Java modifiable view on collection

Question

I'm looking to solve the following problem:
Starting with a collection A, I want to pass some kind of 'view' on that collection (say collection B) to a certain method. The view B does not necessary contain all the elements of the original collection A. If in this method objects are added to or removed from the view (collection B), these changes should also be reflected on the original collection A as well.

For instance (pseudo-code):

1. Start situation:

   Collection A = {1, 2, 3};  
   View-on-collection B = {1, 2};
   

2. Method call:

   someMethod(B) {  
       B.add(4);  
       B.remove(2);  
   }
   

3. End situation:

   Collection A = {1, 3, 4};
   

Does anyone know a neat solution to this problem?

Answer 1

One way is to use List.sublist():

public static void main(String[] args) {
    List<Integer> aList = new ArrayList<Integer>(Arrays.asList(1,2,3));
    List<Integer> view = aList.subList(0, 2);

    view.add(new Integer(4));
    view.remove(new Integer(2));
    System.out.println("aList: " + aList);
    System.out.println("view : " + view);        
}

Another more general way would be through Guavas Collections2.filter(), that lets you define a predicate to control which objects should be in the view:

public static void main(String[] args) {

    List<Integer> aList = new ArrayList<Integer>(Arrays.asList(1,2,3));
    @SuppressWarnings("unchecked")
    Collection<Integer> view = Collections2.filter(aList, new Predicate() {
        public boolean apply(Object arg0) {
            return ((Integer) arg0).intValue() % 3 != 0;
        }});
    view.add(new Integer(4));
    view.remove(new Integer(2));
    System.out.println("aList: " + aList);
    System.out.println("view : " + view);

}

Both examples print

aList: [1, 4, 3]
view : [1, 4]