java-collection java-hashset hash memory

<collection>

Collection collection   = new HashSet();
boolean containsElement = collection.contains("an element");

Collection elements     = new HashSet();
boolean containsAll     = collection.containsAll(elements);


Collection colA = [A,B,C]
Collection colB = [1,2,3]

Collection target = [];

target.addAll(colA);     //target now contains [A,B,C]
target.addAll(colB);     //target now contains [A,B,C,1,2,3]

target.retainAll(colB);  //target now contains [1,2,3]

target.removeAll(colA);  //nothing happens - already removed
target.removeAll(colB);  //target is now empty.