How to Use Collections.sort in Java: Simple Guide

Use Collections.sort(List<T> list) to sort a list in Java in natural order. For custom sorting, use Collections.sort(List<T> list, Comparator<? super T> c) with a comparator.

Syntax

The Collections.sort method has two main forms:

• Collections.sort(List<T> list): Sorts the list elements in their natural order (like numbers ascending or strings alphabetically).
• Collections.sort(List<T> list, Comparator<? super T> c): Sorts the list using a custom rule defined by the comparator.

Here, List<T> is the list you want to sort, and Comparator defines how to compare two elements.

java

Collections.sort(list);
Collections.sort(list, comparator);

Example

This example shows sorting a list of strings alphabetically and then sorting a list of integers in reverse order using a comparator.

java

import java.util.*;

public class SortExample {
    public static void main(String[] args) {
        List<String> fruits = new ArrayList<>(Arrays.asList("Banana", "Apple", "Orange"));
        Collections.sort(fruits);
        System.out.println("Sorted fruits: " + fruits);

        List<Integer> numbers = new ArrayList<>(Arrays.asList(5, 3, 8, 1));
        Collections.sort(numbers, (a, b) -> b.compareTo(a)); // reverse order
        System.out.println("Numbers sorted in reverse: " + numbers);
    }
}

Output

Sorted fruits: [Apple, Banana, Orange]
Numbers sorted in reverse: [8, 5, 3, 1]

Common Pitfalls

• Trying to sort a list of objects without natural order or without providing a comparator causes ClassCastException.
• Modifying the list while sorting can cause unexpected behavior.
• Using Collections.sort on immutable lists throws UnsupportedOperationException.

Always ensure the list is mutable and elements are comparable or provide a comparator.

java

import java.util.*;

class Person {
    String name;
    Person(String name) { this.name = name; }
}

public class SortPitfall {
    public static void main(String[] args) {
        List<Person> people = new ArrayList<>();
        people.add(new Person("Alice"));
        people.add(new Person("Bob"));

        // Wrong: no comparator and Person does not implement Comparable
        // Collections.sort(people); // This causes ClassCastException

        // Right: provide comparator
        Collections.sort(people, (p1, p2) -> p1.name.compareTo(p2.name));
        for (Person p : people) {
            System.out.println(p.name);
        }
    }
}

Output

Alice
Bob

Quick Reference

• Sort natural order: Collections.sort(list);
• Sort with custom rule: Collections.sort(list, comparator);
• List must be mutable: Use ArrayList or similar.
• Elements must be Comparable or use Comparator.

Key Takeaways

Use Collections.sort(list) to sort lists in natural order.

Provide a Comparator to sort with custom rules.

Ensure the list is mutable before sorting.

Elements must implement Comparable or use a Comparator.

Avoid modifying the list during sorting to prevent errors.