An interface is a completely “abstract class” that is used to group related methods with empty bodies:

interface Animal {
  public void animalSound(); // interface method (does not have a body)
  public void run(); // interface method (does not have a body)

To access the interface methods, the interface must be “implemented” (kinda like inherited) by another class with the implements keyword (instead of extends). The body of the interface method is provided by the “implement” class:

// Pig "implements" the Animal interface
class Pig implements Animal {
  public void animalSound() {
    // The body of animalSound() is provided here
    System.out.println("The pig says: wee wee");
  public void sleep() {
    // The body of sleep() is provided here
class Main {
  public static void main(String[] args) {
    Pig myPig = new Pig();  // Create a Pig object

Interface vs Abstract Class

  • An interface cannot have state, whereas the abstract class can have state with instance variables.

Abstract Class

An abstract class in Java is a class that cannot be instantiated, but can be subclassed.
It can have both abstract and non-abstract methods. A normal (non-abstract) class cannot have abstract methods.


Used with compareTo

public class Person implements Comparable<Person> {
    private int age;
    public Person(int age) {
        this.age = age;
    public int getAge() {
        return age;
    public int compareTo(Person otherPerson) {
        return this.age - otherPerson.getAge();