1. Overview

In this tutorial, we're going to take a first look at the Lambda support in Java 8 – specifically at how to leverage it to write the Comparator and sort a Collection.

This article is part of the “Java – Back to Basic” series here on Baeldung.

First, let's define a simple entity class:

public class Human { private String name; private int age; // standard constructors, getters/setters, equals and hashcode }

2. Basic Sort Without Lambdas

Before Java 8, sorting a collection would involve creating an anonymous inner class for the Comparator used in the sort:

new Comparator<Human>() { @Override public int compare(Human h1, Human h2) { return h1.getName().compareTo(h2.getName()); } }

This would simply be used to sort the List of Human entities:

@Test public void givenPreLambda_whenSortingEntitiesByName_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 10), new Human("Jack", 12) ); Collections.sort(humans, new Comparator<Human>() { @Override public int compare(Human h1, Human h2) { return h1.getName().compareTo(h2.getName()); } }); Assert.assertThat(humans.get(0), equalTo(new Human("Jack", 12))); }

3. Basic Sort With Lambda Support

With the introduction of Lambdas, we can now bypass the anonymous inner class and achieve the same result with simple, functional semantics:

(final Human h1, final Human h2) -> h1.getName().compareTo(h2.getName());

Similarly – we can now test the behavior just as before:

@Test public void whenSortingEntitiesByName_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 10), new Human("Jack", 12) ); humans.sort( (Human h1, Human h2) -> h1.getName().compareTo(h2.getName())); assertThat(humans.get(0), equalTo(new Human("Jack", 12))); }

Notice that we're also using the new sort API added to java.util.List in Java 8 – instead of the old Collections.sort API.

4. Basic Sorting With No Type Definitions

We can further simplify the expression by not specifying the type definitions – the compiler is capable of inferring these on its own:

(h1, h2) -> h1.getName().compareTo(h2.getName())

And again, the test remains very similar:

@Test public void givenLambdaShortForm_whenSortingEntitiesByName_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 10), new Human("Jack", 12) ); humans.sort((h1, h2) -> h1.getName().compareTo(h2.getName())); assertThat(humans.get(0), equalTo(new Human("Jack", 12))); }

5. Sort Using Reference to Static Method

Next, we're going to perform the sort using a Lambda Expression with a reference to a static method.

First, we're going to define the method compareByNameThenAge – with the exact same signature as the compare method in a Comparator<Human> object:

public static int compareByNameThenAge(Human lhs, Human rhs) { if (lhs.name.equals(rhs.name)) { return Integer.compare(lhs.age, rhs.age); } else { return lhs.name.compareTo(rhs.name); } }

Now, we're going to call the humans.sort method with this reference:

humans.sort(Human::compareByNameThenAge);

The end result is a working sorting of the collection using the static method as a Comparator:

@Test public void givenMethodDefinition_whenSortingEntitiesByNameThenAge_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 10), new Human("Jack", 12) ); humans.sort(Human::compareByNameThenAge); Assert.assertThat(humans.get(0), equalTo(new Human("Jack", 12))); }

6. Sort Extracted Comparators

We can also avoid defining even the comparison logic itself by using an instance method reference and the Comparator.comparing method – which extracts and creates a Comparable based on that function.

We're going to use the getter getName() to build the Lambda expression and sort the list by name:

@Test public void givenInstanceMethod_whenSortingEntitiesByName_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 10), new Human("Jack", 12) ); Collections.sort( humans, Comparator.comparing(Human::getName)); assertThat(humans.get(0), equalTo(new Human("Jack", 12))); }

7. Reverse Sort

JDK 8 has also introduced a helper method for reversing the comparator – we can make quick use of that to reverse our sort:

@Test public void whenSortingEntitiesByNameReversed_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 10), new Human("Jack", 12) ); Comparator<Human> comparator = (h1, h2) -> h1.getName().compareTo(h2.getName()); humans.sort(comparator.reversed()); Assert.assertThat(humans.get(0), equalTo(new Human("Sarah", 10))); }

8. Sort With Multiple Conditions

The comparison lambda expressions need not be this simple – we can write more complex expressions as well – for example sorting the entities first by name, and then by age:

@Test public void whenSortingEntitiesByNameThenAge_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 12), new Human("Sarah", 10), new Human("Zack", 12) ); humans.sort((lhs, rhs) -> { if (lhs.getName().equals(rhs.getName())) { return Integer.compare(lhs.getAge(), rhs.getAge()); } else { return lhs.getName().compareTo(rhs.getName()); } }); Assert.assertThat(humans.get(0), equalTo(new Human("Sarah", 10))); }

9. Sort With Multiple Conditions – Composition

The same comparison logic – first sorting by name and then, secondarily, by age – can also be implemented by the new composition support for Comparator.

Starting with JDK 8, we can now chain together multiple comparators to build more complex comparison logic:

@Test public void givenComposition_whenSortingEntitiesByNameThenAge_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList( new Human("Sarah", 12), new Human("Sarah", 10), new Human("Zack", 12) ); humans.sort( Comparator.comparing(Human::getName).thenComparing(Human::getAge) ); Assert.assertThat(humans.get(0), equalTo(new Human("Sarah", 10))); }

10. Sorting a List With Stream.sorted()

We can also sort a collection using Java 8's Stream sorted() API.

We can sort the stream using natural ordering as well as ordering provided by a Comparator. For this, we have two overloaded variants of the sorted() API:

sorted() – sorts the elements of a Stream using natural ordering; the element class must implement the Comparable interface.

sorts the elements of a Stream using natural ordering; the element class must implement the Comparable interface. sorted(Comparator<? super T> comparator) – sorts the elements based on a Comparator instance

Let's see an example of how to use the sorted() method with natural ordering:

@Test public final void givenStreamNaturalOrdering_whenSortingEntitiesByName_thenCorrectlySorted() { List<String> letters = Lists.newArrayList("B", "A", "C"); List<String> sortedLetters = letters.stream().sorted().collect(Collectors.toList()); assertThat(sortedLetters.get(0), equalTo("A")); }

Now let's see how we can use a custom Comparator with the sorted() API:

@Test public final void givenStreamCustomOrdering_whenSortingEntitiesByName_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList(new Human("Sarah", 10), new Human("Jack", 12)); Comparator<Human> nameComparator = (h1, h2) -> h1.getName().compareTo(h2.getName()); List<Human> sortedHumans = humans.stream().sorted(nameComparator).collect(Collectors.toList()); assertThat(sortedHumans.get(0), equalTo(new Human("Jack", 12))); }

We can simplify the above example even further if we use the Comparator.comparing() method:

@Test public final void givenStreamComparatorOrdering_whenSortingEntitiesByName_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList(new Human("Sarah", 10), new Human("Jack", 12)); List<Human> sortedHumans = humans.stream() .sorted(Comparator.comparing(Human::getName)) .collect(Collectors.toList()); assertThat(sortedHumans.get(0), equalTo(new Human("Jack", 12))); }

11. Sorting a List in Reverse With Stream.sorted()

We can also use Stream.sorted() to sort a collection in reverse.

First, let's see an example of how to combine the sorted() method with Comparator.reverseOrder() to sort a list in the reverse natural order:

@Test public final void givenStreamNaturalOrdering_whenSortingEntitiesByNameReversed_thenCorrectlySorted() { List<String> letters = Lists.newArrayList("B", "A", "C"); List<String> reverseSortedLetters = letters.stream() .sorted(Comparator.reverseOrder()) .collect(Collectors.toList()); assertThat(reverseSortedLetters.get(0), equalTo("C")); }

Now, let's see how we can use the sorted() method and a custom Comparator:

@Test public final void givenStreamCustomOrdering_whenSortingEntitiesByNameReversed_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList(new Human("Sarah", 10), new Human("Jack", 12)); Comparator<Human> reverseNameComparator = (h1, h2) -> h2.getName().compareTo(h1.getName()); List<Human> reverseSortedHumans = humans.stream().sorted(reverseNameComparator) .collect(Collectors.toList()); assertThat(reverseSortedHumans.get(0), equalTo(new Human("Sarah", 10))); }

Note that the invocation of compareTo is flipped, which is what is doing the reversing.

Finally, let's simplify the above example by using the Comparator.comparing() method:

@Test public final void givenStreamComparatorOrdering_whenSortingEntitiesByNameReversed_thenCorrectlySorted() { List<Human> humans = Lists.newArrayList(new Human("Sarah", 10), new Human("Jack", 12)); List<Human> reverseSortedHumans = humans.stream() .sorted(Comparator.comparing(Human::getName, Comparator.reverseOrder())) .collect(Collectors.toList()); assertThat(reverseSortedHumans.get(0), equalTo(new Human("Sarah", 10))); }

12. Null Values

So far, we implemented our Comparators in a way that they can't sort collections containing null values. That is, if the collection contains at least one null element, then the sort method throws a NullPointerException:

@Test(expected = NullPointerException.class) public void givenANullElement_whenSortingEntitiesByName_thenThrowsNPE() { List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12)); humans.sort((h1, h2) -> h1.getName().compareTo(h2.getName())); }

The simplest solution is to handle the null values manually in our Comparator implementation:

@Test public void givenANullElement_whenSortingEntitiesByNameManually_thenMovesTheNullToLast() { List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12), null); humans.sort((h1, h2) -> { if (h1 == null) { return h2 == null ? 0 : 1; } else if (h2 == null) { return -1; } return h1.getName().compareTo(h2.getName()); }); Assert.assertNotNull(humans.get(0)); Assert.assertNull(humans.get(1)); Assert.assertNull(humans.get(2)); }

Here we're pushing all null elements towards the end of the collection. To do that, the comparator considers null to be greater than non-null values. When both are null, they are considered equal.

Additionally, we can pass any Comparator that is not null-safe into the Comparator.nullsLast() method and achieve the same result:

@Test public void givenANullElement_whenSortingEntitiesByName_thenMovesTheNullToLast() { List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12), null); humans.sort(Comparator.nullsLast(Comparator.comparing(Human::getName))); Assert.assertNotNull(humans.get(0)); Assert.assertNull(humans.get(1)); Assert.assertNull(humans.get(2)); }

Similarly, we can use Comparator.nullsFirst() to move the null elements towards the start of the collection:

@Test public void givenANullElement_whenSortingEntitiesByName_thenMovesTheNullToStart() { List<Human> humans = Lists.newArrayList(null, new Human("Jack", 12), null); humans.sort(Comparator.nullsFirst(Comparator.comparing(Human::getName))); Assert.assertNull(humans.get(0)); Assert.assertNull(humans.get(1)); Assert.assertNotNull(humans.get(2)); }

It's highly recommended to use the nullsFirst() or nullsLast() decorators, as they're more flexible and, above all, more readable.

13. Conclusion

This article illustrated the various and exciting ways that a List can be sorted using Java 8 Lambda Expressions – moving right past syntactic sugar and into real and powerful functional semantics.

The implementation of all these examples and code snippets can be found over on GitHub.