I tried myself on a little "project" . It is basically a survival game. You move by using W,A,S,D and you shoot in different directions by using the arrow keys. You basically have to survive for as long as possible. The enemies follow you and they freeze on hit and start moving again after about 3 seconds.
The code is the following (execute the "window" class)
Window-class
package TestGame;
import java.awt.Graphics;
public class Window extends GameIntern{
public void init(){
setSize(windowX,windowY);
Thread th = new Thread(this);
th.start();
offscreen = createImage(windowX,windowY);
d = offscreen.getGraphics();
addKeyListener(this);
}
public void paint(Graphics g){
d.clearRect(0,0,windowX,windowY);//clear window
d.drawString(numberEnemies.toString(), 10, 10);//"Score" number of enemies displayed
d.drawRect(x, y, playerWidth, playerHeight);//draw player
for(Enemy e : enemies){//draw all enemies
d.drawRect(e.getx(), e.gety(), playerWidth, playerHeight);
}
for(Bullet b : bullets){//draw all bullets
d.drawOval(b.getx(), b.gety(), bulletSize, bulletSize);
}
g.drawImage(offscreen,0,0,this);
}
public void update(Graphics g){
paint(g);
}
}
GameIntern-class
package TestGame;
import java.applet.Applet;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.util.ArrayList;
import java.util.Random;
public class GameIntern extends Applet implements Runnable , KeyListener {
public int windowX = 854;//size of the window in x direction
public int windowY = 460;//size of the window in y direction
public static int x;//x-coordinate of player
public static int y;//y-coordinate of player
public int playerpositionX = x;
public int playerpositionY = y;
public int playerHeight = 20;//player height
public int playerWidth = 20;//player width
public int playerSpeed = 3;//pixel per frame
public int bulletSize = 5;//diameter of bullets
public int spawnTime = 4;//time for new enemies to spawn in seconds
public int enemySleepTime = 180;//Time an enemy does nothing in Frames per second (180 in 60fps = 3sec)
public boolean alive = true;
public Image offscreen;
public Graphics d;
public boolean up,down,left,right;
private int delay;
private Random random= new Random();
public Integer numberEnemies = new Integer(enemies.size());
protected static ArrayList<Enemy> enemies = new ArrayList<Enemy>(); //List of all enemies
protected static ArrayList<Bullet> bullets = new ArrayList<Bullet>();//List of all bullets
protected static ArrayList<PowerUps> pUps = new ArrayList<PowerUps>();//List of all powerUps
public void run() {
this.x = 400;//startingposition x
this.y = 240;//startingposition y
double ns = 1000000000.0 / 60.0; //60 "frames"
double delta = 0;
long lastTime = System.nanoTime();
while (alive) {
long now = System.nanoTime();
delta += (now - lastTime) / ns;
lastTime = now;
while (delta >= 1) {
repaint();
tick();
collisionEnemy();
// collisionPowerUp();
delta--;
}
}
}
/**
* Method to calculate all objects and their positions per frame
*/
private void tick() {
if(left == true){
if(x>=0 + playerSpeed){
x-=playerSpeed;
}else{ x=0;}//Farthest left x-coordinate
repaint();
}
if(right == true){
if(x<=windowX - playerWidth - playerSpeed){
x+=playerSpeed;
}else{ x=windowX - playerWidth;}//Farthest right x-coordinate
repaint();
}
if(up == true){
if(y>=0 + playerSpeed){
y-=playerSpeed;
}else{ y=0;}//Highest y-coordinate
repaint();
}
if(down == true){
if(y<=windowY - playerHeight - playerSpeed){
y+=playerSpeed;
}else{y=windowY - playerHeight;}//Lowest y-coordinate
repaint();
}
for (Enemy e : enemies) { //Tick every enemy
e.tick();
}
for (Bullet b : bullets){ //Tick every bullet
b.tick();
}
if(delay % (60 * spawnTime) == 0){ //Spawn enemy
enemies.add(new Enemy(random.nextInt(windowX), random.nextInt(windowY)));
numberEnemies++;
}
delay++;
for(Enemy e : enemies){ //collision : enemy & bullet
for(Bullet b : bullets){
if(b.getx()+bulletSize >= e.getx() && b.getx() <= e.getx()+20){
if(b.gety()+bulletSize >= e.gety() && b.gety() <= e.gety()+20){
e.setHit();
b.setRemove();
}
}
}
}
for(int i = 0; i< bullets.size(); i++){ //Remove bullets from ArrayList
if(bullets.get(i).remove){
bullets.remove(i);
}
}
}
public void keyPressed(KeyEvent e) {
if(e.getKeyCode() == 65){//W
left=true;
}
if(e.getKeyCode() == 87){//A
up=true;
}
if(e.getKeyCode() == 68){//S
right=true;
}
if(e.getKeyCode() == 83){//D
down=true;
}
}
public void keyReleased(KeyEvent e) {
if(e.getKeyCode() == 65){//Arrowkey left
left=false;
}
if(e.getKeyCode() == 87){//Arrowkey up
up=false;
}
if(e.getKeyCode() == 68){//Arrowkey right
right=false;
}
if(e.getKeyCode() == 83){//Arrowkey dowm
down=false;
}
if(e.getKeyCode() == 37){//Arrowkey left
bullets.add(new Bullet(x,y,false,false,true,false)); //Direction the bullet has to go
}
if(e.getKeyCode() == 38){//Arrowkey up
bullets.add(new Bullet(x,y,true,false,false,false));//Direction the bullet has to go
}
if(e.getKeyCode() == 39){//Arrowkey right
bullets.add(new Bullet(x,y,false,false,false,true));//Direction the bullet has to go
}
if(e.getKeyCode() == 40){//Arrowkey down
bullets.add(new Bullet(x,y,false,true,false,false));//Direction the bullet has to go
}
}
public void keyTyped(KeyEvent e){}
/**
* Method to see if the player collided with an enemy
*/
public void collisionEnemy(){
for(Enemy e : enemies){
for(int i = 0;i <= playerWidth; i++){
if(GameIntern.x+i >= e.getx() && GameIntern.x+i <= e.getx()+playerWidth){
if(GameIntern.y+i >= e.gety() && GameIntern.y+i <= e.gety()+playerHeight){
alive = false;
}
}
}
}
}
// public void addEnemy(){
// enemies.add(new Enemy(random.nextInt(windowX), random.nextInt(windowY)));
//
// //Spawn enemies inside the filed, not outside the boarder
// if (playerpositionX < playerWidth * 2 || playerpositionX * 2 > windowX - 2*playerWidth || playerpositionY * 2 > windowY - 2*playerHeight || playerpositionY < playerHeight * 2){
// enemies.add(new Enemy(random.nextInt(windowX - 3*playerWidth), random.nextInt(windowY - 3*playerHeight)+3*playerHeight));
// }else {
// int temp1 = random.nextInt(windowX-3*playerWidth);
// if (temp < playerpositionX){
//
// }
// enemies.add(new Enemy(random.nextInt(windowX), random.nextInt(windowY)));
// }
//
// }
}
Bullet-class
package TestGame;
public class Bullet extends GameIntern{
public int x,y;
public boolean up,down,left,right;
public boolean remove;
public Bullet(int x, int y,boolean up,boolean down, boolean left, boolean right){
this.x = x + 8;
this.y = y + 8;
this.up = up;
this.down = down;
this.left = left;
this.right = right;
}
public int getx(){
return this.x;
}
public int gety(){
return this.y;
}
public void setRemove(){
remove=true;
}
public void tick() {
if (up == true) y-=2;
if (down == true) y+=2;
if (left == true) x-=2;
if (right == true) x+=2;
if(x < 0){
remove = true;
}
if(x > 840){
remove = true;
}
if(y < 0){
remove = true;
}
if(y > 470){
remove = true;
}
}
}
Enemy-class
package TestGame;
public class Enemy extends GameIntern {
public int x,y;
public boolean hit = false;
public int counter = 0;
public Enemy(int x, int y) {
this.x = x;
this.y = y;
}
public int getx(){
return this.x;
}
public int gety(){
return this.y;
}
public void setHit(){
hit = true;
counter = enemySleepTime;
}
public void tick() {
if(counter == 0){
if(hit == true){
hit=false;
}
if (x < GameIntern.x) x++;
if (x > GameIntern.x) x--;
if (y < GameIntern.y) y++;
if (y > GameIntern.y) y--;
}else {counter--;}
}
}
After playing for a while i get a
java.util.ConcurrentModificationException
What does that mean?
Also, I struggle to improve the enemy spawn prcess. Right now it sometimes happens, that enemies spawn inside the player. I want to have an imaginary box around the player where enemies dont spawn inside and enemies should always spawn inside the window.
If you have any questions, please feeld free to ask :)
sincerely Viktor
From the JavaDocs:
The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException.
The problem is likely caused because you're removing bullets from the ArrayList while trying to paint them in a separate thread. The issue is that the two threads are iterating over the bullets list simultaneously.
//The following for loop is likely the cause
for(int i = 0; i< bullets.size(); i++){
if(bullets.get(i).remove){
bullets.remove(i); // <-- this is the problem
}
}
Instead, try using a thread-safe implementation, e.g.
protected static List<Enemy> enemies =
Collections.synchronizedList(new ArrayList<Enemy>()); //List of all enemies
protected static List<Bullet> bullets =
Collections.synchronizedList(new ArrayList<Bullet>());//List of all bullets
Furthermore, you should change the remove code to avoid an IndexOutOfBoundsException:
LinkedList<Bullet> bulletsToRemove = new LinkedList<>();
for(Enemy e : enemies){ //collision : enemy & bullet
for(Bullet b : bullets){
if(b.getx()+bulletSize >= e.getx() && b.getx() <= e.getx()+20){
if(b.gety()+bulletSize >= e.gety() && b.gety() <= e.gety()+20){
e.setHit();
bulletsToRemove.add(b);
}
}
}
}
for(Bullet b : bulletsToRemove){ //Remove bullets from ArrayList
bullets.remove(b);
}
Regarding the spawning process, a simplistic approach is to just define a minimum distance from the player, call it minDist. Now just pick a random location anywhere on the screen, call it p1. If p1 is less than minDist away from the player, pick a new random location. As long as the player and enemy sprites are relatively small compared to the screen area, this approach should work well.