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Java Collections - Comparator.thenComparing() Examples

Java Collections Java Java API 

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Interface:

java.util.Comparator

java.util.ComparatorComparatorLogicBig

Methods:

default Comparator<T> thenComparing(Comparator<? super T> other)

This default method returns a comparator with this comparator and the specified comparator are chained together. If this Comparator considers two elements equal, i.e. compare(a, b) == 0, then the other specified comparator is used to determine the order. 



default <U extends Comparable<? super U>> Comparator<T> thenComparing(Function<? super T,? extends U> keyExtractor)

This default method returns a comparator chained with this comparator. The returned comparator uses the specified function to extract the Comparable sort key.


Examples


package com.logicbig.example.comparator;

import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class ThenComparingExample {

    public static void main(String... args) {
        System.out.println("-- without thenComparing --");
        withoutThenComparing();
        System.out.println("-- with thenComparing --");
        withThenComparing();
    }


    private static void withoutThenComparing() {
        List<Book> list = createExampleBooks();
        System.out.printf("before sort: %s%n", list);
        Collections.sort(list, Comparator.comparing(Book::getAuthor));
        System.out.printf("after sort: %s%n", list);
    }


    private static void withThenComparing() {
        List<Book> list = createExampleBooks();
        System.out.printf("before sort: %s%n", list);
        Collections.sort(list, Comparator.comparing(Book::getAuthor)
                                         .thenComparing(Comparator.comparing(Book::getName)));
        System.out.printf("after sort: %s%n", list);
    }

    private static List<Book> createExampleBooks() {
        return Arrays.asList(
                new Book("Sara", "book1"),
                new Book("Sara", "book3"),
                new Book("Sara", "book2"),
                new Book("John", "book5"),
                new Book("John", "book4")
        );
    }

    private static class Book {
        private String author;
        private String name;

        public Book(String author, String name) {
            this.author = author;
            this.name = name;
        }

        public String getAuthor() {
            return author;
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return author + " - " + name;
        }
    }
}

Output

-- without thenComparing --
before sort: [Sara - book1, Sara - book3, Sara - book2, John - book5, John - book4]
after sort: [John - book5, John - book4, Sara - book1, Sara - book3, Sara - book2]
-- with thenComparing --
before sort: [Sara - book1, Sara - book3, Sara - book2, John - book5, John - book4]
after sort: [John - book4, John - book5, Sara - book1, Sara - book2, Sara - book3]




Using KeyExtractor function 

package com.logicbig.example.comparator;

import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class ThenComparingExample2 {

    public static void main(String... args) {
        List<Book> list = createExampleBooks();
        System.out.printf("before sort: %s%n", list);
        Collections.sort(list, Comparator.comparing(Book::getAuthor)
                                         .thenComparing(Book::getName));
        System.out.printf("after sort: %s%n", list);
    }

    private static List<Book> createExampleBooks() {
        return Arrays.asList(
                new Book("Sara", "book1"),
                new Book("Sara", "book3"),
                new Book("Sara", "book2"),
                new Book("John", "book5"),
                new Book("John", "book4")
        );
    }

    private static class Book {
        private String author;
        private String name;

        public Book(String author, String name) {
            this.author = author;
            this.name = name;
        }

        public String getAuthor() {
            return author;
        }

        public String getName() {
            return name;
        }

        @Override
        public String toString() {
            return author + " - " + name;
        }
    }
}

Output

before sort: [Sara - book1, Sara - book3, Sara - book2, John - book5, John - book4]
after sort: [John - book4, John - book5, Sara - book1, Sara - book2, Sara - book3]




Using streams: 

package com.logicbig.example.comparator;

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class ThenComparingExample3 {
    public static void main(String... args) {
        //original list
        List<String> list = Arrays.asList("abcd3", "ac2", "abcd1", "ac1");
        System.out.println(list);

        //sorting by string length
        List<String> list2 = list.stream().sorted(Comparator.comparing(String::length))
                                 .collect(Collectors.toList());
        System.out.println(list2);

        //sorting by string length and then by natural order
        List<String> list3 = list.stream().sorted(
                Comparator.comparing(String::length)
                          .thenComparing(Comparator.naturalOrder()))
                                 .collect(Collectors.toList());
        System.out.println(list3);
    }
}

Output

[abcd3, ac2, abcd1, ac1]
[ac2, ac1, abcd3, abcd1]
[ac1, ac2, abcd1, abcd3]

See Also