Question

I’m using a singleton for granting a unique copy of each object in runtime:

Car object1= CarFactory.createCar(id);

where the createCar method is:

private static ArrayList<Car> cars= new ArrayList<Car>();

public static synchronized Car createCar(int id){
   Car result= new Car(id);
   int index= cars.indexOf(result);
   if (index==-1){
      cars.add(result);
      return result;
   } else {
      return cars.get(index);
   }
}

The problem is that with this method each Car have always an reference due to the “cars” collection and the object’s memory is never released. How can I improve it?

Answer 1

Use a WeakReference to wrap the objects before putting them in the list. Example:

private static List<WeakReference<Car>> cars = new ArrayList<WeakReference<Car>>();

public static synchronized Car createCar(int id) {
   WeakReference<Car> result= new WeakReference<Car>(new Car(id));
   int index= cars.indexOf(result);
   if (index==-1){
      cars.add(result);
      return result.get();
   } else {
      return cars.get(index).get();
   }
}

Note: This is not production code. You will have to build some more safeguards before using it.