Differences and Real-World Usage Between Java Interface and Abstract Class

1. Java Interface

• Fully Abstract: Interfaces cannot have method implementations (before Java 8); only method declarations. Since Java 8, default and static methods are allowed with bodies.
• Multiple Inheritance: A class can implement multiple interfaces (solves the single inheritance limitation).
• All variables are implicitly public static final constants.
• All methods are implicitly public abstract (except default and static).
• Used to define behaviors and capabilities.

Application Scenarios:

• When you want to define a set of capabilities or behaviors without enforcing a specific implementation:
  • Comparable for comparison logic
  • Runnable for thread task execution
  • Serializable for object serialization
• When multiple inheritance is needed

2. Java Abstract Class

• Can have both abstract and concrete methods (with bodies).
• Can include member variables, constructors, and static blocks.
• Single inheritance only: A class can only extend one abstract class.
• Used for code reuse and structural design.

Application Scenarios:

• When multiple classes share common implementation, but still need customization.
• When you want to provide a default implementation or template method.

3. Comparison Table

4. An Interview Question – Zoo System Design

Design a Zoo System where different animals (e.g., Tiger, Penguin, Parrot) have shared behaviors (like eating and sleeping) and unique capabilities (like flying or swimming). Some animals can fly, others can swim.

Solution Design:

1. Use an abstract class Animal to hold common properties and behaviors (name, eat(), sleep()).
2. Use interfaces (Flyable, Swimmable) to represent optional capabilities.
3. Concrete animals extend Animal and implement the appropriate interfaces.

// Interfaces
public interface Flyable {
    void fly();
}

public interface Swimmable {
    void swim();
}

// Abstract Class
public abstract class Animal {
    protected String name;

    public Animal(String name) {
        this.name = name;
    }

    public void eat() {
        System.out.println(name + " is eating");
    }

    public void sleep() {
        System.out.println(name + " is sleeping");
    }

    public abstract void makeSound();
}

// Concrete Animal Classes
public class Tiger extends Animal implements Swimmable {
    public Tiger(String name) {
        super(name);
    }

    @Override
    public void makeSound() {
        System.out.println("Roar!");
    }

    @Override
    public void swim() {
        System.out.println("Tiger swims powerfully.");
    }
}

public class Penguin extends Animal implements Swimmable {
    public Penguin(String name) {
        super(name);
    }

    @Override
    public void makeSound() {
        System.out.println("Penguin squawks!");
    }

    @Override
    public void swim() {
        System.out.println("Penguin swims fast!");
    }
}

public class Parrot extends Animal implements Flyable {
    public Parrot(String name) {
        super(name);
    }

    @Override
    public void makeSound() {
        System.out.println("Parrot says hello!");
    }

    @Override
    public void fly() {
        System.out.println("Parrot is flying!");
    }
}

Conclusion:

I used an abstract class Animal to provide common behavior like eat() and sleep(), and kept `makeSound() as an abstract method because each animal makes a different sound.

For flying and swimming capabilities, I created interfaces (Flyable, Swimmable) so that these behaviors can be flexibly added to specific animal classes, without affecting inheritance structure.

This follows the object-oriented principle of “composition over inheritance” and improves code reusability and maintainability.