public class Enemy extends Actor {

    Rectangle bounds;
    public Enemy() {
        bounds=new Rectangle((int)getX(), (int)getY(), (int)getWidth(), (int)getHeight());
    }

    public Rectangle getBounds() {
        return bounds;
    }

    private void setXY(float pX,float pY) {
        setPosition(pX, pY);
        bounds.setX((int)pX);
        bounds.setY((int)pY);
    }
}

//init lists
List<Enemy> enemies = new ArrayList<Enemy>();
List<Bullet> bullets = new ArrayList<Bullet>();

//add actors to lists
enemies.Add(new Enemy());
bullets.Add(new Bullet());

for (int i = enemies.size() - 1; i >= 0; i--) {
    Enemy enemy = enemies[i];
    for (int j = bullets.size() - 1; j >= 0; j--) {
        Bullet bullet = bullets[j];
        if(enemy.getBounds().overlaps(bullet.getBounds())) {
            //handle collision
        }
    }
}