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Java Abstraction & Interfaces

Abstract classes and interfaces.

✍️ 2 exercises in this lesson
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  • An abstract class cannot be instantiated and can have abstract methods without a body.
  • An interface defines methods that implementing classes must provide: class Card implements Payment.
  • A class can implement many interfaces but extend only one class.
  • Interfaces can have default methods with a body.

Key Takeaways: Java Abstraction & Interfaces

Mastering Java Abstraction & Interfaces helps you write clean and reliable Java code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.

Java Abstraction & Interfaces Code Example

Java 
interface Payment {
    double fee();
    default String describe() { return getClass().getSimpleName() + " fee: " + fee(); }
}

class Card implements Payment {
    public double fee() { return 2.5; }
}

class BankTransfer implements Payment {
    public double fee() { return 0; }
}

abstract class Shape {
    abstract double area();
}

class Square extends Shape {
    double side = 4;
    double area() { return side * side; }
}

public class Main {
    public static void main(String[] args) {
        Payment[] methods = {new Card(), new BankTransfer()};
        for (Payment p : methods) System.out.println(p.describe());
        System.out.println(new Square().area());
    }
}
Output
Card fee: 2.5
BankTransfer fee: 0.0
16.0
Run this example on your own computer. The "Get Started" lesson shows how to set it up.

Java Abstraction & Interfaces Practice Exercises

Check what you learned in “Java Abstraction & Interfaces”.

/ 2 solved
Fill in the blank

Implement an interface.

class Card  Payment { }
✓ Correct! Not quite. Try again.
Multiple choice

How many classes can a Java class extend?

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