Cant remove custom object from HashSet

Question

Why does removeAll(Collection<?> a) returns true when the object inside the collection was not removed? I overrid the hashcode and equals method for my custom object. The hashcode of the Object from the input is the same as the hashcode of the object inside the selection.

 private void onRemove(IStructuredSelection selection) {
            boolean removeAll = getInput().removeAll(selection.toList()); // returns true
            Set<ReadingNodeCfg> input = getInput(); // Object from selection is still there
    }

The object from the input is the same as the object in the selection, so why wont he delete that?

Best regards

edit1 : i generated equals and hashcode by eclipse, getInput() returns a HashSet<ReadingNodeCfg> , selection.toList() returns a List<ReadingNodeCfg>

edit2 :

   for (Object object : selection.toList()) {
        boolean remove = getInput().remove(object); // returns false
        int hashCode = object.hashCode(); // returns 1130504316
        int hashCode2 = getInput().iterator().next().hashCode(); // returns 1130504316
        boolean equals = object.equals(getInput().iterator().next()); // returns true

    }

edit3 : I am using now an IObservableList. That works fine now!

The hashcode changed in the input after modifying. And the object i wanted to delete had the same hashcode as the modified input object, but anyway it didnt worked and im still not sure why.

SOLUTION:

Well i solved the problem now. I created an example for any other people who got the same issues.

@Test
    public void derp() {
        Person person = new Person();
        person.name = "jay";

        Set<Person> humans = new HashSet<>();
        humans.add(person);

        person.name = "fred";
        assertTrue(!humans.remove(person));
    }

Its impossible to remove this person object after it was modified. The hashset cant find the hashcode of the object person in itself.

1. Hashcode of person : 1337
2. Adding it to the Hashcode will create something like a reference between this hashcode and the object
3. Changing the name of the person will change the hashcode: 1338
4. Removing this object from the hashcode will try to delete the object with the hashcode 1337, that wont work
5. If you dont override quals/hashcode, you will be able to remove this item without problems

Best regards

Answer 1

Why does removeAll(Collection a) returns true when the object inside the collection was not removed?

It only returns true when the collection is modified. If it appears your collection is not modified, you are looking the wrong collections. Instead of modifying the result of a getter (which is bad practice) I suggest passing the task to the caller. eg

boolean removeAll = removeAllFromInput(selection.toList()); // returns true

My bet is that getInput() takes a defensive copy of the collection to avoid you incorrectly trying to change it.

Answer 2

SOLUTION:

Well i solved the problem now. I created an example for any other people who got the same issues.

@Test
    public void derp() {
        Person person = new Person();
        person.name = "jay";

        Set<Person> humans = new HashSet<>();
        humans.add(person);

        person.name = "fred";
        assertTrue(!humans.remove(person));
    }

Its impossible to remove this person object after it was modified. The hashset cant find the hashcode of the object person in itself.

1. Hashcode of person: 1337
2. Adding it to the Hashcode will create something like a reference between this hashcode and the object
3. Changing the name of the person will change the hashcode: 1338
4. Removing this object from the hashcode will try to delete the object with the hashcode 1337, that wont work
5. If you dont override equals/hashcode, you will be able to remove this item without problems

Best regards