//for Entity.java
public boolean isColliding(Entity that){
    return Entity.intersectsAABB(this, that);
}

public void onCollision(Entity that){
    // no default implementation
}

public static boolean intersectsAABB(Entity a, Entity b) {
    return !(a.right() < b.left()
            || b.right() < a.left()
            || a.bottom() < b.top()
            || b.bottom() < a.top());
}