Get Duplicate Elements https://www.v2ex.com/t/333143

DuplicateElementUtils.java

package com.v2ex.utils;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

public class DuplicateElementUtils{
  public static <T> List<T> getDuplicateElements(List<T> list){
    List<T> result = new ArrayList<T>();
    Set<T> set = new HashSet<T>();
    for(T e : list){
      if(!set.add(e)){
        result.add(e);
      }
    }
    return result;
  }
}