How to compare two large array lists efficient in java

Question

so I have 2 large arraylists of objects of A 500k and B 900k in size and I want to create a 3rd list (c) of all the dups of A in B.

So I've been doing

    for(obj as:B)
    {

        if(!A.contains(as.getA()))
        {
            C.add(as);
        }

    }

But this takes quite a bit of time to process. Would maps or sets be quicker or any other suggestions? Thanks.

If they are sortable, sort them first then use a binary search (see Collections class). If you can deal with the overhead of putting them in a Set then see the answer by @Kushan. — john16384
– john16384, Commented Mar 20, 2017 at 17:11

Kushan · Accepted Answer · 2017-03-20 17:08:07Z

3

Use HashSet collection

List<MyClass> myList = new ArrayList<MyClass>();
myList.addAll(listA);
myList.addAll(listB);

Set<MyClass> mySet = new HashSet<MyClass>(myList);

answered Mar 20, 2017 at 17:08

Kushan

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2 Comments

Since duplicates are desired should use myList.retainAll(listB) ?

retainAll means removes from the list all of its elements that are not contained in the specified collection