How to select duplicate values from a list in java?

Question

For example my list contains {4, 6, 6, 7, 7, 8} and I want final result = {6, 6, 7, 7}

One way is to loop through the list and eliminate unique values (4, 8 in this

Answer 1

Given that you can do this by looping through the list only once, I wouldn't worry about performance too much. If you search for more performant solutions then you'll probably end up over-complicating the code and the readability and maintainability will suffer. At the end of the day, if you want to check the whole list for duplicates then you have to visit every element.

I'd advise writing the obvious solution and see how it performs. You'll probably be surprised how fast Java can iterate over a list, even if it is particularly large.