Collection.addAll?

Question

I have an arbitary object an override the equals() method on the object.
eg.
public class Thing {
public boolean equals() {
I have a Vector of these Thing objects.
Now the problem I get is when I create a HashSet and add the Vector using addAll(Collection c) method, the signature of the collection is added correctly, however when I call the contains(Object o) method, the signature of the equals() method points to the Object.equals() method instead of Thing.equals().
eg.
Vector v = new Vector();
Thing thing1 = new Thing();
thing1.setUniqueID("1");
Thing thing2 = new Thing();
thing2.setUniqueID("2");
v.add(thing1);
v.add(thing2);
HashSet hs = new HashSet();
hs.addAll(v);
Thing thing3 = new Thing();
thing3.setID("2");
boolean flagFound = hs.contains(thing);
... flagFound returns false as hs.contains calls the Object.equals() method Not Thing's equals method.
My question:
How do I add a collection such that the function pointer of equals() method points to the derived class rather than the Object.equals() method (that just does a reference comparision).

Answer 1

you need to also override the hashCode method in your class:
public class Thing {
    public boolean equals() {
    public int hashCode() {
        return id;   // or something similar
}that's because the HashSet first uses the hashCode to find an object and then calls equals. If you don't override it, the object's address is used, so thing3 ends up with a different hashcode than thing2.
regards,
paris