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

again lambda saves the day:

List duplicates = duplicate.stream()
  .collect( Collectors.collectingAndThen( Collectors.groupingBy( Function.identity() ),
    map -> {
      map.values().removeIf( v -> v.size() < 2 );  // eliminate unique values (4, 8 in this case)
      return( map.values().stream().flatMap( List::stream ).collect( Collectors.toList() ) );
    } ) );  // [6, 6, 7, 7]

the speed-optimized version of the above solution:

List duplicates = duplicate.stream().collect( Collectors.collectingAndThen(
    Collectors.groupingBy( Function.identity(), Collectors.counting() ),
    map -> {
      map.values().removeIf( v -> v < 2 );  // eliminate unique values (4, 8 in this case)
      return( map.entrySet().stream().collect( Collector.of( ArrayList::new, (list, e) -> {
        for( long n = 0; n < e.getValue(); n++ )
          list.add( e.getKey() );
      }, (l1, l2) -> null ) ) );
    } ) );  // [6, 6, 7, 7]

the long values of duplicate are not saved but counted — quite certainly the fastest and most space-saving variant