How to compare two arraylist?

Question

I have two ArrayList. Each is of size 100000. I want to compare them and count matched elements.

Here's my code:

for (int i = 0; i < mArryLst2.size(); i++) {
   if (ArryLst1.contains(mArryLst2.get(i))) {
       matchedPixels++;         
   }
}

Here comparison process is taking lot of time.

How to solve and optimize this problem.

Answer 1

you should use CollectionUtils.retainAll : Returns a collection containing all the elements in collection1 that are also in collection2.

ArrayList commonList = CollectionUtils.retainAll(list1,list2);

Answer 2

You should transform you first list into a HashSet. HashSet lookups are O(1), and List lookups are O(n). This makes the whole algorithm O(n) rather than O(n^2)

Set<Foo> set1 = new HashSet<Foo>(list1);
for (Foo foo : list2) {
    if (set1.contains(foo)) {
        matchedPixels++;
    }
}

Answer 3

you should look at this link How to compare two Arraylist values in java?. make a copy of one of the list and then call remove all for the list against the other list

List result = new ArrayList(mArryLst2);
result.removeAll(ArryLst1);