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

Here's my version of the solution:

import java.util.ArrayList;

public class Main {

public static void main(String[] args) {

    ArrayList randomNumbers = new ArrayList();
    ArrayList expandingPlace = new ArrayList();
    ArrayList sequenceOfDuplicates = new ArrayList();

    for (int i = 0; i < 100; i++) {
        randomNumbers.add((int) (Math.random() * 10));
        expandingPlace.add(randomNumbers.get(i));
    }

    System.out.println(randomNumbers); // Original list.

    for (int i = 0; i < randomNumbers.size(); i++) {
        if (expandingPlace.get(i) == expandingPlace.get(i + 1)) {
            expandingPlace.add(0);
            sequenceOfDuplicates.add(expandingPlace.get(i)); 
            sequenceOfDuplicates.add(expandingPlace.get(i + 1));
        }
    }

    System.out.println(sequenceOfDuplicates); // What was in duplicate there.

}

}

It adds numbers from 0 to 9 to a list, and it adds to another list what is in "duplicate" (a number followed by the same number). You can use your big list instead of my randomNumbers ArrayList.