HashSet.remove() and Iterator.remove() not working

Under the covers, HashSet uses HashMap, which calls HashMap.removeEntryForKey(Object) when either HashSet.remove(Object) or Iterator.remove() is called. This method uses both hashCode() and equals() to validate that it is removing the proper object from the collection.

If both Iterator.remove() and HashSet.remove(Object) are not working, then something is definitely wrong with your equals() or hashCode() methods. Posting the code for these would be helpful in diagnosis of your issue.

I was very curious about this one still, and wrote the following test:

import java.util.HashSet;
import java.util.Iterator;
import java.util.Random;
import java.util.Set;

public class HashCodeTest {
    private int hashCode = 0;

    @Override public int hashCode() {
        return hashCode ++;
    }

    public static void main(String[] args) {
        Set<HashCodeTest> set = new HashSet<HashCodeTest>();

        set.add(new HashCodeTest());
        System.out.println(set.size());
        for (Iterator<HashCodeTest> iter = set.iterator();
                iter.hasNext();) {
            iter.next();
            iter.remove();
        }
        System.out.println(set.size());
    }
}

which results in:

1
1

If the hashCode() value of an object has changed since it was added to the HashSet, it seems to render the object unremovable.

I'm not sure if that's the problem you're running into, but it's something to look into if you decide to re-visit this.

Are you absolutely certain that DataResult is immutable? What is the type of the timestamp? If it's a java.util.Date are you making copies of it when you're initializing the DataResult? Keep in mind that java.util.Date is mutable.

For instance:

Date timestamp = new Date();
DataResult d = new DataResult(timestamp);
System.out.println(d.getTimestamp());
timestamp.setTime(System.currentTimeMillis());
System.out.println(d.getTimestamp());

Would print two different times.

It would also help if you could post some source code.