall() { setLocation(getX(), getY() + velocity); if (isOnSolidGround()) { velocity = 0; while (isOnSolidGround()) { setLocation(getX(), getY() - 1); } setLocation(getX(), getY() + 1); } else if (velocity < 0 && didBumpHead()) velocity = 0; else velocity += GRAVITY; } public void jump () { velocity = -20; } public void move(){ int y = getY(); int x = getX(); if (Greenfoot.isKeyDown("A")) x-= 3; if (Greenfoot.isKeyDown("D")) x += 3; setLocation(x, y); } public boolean isOnSolidGround() { boolean isOnGround = false; if (getY() > getWorld().getHeight() - 100) isOnGround = true; int imageWidth = getImage().getWidth(); int imageHeight = getImage().getHeight(); if (getOneObjectAtOffset(imageWidth / -2, imageHeight / 2, Platform.class) != null || getOneObjectAtOffset(imageWidth / 2, imageHeight / 2, Platform.class) != null) isOnGround = true; return isOnGround; } public boolean didBumpHead() { boolean bumpedHead = false; int imageWidth = getImage().getWidth(); int imageHeight = getImage().getHeight(); if (getOneObjectAtOffset(imageWidth / -2, imageHeight / 2, Platform.class) != null || getOneObjectAtOffset(imageWidth / 2, imageHeight / 2, Platform.class) != null) bumpedHead = true; return bumpedHead; } }

The following part: is probably the problem. Unless you are sure that at some point the actor will go from being on solid ground to NOT being on solid ground, this will loop forever.

I do have a Jump and Run Demo w/Moving Platform scenario you might be interested in looking at. The Who button along the bottom of the window shows the codes I used for the player.

I deleted while (isOnSolidGround()) { setLocation(getX(), getY() - 1); } setLocation(getX(), getY() + 1); but it still freezes. I'll check out your scenario though.