Differences Between Abstract Classes and Interfaces in Java 8

What are the key differences between abstract classes and interfaces in Java 8?

Java 8 introduced significant changes to interfaces, primarily with the addition of default methods. This article outlines the essential differences between abstract classes and interfaces in Java 8, including handling multiple inheritance and the diamond problem.

// Example of an abstract class in Java
abstract class Animal {
    abstract void sound();
    void eat() {
        System.out.println("Eating...");
    }
}

class Dog extends Animal {
    void sound() {
        System.out.println("Bark!");
    }
}

// Example of an interface in Java 8
interface Playable {
    void play();
    default void stop() {
        System.out.println("Stopped playing.");
    }
}

class Game implements Playable {
    public void play() {
        System.out.println("Playing the game!");
    }
}

Causes

• Abstract classes can have method implementations and member variables, while interfaces primarily define method signatures without an implementation (though this changed with Java 8).
• Abstract classes support single inheritance, which means a class can extend only one abstract class, while multiple interfaces can be implemented by a single class.

Solutions

• Use abstract classes when you want to define a template with some implementation and keep track of state through fields.
• Utilize interfaces primarily for defining contracts that can be implemented by multiple classes, emphasizing behavior over state.

Mistake: Overusing default methods in interfaces, leading to unclear designs.

Solution: Use default methods sparingly; prefer abstract classes for shared code implementation when complex behavior is required.

Mistake: Confusing interfaces with abstract classes in terms of state management and implementation.

Solution: Remember that interfaces should define what an object can do, while abstract classes can provide reusable code.