Java Stream sorted() Example with/without Comparator

Since Java 8, the sorted() method is part of the Stream API and is used to sort the elements of a stream. By default, elements are sorted in the natural order, but we can apply for a custom order using a Comparator.

//Default sorting
List sortedList = unsortedList.stream().sorted().toList();

//Custom Sorting
Comparator comparator = ...;
List sortedList = unsortedList.stream().sorted(comparator).toList();

1. Stream sort() is an Overloaded Method

The Stream interface provides two methods for sorting the elements:

• sorted() – Provides the default sorting
• sorted(Comparator) – Sorting based on the provided comparator.

1.1. Stream sorted()

Stream<T> sorted()

• sorted() is a stateful intermediate operation that returns a new Stream.
• It returns a stream consisting of the elements of this stream, sorted according to the natural order.
• If the elements of this stream are not Comparable, a java.lang.ClassCastException may be thrown when the terminal operation is executed.
• For ordered streams, the sort is stable.
• For unordered streams, no stability guarantees are made.

1.2. Stream sorted(comparator)

Stream<T> sorted(Comparator<? super T> comparator)

• This is a stateful intermediate operation that returns a new stream.
• It returns a stream consisting of the elements of this stream, sorted according to the provided Comparator..
• For ordered streams, the sort is stable.
• For unordered streams, no stability guarantees are made.

2. Stream sorted() Examples

2.1. Sorting Stream Elements in Natural Order

In the given Java example, we are sorting a List of integers in the natural order and printing them into the standard output.

List<Integer> list = Arrays.asList(2, 4, 1, 3, 7, 5, 9, 6, 8);

List<Integer> sortedList = list.stream()
      .sorted()
      .collect(Collectors.toList());

System.out.println(sortedList);

Program output.

[1, 2, 3, 4, 5, 6, 7, 8, 9]

2.2. Sorting Stream Elements in Reverse Order

In the given Java example, we are sorting a stream of integers in reverse order using a Comparator.reverseOrder() and printing them into the standard output.

List<Integer> list = Arrays.asList(2, 4, 1, 3, 7, 5, 9, 6, 8);

List<Integer> sortedList = list.stream()
        .sorted(Comparator.reverseOrder())
        .collect(Collectors.toList());

System.out.println(sortedList);

Program output.

[9, 8, 7, 6, 5, 4, 3, 2, 1]

2.3. Sort Stream Elements in Custom Order using Comparator

In the given Java example, we are sorting a stream of integers using a custom Comparator.

List<Integer> list = Arrays.asList(2, 4, 1, 3, 7, 5, 9, 6, 8);
 
Comparator<Integer> reverseComparator = new Comparator<Integer>() {
  @Override
  public int compare(Integer i1, Integer i2) {
    return i2.compareTo(i1);
  }
}; 

List<Integer> sortedList = list.stream()
      .sorted(reverseComparator)
      .collect(Collectors.toList());

System.out.println(sortedList);

Program output.

[9, 8, 7, 6, 5, 4, 3, 2, 1]

2.4. Stream Sorting using Lambda Expressions

Java example to sort a stream of integers in reverse order using lambda expression to specify the comparison logic.

We are rewriting the previous Comparator logic with an inline lambda expression.

List<Integer> list = Arrays.asList(2, 4, 1, 3, 7, 5, 9, 6, 8);

List<Integer> sortedList = list.stream()
      .sorted( (i1, i2) -> i2.compareTo(i1) )
      .collect(Collectors.toList());

System.out.println(sortedList);

Program output.

[9, 8, 7, 6, 5, 4, 3, 2, 1]

3. Conclusion

In this Java 8 Stream tutorial, we learned the basics of sorting the stream elements using sorted() method. We learned to sort in default ascending order as well as custom order using a Comparator.

Happy Learning !!

Source Code on Github