How to sort a List in Java?

Lewis D.

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:

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);
        }
    }
}
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Output:

23678
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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:

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);
        }
    }
}
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Output:

87632
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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:

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);
        }
    }
}
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Output:

23678
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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.

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);
        }
    }
}
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Output:

23678
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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).