How to Sort List in Java: Simple Guide with Examples

To sort a list in Java, use Collections.sort(list) for natural order sorting or list.sort(Comparator) for custom order. Both methods modify the original list to be sorted.

Syntax

There are two common ways to sort a list in Java:

• Collections.sort(list): Sorts the list in natural order (like numbers ascending or strings alphabetically).
• list.sort(Comparator): Sorts the list using a custom rule you provide with a Comparator.

java

import java.util.*;

// Sort list in natural order
Collections.sort(list);

// Sort list with custom comparator
list.sort(Comparator.reverseOrder());

Example

This example shows how to sort a list of numbers in ascending order using Collections.sort() and in descending order using list.sort() with a comparator.

java

import java.util.*;

public class SortListExample {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(Arrays.asList(5, 3, 8, 1, 9));

        // Sort in ascending order
        Collections.sort(numbers);
        System.out.println("Ascending: " + numbers);

        // Sort in descending order
        numbers.sort(Comparator.reverseOrder());
        System.out.println("Descending: " + numbers);
    }
}

Output

Ascending: [1, 3, 5, 8, 9]
Descending: [9, 8, 5, 3, 1]

Common Pitfalls

Common mistakes when sorting lists in Java include:

• Trying to sort a list of objects without implementing Comparable or providing a Comparator.
• Expecting Collections.sort() or list.sort() to return a new sorted list (they sort the list in place).
• Using Arrays.sort() on a list instead of an array.

java

import java.util.*;

public class SortPitfall {
    public static void main(String[] args) {
        List<Object> items = new ArrayList<>();
        items.add(new Object());
        items.add(new Object());

        // Wrong: This will throw ClassCastException
        // Collections.sort(items);

        // Right: Provide a Comparator or use Comparable objects
    }
}

Quick Reference

Summary tips for sorting lists in Java:

• Use Collections.sort(list) for natural order sorting.
• Use list.sort(Comparator) for custom sorting rules.
• Sorting modifies the original list; it does not create a new one.
• Ensure list elements implement Comparable or provide a Comparator.

Key Takeaways

Use Collections.sort(list) to sort a list in natural order in Java.

Use list.sort(Comparator) to sort with custom rules.

Sorting changes the original list; it does not return a new list.

List elements must implement Comparable or you must provide a Comparator.

Avoid sorting lists of objects without proper comparison logic to prevent errors.