// fields (variables)
int tankCount;
int spawnRate; // secs between spawn during a wave
int pauseTime = 15; // time between waves in secs
int actCount;

// initializing a wave
tankCount = 10;
spawnRate = 3;
actCount = 60*(pauseTime+tankCount*spawnRate);

// in act
if (actCount != 60*pauseTime) // spawning or resting
{
    if (--actCount == 0) newWave(); // end resting
    else if (actCount >= 60*pauseTime && (actCount-60*pauseTime)%(60*spawnRate) == 0) spawnTank();
}
else if (getObjects(Tank.class).isEmpty()) actCount--; // start resting

    int tankCount;
    int spawnRate; //spawning tanks
    int interval = 15; //time between waves
    int actCount;
   
    public void act()
    {
      //new wave
      actCount = 60*(interval+tankCount*spawnRate);
      tankCount = 10;
      spawnRate = 3;
      
      if (actCount != 60*interval) // spawning or resting
      {
            if (--actCount == 0)
            {
             newWave(); // end resting
            } 
            else if (actCount >= 60*interval && (actCount-60*interval)%(60*spawnRate) == 0) 
            {
              spawnTank();
            }
       }
      else if (getObjects(tank.class).isEmpty())
       {
           actCount--; // start resting
       }
    }
   
    public void newWave()
    {
    }
    
        public void spawnTank()
    {
    
        addObject(new tank(),681, 756);
     
    }

public MyWorld()
{
    super(/** whatever */);
    // the following lines go here
    tankCount = 10;
    spawnRate = 3;
    newWave();
}

// the newWave method can just be this
public void newWave()
{
    actCount = 60*(interval+tankCount*spawnRate);
}