Java Interfaces

intermediate

Part of Core Java

Theory

An interface in Java is a reference type that defines a contract of abstract methods. Classes implement interfaces to fulfill that contract.

Defining and Implementing Interfaces

interface Flyable {
    void fly();
}
 
interface NoiseMaker {
    void makeSound();
}
 
class Bird implements Flyable, NoiseMaker {
    public void fly() {
        System.out.println("Bird is flying");
    }
    public void makeSound() {
        System.out.println("Chirp chirp");
    }
}

A class can implement multiple interfaces — this is Java's way of achieving multiple inheritance.

Default Methods (Java 8+)

Interfaces can have default methods with a body. Implementing classes can override them or use the default:

interface Vehicle {
    void start();
    default void honk() {
        System.out.println("Beep beep!");
    }
}

Functional Interfaces and Lambdas

A functional interface has exactly one abstract method. It can be implemented with a lambda expression:

@FunctionalInterface
interface Greeter {
    String greet(String name);
}
 
Greeter g = name -> "Hello, " + name;
System.out.println(g.greet("Alice"));

Comparator vs Comparable

Comparable — defines natural ordering (compareTo in the class itself)
Comparator — defines custom ordering (separate class or lambda)

Comparable and Comparator Example

java

Practical Examples

Example: Functional Interface with Lambda

java

Exercises

Shape Interface with Comparator

easy

Define a Shape interface with a getArea() method. Create Circle and Rectangle classes that implement it. Then sort a list of shapes by area using Comparator.

Expected Output:

Circle(r=2): 12.57\nRectangle(3x4): 12.00\nCircle(r=5): 78.54