How to sort items in a stream with Stream.sorted()

Just like lists or arrays, you would also like to sort streams. Of course, the Java 8 Stream API offers a simple solution.
 

Sort stream with Comparator as lambda expression

The Stream.sorted() method expects a comparator. Since Comparator is a functional interface, we can pass a lambda expression instead of a class:

List myList = Arrays.asList(
        new Person("1", "Alex", "Kourti"),
        new Person("2", "Thomas", "Nibosi"),
        new Person("3", "Jean", "Fawdor"),
        new Person("4", "Emily", "Somilaro")
);

List firstNames = myList.stream()
    .sorted((p1, p2) -> p1.getFirstName().compareTo(p2.getFirstName()))
    .map(p -> p.getFirstName())
    .collect(Collectors.toList());

System.out.println(firstNames);

Output:

[Alex, Emily, Jean, Thomas]

The list “myList” is easily sorted by first name using the lambda expression :

(p1, p2) -> p1.getFirstName().compareTo(p2.getFirstName())

 

Sort the stream in the natural order of the elements

If the class itself implements the Comparable interface, we don’t need to pass the comparator or lambda expression:

List firstNames = myList.stream()
    .sorted()
    .map(p -> p.getFirstName())
    .collect(Collectors.toList());

System.out.println(firstNames);

Output:

[Alex, Emily, Jean, Thomas]

In this case, the Person class must implement the Comparable interface, which is provided by the compareTo() method:

@Override
public int compareTo(@NotNull Object o) {
    return firstName.compareTo(((Person) o).getFirstName());
}

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