Comparator with lambda

Learn sorting with Comparator.comparing.

Comparator with a lambda

The modern way to sort a list is to give sort a Comparator lambda. names.sort((a, b) -> a.length() - b.length()); sorts names by length. A Comparator takes two parameters; it returns negative, zero or positive.

For ascending order write a - b; for descending write b - a. A more readable way is Comparator.comparingInt(String::length). This uses a method reference to make a field the sort key. To reverse, append .reversed().

Comparator.comparingInt

Comparator.comparingInt(String::length) makes a field a sort key. The method reference String::length is the key extractor function. This is more readable than the (a, b) -> a.length() - b.length() lambda.

To sort by multiple keys use .thenComparing. For example first by length then alphabetically: Comparator.comparingInt(String::length).thenComparing(s -> s). For reverse order .reversed() flips the whole comparison.

Exercise Sort names by length

Declare List<String> names = new ArrayList<>();; add "Mehmet", "Ayşe", "Alex". Sort ascending by length with names.sort((a, b) -> a.length() - b.length());. Print the result with names.forEach(System.out::println);.

Show example solution

import java.util.List;
import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        List<String> names = new ArrayList<>();
        names.add("Mehmet");
        names.add("Ayşe");
        names.add("Alex");
        names.sort((a, b) -> a.length() - b.length());
        names.forEach(System.out::println);
    }
}

Test yourself

Complete the length Comparator

names.sort((a, b) -> a.____() - b.length());

Show answer

names.sort((a, b) -> a.length() - b.length());

Match the Comparator lambda parts

(a, b)

Two parameters
->

Arrow token
a.length() - b.length()

Ascending
b.length() - a.length()

Descending
What order does (a, b) -> a - b produce?
- Descending
- Ascending
- Random
- Shuffled
Show answer

Ascending — a - b sorts ascending; smaller values come first.

Next lesson: Summary - Lambda expressions

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