How in Java sort List? Java allows to sort list elements using their natural ordering or using custom Comparators. In the following example we present both approaches.

java.util.Collections has two sort methods:

• <T extends Comparable<? super T>> void sort(List<T> list): sorts according to the natural ordering of the elements (strings, numbers, etc.)
• <T> void sort(List<T> list, Comparator<? super T> c): sorts the list according to the ordering specified by the Comparator.

In the following example you can see both sort methods in use:

package com.farenda.java;

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

public class CollectionsSort {

    public static void main(String[] args) {
        sortUsingDefaultComparator();

        System.out.println();

        sortUsingOwnComparator();
    }

    private static void sortUsingOwnComparator() {
        List<User> kings = new LinkedList<>(Arrays.asList(
                new User("Jagiełło"),
                new User("Batory"),
                new User("Chrobry"),
                new User("Sobieski")));
        Collections.shuffle(kings);
        System.out.println("Kings:");
        kings.forEach(System.out::println);

        System.out.println();

        List<User> sorted = new LinkedList<>(kings);
        Collections.sort(sorted, new NameComparator());
        System.out.println("Kings sorted by name:");
        sorted.forEach(System.out::println);

        System.out.println();

        sorted = new LinkedList<>(kings);
        Collections.sort(sorted, new IdComparator());
        System.out.println("Kings sorted by id:");
        sorted.forEach(System.out::println);
    }

    private static void sortUsingDefaultComparator() {
        List<Integer> temperatures = Arrays.asList(34, 30, 31, 30, 32, 35, 34);
        System.out.println("Temperatures: " + temperatures);

        Collections.sort(temperatures);
        System.out.println("Sorted: " + temperatures);
    }
}

Method sortUsingDefaultComparator() shows how Java sort List using default Comparator. In the second method sort is done using own Comparator.

The User class:

package com.farenda.java;

public class User {
    private static long userCounter;
    private long id;
    private final String name;

    public User(String name) {
        this.id = userCounter++;
        this.name = name;
    }

    public long getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    @Override
    public String toString() {
        return "User(id:" + id + ", name: " + name + ")";
    }
}

Custom comparator that sorts Users by their names:

package com.farenda.java;

import java.util.Comparator;

/**
 * Compares Users using only their names.
 */
public class NameComparator implements Comparator<User> {
    @Override
    public int compare(User u1, User u2) {
        return u1.getName().compareTo(u2.getName());
    }
}

Another Comparator that sorts Users, but this time by their IDs:

package com.farenda.java;

import java.util.Comparator;

/**
 * Compares Users using only their Ids.
 */
public class IdComparator implements Comparator<User> {
    @Override
    public int compare(User u1, User u2) {
        return Long.compare(u1.getId(), u2.getId());
    }
}

Running the above Java example prints the following output:

Temperatures: [34, 30, 31, 30, 32, 35, 34]
Sorted: [30, 30, 31, 32, 34, 34, 35]

Kings:
User(id:3, name: Sobieski)
User(id:2, name: Chrobry)
User(id:0, name: Jagiełło)
User(id:1, name: Batory)

Kings sorted by name:
User(id:1, name: Batory)
User(id:2, name: Chrobry)
User(id:0, name: Jagiełło)
User(id:3, name: Sobieski)

Kings sorted by id:
User(id:0, name: Jagiełło)
User(id:1, name: Batory)
User(id:2, name: Chrobry)
User(id:3, name: Sobieski)