In this post, we’re going to explore different ways to sort lists alphabetically in Java. We’ll cover various sorting approaches using Java Streams, Java Collections, TreeSet, Comparator, and the RuleBasedCollator functionality.

Sort Lists Alphabetically with Java Streams

With the introduction of Java 8, Streams were introduced in Java. Let’s see how we can sort lists alphabetically in Java using streams.

To perform sorting with Streams, we use the sorted method, which returns a Stream of Strings.

Natural Alphabetical Sorting

In the following example, we use sorted() to obtain natural alphabetical sorting. Let’s see it in the following test:

@Test
void given_name_list_when_sort_then_return_a_list_sorted() {
    List<String> names = List.of("Pepe","Pablo","Noel","Susan","John","Aman");
    List<String> sortedNames = names.stream()
      .sorted()
      .collect(Collectors.toList());

    List<String> expectedNames = List.of("Aman","John","Noel","Pablo","Pepe","Susan");
    assertThat(sortedNames).isEqualTo(expectedNames);
}

Reverse Alphabetical Sorting

To perform reverse sorting, we can use the reverseOrder method of Comparator or a lambda function.

@Test
void given_name_list_when_reverse_sort_then_return_a_list_sorted() {
    List<String> names = List.of("Pepe","Pablo","Noel","Susan","John","Aman");
    List<String> sortedNames = names.stream()
      .sorted(Comparator.reverseOrder())
      .collect(Collectors.toList());

    List<String> expectedNames = List.of("Susan","Pepe", "Pablo", "Noel", "John", "Aman");
    assertThat(sortedNames).isEqualTo(expectedNames);
}

Alphabetical Sorting with Java Collection

Java Collection provides the sort method for performing natural sorting and the Comparator for performing reverse sorting.

Natural Sorting with Collection in Java

The sort method can be used on any Collection and it returns the fully sorted list. If no comparator is applied, the list is sorted in natural order (A to Z).

Let’s see how to use sort() on Java collections:

@Test
void given_name_list_when_sort_then_return_a_list_sorted() {
    List<String> names = List.of("Pepe","Pablo","Noel","Susan","John","Aman");
    
    Collections.sort(names);

    List<String> expectedNames = List.of("Aman","John","Noel","Pablo","Pepe","Susan");
    assertThat(sortedNames).isEqualTo(expectedNames);
}

Reverse Sorting with Collection in Java

Let’s perform reverse sorting using Comparator. Since Comparator is a functional interface, it can be used in a lambda expression.

@Test
void given_name_list_when_reverse_sort_then_return_a_list_sorted() {
    List<String> names = List.of("Pepe","Pablo","Noel","Susan","John","Aman");

    Collections.sort(names, Comparator.reverseOrder());


    List<String> expectedNames = List.of("Susan","Pepe", "Pablo", "Noel", "John", "Aman");
    assertThat(sortedNames).isEqualTo(expectedNames);
}

Natural Sorting in Lists in Java

Java lists have their own sort method for sorting instead of using Collection.sort().

@Test
void given_name_list_when_sort_then_return_a_list_sorted() {
    List<String> names = List.of("Pepe","Pablo","Noel","Susan","John","Aman");
    
    names.sort();

    List<String> expectedNames = List.of("Aman","John","Noel","Pablo","Pepe","Susan");
    assertThat(sortedNames).isEqualTo(expectedNames);
}

Reverse Sorting with List in Java

To perform reverse sorting of a list, we can apply Comparator.reverseOrder().

@Test
void given_name_list_when_inverse_sort_then_return_a_list_sorted() {
    List<String> names = List.of("Pepe","Pablo","Noel","Susan","John","Aman");
    
    names.sort(Comparator.reverseOrder());

    List<String> expectedNames = List.of("Susan","Pepe", "Pablo", "Noel", "John", "Aman");
    assertThat(sortedNames).isEqualTo(expectedNames);
}

Performing Sorting in Java with Rule-Based Collator

When using the RuleBasedCollator class, we can define a series of steps or rules to define an order in a list.

Let’s see an example where we use a rule to create a new sorting:

  @Test
  void given_name_list_when_request_to_sort_with_rule_based_collector_then_list_is_returned() throws ParseException {
    List<String> names = Arrays.asList(
        "Pepe","Pablo","Noel","Susan","John","Aman");

    Collections.sort(names);

    List<String> namesOrderExpected = Arrays.asList(
        "Aman","John","Noel","Pablo","Pepe","Susan");

    assertThat(names).isEqualTo(namesOrderExpected);

    List<String> rulesBasedExpected = Arrays.asList(
        "Noel","Pablo","Pepe","Susan","Aman","John");

    String rule = "< n, N < P, S < a, A < j, J";

    RuleBasedCollator rulesCollator = new RuleBasedCollator(rule);

    names.sort(rulesCollator);

    assertThat(names).isEqualTo(rulesBasedExpected);
  }

In the above example, we have altered the logical order using the RuleBasedCollector object, where we have established the order: “< n, N < P, S < a, A < j, J”.

Sorting in Java with TreeSet

It’s not very common to use TreeSet, but using this class allows us to perform alphabetical sorting in Java.

TreeSet enables natural and reverse alphabetical sorting. The only condition is that it does not retain duplicate elements.

@Test
void given_name_list_when_sort_then_return_a_list_sorted() {
    List<String> names = List.of("Pepe","Pablo","Noel","Susan","John","Aman");
    
    SortedSet<String> sortedTreeSet = new TreeSet<>(names);
    List<String> sortedList = new ArrayList<>(sortedTreeSet);

    List<String> expectedNames = List.of("Aman","John","Noel","Pablo","Pepe","Susan");
    assertThat(sortedNames).isEqualTo(expectedNames);
}

Conclusion

In this post about sort lists alphabetically in Java, we have seen different approaches to perform list sorting.