Respawning???????

You could write aa method into your world class, which spawns them randomly at the right side of the screen. Like this: And if you don't want to have it like this and you want to appear a new alien if one alien dies, then write into your method, where your alien dies by hitting a bullet, only this: addObject(new Alien(), getWidth(), Greenfoot.getRandomNumber(getHeight()+1)); And afterwards the dying alien can remove itself.

It should be like a new wave. Like in the first wave three for eg:- (* means aliens) * (! means plane) ! * * second wave six:- * * ! * * * *

Then you need: - an integer variable in the world called (perhaps) wave - a method that spawns the waves All the code should be in your world.

import greenfoot.*; // (World, Actor, GreenfootImage, Greenfoot and MouseInfo) /** * Write a description of class airport here. * * @author (your name) * @version (a version number or a date) */ public class airport extends World { int wave = 0; public void act() { scrollPosition -= scrollSpeed; while(scrollSpeed > 0 && scrollPosition < -picWidth) scrollPosition += picWidth; while(scrollSpeed < 0 && scrollPosition > 0) scrollPosition -= picWidth; paint(scrollPosition); if(numOfaliens()==0){ wave++; nextWave(wave); } } public int numOfaliens(){{ return getObjects(alien.class).size(); } } public void nextWave(int wave){ if(wave==1){ //insert here the spawning of the aliens like: addObject(new alien(), getWidth(), getHeight()/4); addObject(new alien(), getWidth(), getHeight()/2); addObject(new alien(), getWidth(), 3*getHeight()/4); //this is like you "painted" the first wave }else if(wave==2){ //same here. change the number of aliens and their place }else if(wave==3){ //and here } //etc. } private static final String bgImageName = "sky.png"; private static final double scrollSpeed = 2.5; private static final int picWidth = (new GreenfootImage(bgImageName)).getWidth(); private GreenfootImage bgImage, bgBase; private int scrollPosition = 0; /** * Constructor for objects of class airport. * */ public airport() { // Create a new world with 600x400 cells with a cell size of 1x1 pixels. super(812, 452, 1); setBackground(bgImageName); bgImage = new GreenfootImage(getBackground()); bgBase = new GreenfootImage(picWidth, getHeight()); bgBase.drawImage(bgImage, 0, 0); addObject (new alien (), 550, 50); addObject (new alien (), 550, 100); addObject (new alien (), 550, 150); addObject (new alien (), 550, 250); addObject (new alien (), 550, 300); addObject (new alien (), 550, 350); prepare(); } private void prepare() { Plane plane= new Plane(); addObject(plane, 156, 245); plane.setLocation(100, 300); } private void paint(int position) { if (!getObjects(Display.class).isEmpty()) {removeObjects(getObjects(Display.class)); } GreenfootImage bg = getBackground(); bg.drawImage(bgBase, position, 0); bg.drawImage(bgImage, position + picWidth, 0); } }

1.) import java.util.*; 2.) change the numOfAliens() method to this: 3.) You have to insert your own things into the nextWave method. You have to design your own waves like in the constructor. 4. If it doesn't work again, inspect the world an look what value is wave and tell me.

Thank you very much , it worked. Can u help me in two more things ??? 1. Display the text "Wave - 1 " and so on..... 2. My aliens are moving but I want them to move just half of the screen and return back.

1.) Make a new class. Name it text or however you want. Then the world has to tell the text, what value wave is. There are more ways to do this. I pick one of them: The world has to add the text now into it. It's the best, if you do that in the prepare() method, like with all other objects. Text counter = new Text(this); //here you give the airport to the text addObject(counter, getWidth()-counter.getWidth()/2, getHeight()-counter.getHeight()/2); Try this, then I will watch on your other problem.

It is really wasteful (as far as resources are concerned) to update the image of an object every act cycle for something that will change 'once in a blue moon' (as far as how often the wave value will change compared to passing act cycles). It would be better to keep a reference to the 'Text' object that displays the wave number in the world class and reset its text when the wave number changes.