How to sort a List in Java?

Lewis D.

—October 21, 2022

The Problem

You’ve created a List object in your Java project, but some functionality requires the values to be sorted in a specific order. How can you do this?

The Solution

There are several solutions to this problem. Let’s take a look at some of the more common ones.

Using the `Collections` Class

The java.utils.Collections library provides the method sort(List<T>), which will take an implemented List object and sort it based on the natural order of the value type:

Click to Copy

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

public class Example {
    public static void main(String[] args) {
        List<Integer> test = new ArrayList<>(Arrays.asList(8, 6, 3, 7, 2));
        Collections.sort(test);
        for (Integer i : test) {
            System.out.print(i);
        }
    }
}

Output:

Click to Copy

If you want to change the ordering of your sort, use the overloaded sort(List<T> list, Comparator<? super T> c) method and provide a comparator:

Click to Copy

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

public class Example {
    public static void main(String[] args) {
        List<Integer> test = new ArrayList<>(Arrays.asList(8, 6, 3, 7, 2));
        Collections.sort(test, Comparator.reverseOrder());
        for (Integer i : test) {
            System.out.print(i);
        }
    }
}

Output:

Click to Copy

Using Comparator with `List.sort()`

The comparator interface provides the framework to compare different comparable objects.

The List.sort(Comparator<? super T> c) method takes a comparator as an argument, and will return a sorted array based on the comparator order specified:

Click to Copy

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

public class Example {
    public static void main(String[] args) {
        List<Integer> test = new ArrayList<>(Arrays.asList(8, 6, 3, 7, 2));
        test.sort(Comparator.naturalOrder());
        for (Integer i : test) {
            System.out.print(i);
        }
    }
}

Output:

Click to Copy

This method in the List class is practically identical to the Collections class implementation, and so the order can be changed with whatever comparator it receives as an argument.

Using a `Stream` Object

You can also use the List.stream() method to sort your list. This method will return a sequential Stream object containing the values from the list. You can then use the Stream.sorted(Comparator<? super T> c) method to order your list based on a provided comparator. You will then need to convert the Stream object back into a List.

Click to Copy

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

public class Example {
    public static void main(String[] args) {
        List<Integer> test = new ArrayList<>(Arrays.asList(8, 6, 3, 7, 2));
        test = test.stream()
                .sorted(Comparator.naturalOrder())
                .collect(Collectors.toList());
        for (Integer i : test) {
            System.out.print(i);
        }
    }
}

Output:

Click to Copy

These methods all behave in a similar way, and they are mostly interchangeable.

All three implement a version of a merge sort and have an average time complexity of O(n*Log n).

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How to sort a List in Java?

The Problem

The Solution

Using the `Collections` Class

Using Comparator with `List.sort()`

Using a `Stream` Object

How to sort a List in Java?

The Solution

Using the Collections Class

Using Comparator with List.sort()

Using a Stream Object

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Using the `Collections` Class

Using Comparator with `List.sort()`

Using a `Stream` Object