Java Comparable Interface

The java.lang package includes the Comparable interface. The Comparable interface allows developers to specify the natural order of Java objects. When a class implements Comparable it enables its objects to be arranged in order through sorting methods such as Collections.sort() or Arrays.sort().

Key Features

• Enables default sorting logic
• Requires implementation of compareTo() method
• The Comparable interface supports sorting using Collections.sort() and Arrays.sort().
• Belongs to java.lang (no need to import)

Interface Declaration

public interface Comparable {
    public int compareTo(T o);
} 

Method Signature

int compareTo(T other);

Return values:

• 0 – current object is equal to other
• < 0 – current object is less than other
• > 0 – current object is greater than other

Example

import java.util.*;

public class ComparableExample {
    public static void main(String[] args) {
        List list = new ArrayList<>();
        list.add(new Student(102, "Alice"));
        list.add(new Student(101, "Bob"));
        list.add(new Student(103, "Charlie"));

        Collections.sort(list);  // uses compareTo()

        for (Student s : list) {
            System.out.println(s.id + " - " + s.name);
        }
    }
}

Note

• Comparable functionality requires you to override the compareTo() method.
• Use Comparator when:
• You need multiple sort orders.
• Modifying the class is not an option such as in third-party classes.

Comparable vs Comparator