Its not adding the actor again to the world when its null
//world
//Globally
int timer;
//act method
if(Greenfoot.isKeyDown("4"))
{
addObj(tank,40,40);
if(timer>0 && --timer ==0) addObj(new tank(),40,40);
if(timer==0 && getObjects(tank.class).isEmpty())timer=100;
}


