protected Shell[] shells = new Shell[5];
    public Tank() {
        for(int i=0;i<shells.length;i++) {
            //shells[i] = new Shell(this);
        }
    }
public void shoot() {
        if(shells!=null) {
            System.out.println("My Ammo isnt empty"+shells.length);
            for(int i=0;i<shells.length;i++) {
                if(shells[i]!=null) {
                    Shell shootShell = shells[i]; 
                    shootShell.executeShell(); //move the shell bla bla
                    shells[i] = null;
                    break;
                }
            }
        }
    }

protected Shell[] shells = new Shell[5];
protected int coolDown = 0;

public void shoot() {
        if(shells[0]!=null && coolDown == 0) { //is there shells left and coolDown = 0?
            //shoot
            getWorld().addObject((Actor)shells[0], getX(), getY());
            shells[0] = null;
            coolDown = 50;
            //reload
            for(int i=0;i<shells.length-1;i++){
                shells[i] = shells[i+1];
                shells[shells.length-1] = null; //necessary?
            }
        } 
    }