HashSet addAll method modifies class internal field

Question

I have a class which extends a parametrized version of HashSet. This class has an internal field currentSize which keeps track of how many elements have been added so far. The child class overrides the base class' add and addAll methods so the currentSize increases accordingly. My problem is that inside addAll, the size of the set is added two times to the currentSize counter, not once. Below is my class' code:

public class InheritHashSet extends HashSet<Integer> {
    private int currentSize = 0;

    @Override
    public boolean add(Integer e) {
        currentSize++;
        super.add(e);
    }

    @Override
    public boolean addAll(Collection<? extends Integer> e) {
        currentSize += e.size();
        super.addAll(e);
    }

    public int getCurrentSize() {
        return currentSize;
    }
}

The add method seems to be working fine, since after adding three elements, currentSize is 3, but the following will add 6 to the currentSize instead of 3:

InheritHashSet ihs = new InheritHashSet();
Collection<Integer> c = new LinkedList<Integer>();
c.add(new Integer(0));
c.add(new Integer(1));
c.add(new Integer(2));
ihs.addAll(c);
System.out.println(ihs.getCurrentSize()); // prints 6 instead of 3

It looks like the call to super.addAll(e) also modifies the currentSize (this does not seem to make any sense at all).

Answer 1

The addAll() method of the InheritHashSet class first set currentSize = 3, with currentSize += e.size(); instruction. Then super.addAll(e); calls add(e) method three times. For dynamic binding the public boolean add(Integer e) of class InheritHashSet is invoked first and then currentSize is then incremented to 6.

for this reason you should implement addAll method in this way:

@Override
public boolean addAll(Collection<? extends Integer> e) {
    return super.addAll(e);
}