# Sort a List in Java Using Collections, Comparator, or Stream

> Sort a Java List using Collections.sort, List.sort with Comparator, or Stream.sorted with examples for natural order, reverse order, and custom comparators

**URL:** https://sentry.io/answers/java-sort-list/

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## 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`](https://docs.oracle.com/en/java/javase/21/docs/api/java.base/java/util/Collections.html) 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:

```java
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:**

```
23678
```

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](https://docs.oracle.com/javase/8/docs/api/java/util/Comparator.html):

```java
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:**

```
87632
```

### Using Comparator with `List.sort()`

The comparator interface provides the framework to compare different [comparable](https://docs.oracle.com/javase/8/docs/api/java/lang/Comparable.html) 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:

```java
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:**

```
23678
```

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`.

```java
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:**

```
23678
```

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

All three implement a version of a [merge sort](https://en.wikipedia.org/wiki/Merge_sort) and have an average time complexity of `O(n*Log n)`.

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*Source: [sentry.io/answers/java-sort-list/](https://sentry.io/answers/java-sort-list/)*