Chapter 5: Other actors and collision

A game needs more than one object. Add a second Actor, place it with addObject, then ask the player whether it is touching something.

A collectible

Coins, walls, and enemies are all subclasses of Actor. If the object never needs per-tick logic, override isTickInert() so the world skips act() (cheaper when you have many tiles). Decorative scenery can also use setStatic(true) so it stays off the debug list. 

import ActiverseEngine.*;

public class Coin extends Actor {
    public Coin() {
        setImage(new ActiverseImage("assets/images/coin.png"));
    }

    @Override
    public void act() {
        // no per-tick behavior
    }

    @Override
    public boolean isTickInert() {
        return true;
    }
}
public class MyWorld extends World {
    public MyWorld() {
        super(400, 400, 1);
        addObject(new Player(), 100, 100);
        addObject(new Coin(), 200, 180);
        addObject(new Coin(), 280, 90);
    }
}

Detecting a hit

Two methods beginners actually use:

Pixel-perfect checks need loaded images. If an image is still missing, collision falls back to boxes. move() already refuses to walk off the world edge; you do not need extra bounds code for that. 

@Override
public void act() {
    movementViaKey();

    Actor hit = getOneIntersectingObject();
    if (hit instanceof Coin) {
        playPickup();
        getWorld().removeObject(hit);
    }
}

removeObject takes the actor out of the world. After that, do not keep using the removed reference. getWorld() is null-safe to call from act() only while the actor is still in a world — call it before you remove yourself.

instanceof is the filter

getOneIntersectingObject() returns any overlapping actor, including other players or walls. Check the type (or a field you add) before you treat it as a coin. For several overlapping objects in one tick, the method returns the first match; if you need all of them, loop getWorld().getActors() and test intersects yourself. 

for (Actor other : getWorld().getActors()) {
    if (other != this && other instanceof Coin && intersects(other)) {
        getWorld().removeObject(other);
        break;
    }
}

Showing a score

World.showText(x, y, "Score: 3") draws a string each frame. Keep a counter on the world or the player and update the text when a coin disappears. 

private int score = 0;

public void addScore(int amount) {
    score += amount;
    showText(10, 20, "Score: " + score);
}

Call ((MyWorld) getWorld()).addScore(1) from the player when a coin is collected, or keep the counter on the player and have the world read it. Either is fine for a first game.

What to do next