How to add all elements of an ArrayList into another ArrayList in java

Question

I'm trying to add all the elements of an ArrayList into another ArrayList,

I've tried using

if (!listTwo.isEmpty()){
    finalList.addAll(listTwo);
    }

finalList.addAll(listTwo);

However, this keeps sending me an NullPointerException error. Both are ArryList, and listTwo does have elements inside of it.

Any idea why it's sending this exception? Thanks a lot

Answer 1

finalList.addAll(listTwo);

Will add the elements from listTwo into finalList .

If you are getting a NullPointerException it is because one of your lists is null. Note that isEmpty() will throw a NullPointerException if the list is null, so that probably won't help you.

To check if the lists are null, do listTwo == null and/or finalList == null .