Question

I have an ArrayList<MyObject> that may (or may not) contain duplicates of MyObject I need to remove from the List. How can I do this in a way that I don’t have to check duplication twice as I would do if I were to iterate the list in two for-loops and cross checking every item with every other item.

I just need to check every item once, so comparing A:B is enough – I don’t want to compare B:A again, as I already did that.

Furthermore; can I just remove duplicates from the list while looping? Or will that somehow break the list and my loop?

Edit: Okay, I forgot an important part looking through the first answers: A duplicate of MyObject is not just meant in the Java way meaning Object.equals(Object), but I need to be able to compare objects using my own algorithm, as the equality of MyObjects is calculated using an algorithm that checks the Object’s fields in a special way that I need to implement!

Furthermore, I can’t just override euqals in MyObject as there are several, different Algorithms that implement different strategies for checking the equality of two MyObjects – e.g. there is a simple HashComparer and a more complex EuclidDistanceComparer, both being AbstractComparers implementing different algorithms for the public abstract boolean isEqual(MyObject obj1, MyObject obj2);

Answer 1

Sort the list, and the duplicates will be adjacent to each other, making them easy to identify and remove. Just go through the list remembering the value of the previous item so you can compare it with the current one. If they are the same, remove the current item.

And if you use an ordinary for-loop to go through the list, you control the current position. That means that when you remove an item, you can decrement the position (n--) so that the next time around the loop will visit the same position (which will now be the next item).

You need to provide a custom comparison in your sort? That’s not so hard:

Collections.sort(myArrayList, new Comparator<MyObject>() {

    public int compare(MyObject o1, MyObject o2) {
        return o1.getThing().compareTo(o2.getThing());
    }
});

I’ve written this example so that getThing().compareTo() stands in for whatever you want to do to compare the two objects. You must return an integer that is zero if they are the same, greater than 1 if o1 is greater than o2 and -1 if o1 is less than o2. If getThing() returned a String or a Date, you’d be all set because those classes have a compareTo method already. But you can put whatever code you need to in your custom Comparator.