Interface vs. Abstract Class in Java

In Java, both interfaces and abstract classes are powerful tools for achieving abstraction and defining common behavior among classes. They allow you to create a contract that other classes must follow. However, each has its unique characteristics and use cases. In this article, we will explore the differences between interfaces and abstract classes in Java and when to use each of them.

1. Introduction to Abstraction in Java

Abstraction is a fundamental concept in object-oriented programming that allows you to define the structure and behavior of objects without specifying their implementation details. In Java, both interfaces and abstract classes serve as mechanisms for achieving abstraction.

2. Understanding Interfaces

An interface in Java is a collection of abstract methods and constants. It defines a contract that a class must follow, specifying the methods it should implement. An interface can be seen as a blueprint for a class, outlining the required methods without providing any implementation details.

2.1. Declaring Interfaces

In Java, interfaces are declared using the interface keyword.

public interface Shape {
    void draw();
    double getArea();
}

In this example, we define an interface Shape with two abstract methods: draw() and getArea(). Any class that implements the Shape interface must provide concrete implementations for these methods.

2.2. Implementing Interfaces

To implement an interface, a class must use the implements keyword and provide implementations for all the abstract methods defined in the interface.

public class Circle implements Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public void draw() {
        System.out.println("Drawing a circle");
    }

    @Override
    public double getArea() {
        return Math.PI * radius * radius;
    }
}

In this example, the Circle class implements the Shape interface by providing concrete implementations for the draw() and getArea() methods.

3. Understanding Abstract Classes

An abstract class in Java is a class that cannot be instantiated on its own and may contain abstract methods (methods without implementation) alongside concrete methods. Abstract classes are designed to be extended by other classes, allowing them to inherit and override behavior.

3.1. Declaring Abstract Classes

In Java, abstract classes are declared using the abstract keyword.

public abstract class Vehicle {
    protected String brand;

    public Vehicle(String brand) {
        this.brand = brand;
    }

    public abstract void start();

    public void stop() {
        System.out.println("Vehicle stopped.");
    }
}

In this example, we define an abstract class Vehicle with a constructor and two methods: start() (an abstract method) and stop() (a concrete method with implementation).

3.2. Extending Abstract Classes

To use an abstract class, a subclass must extend it using the extends keyword and provide concrete implementations for any abstract methods defined in the abstract class.

public class Car extends Vehicle {
    public Car(String brand) {
        super(brand);
    }

    @Override
    public void start() {
        System.out.println("Car started.");
    }
}

In this example, the Car class extends the Vehicle abstract class and provides a concrete implementation for the start() method.

4. Interface vs. Abstract Class: Key Differences

While both interfaces and abstract classes allow you to achieve abstraction and define contracts, there are some key differences between the two:

• Multiple Inheritance: In Java, a class can implement multiple interfaces, allowing it to inherit behavior from multiple sources. However, a class can only extend one abstract class, making interfaces more flexible in terms of multiple inheritance.
• Method Implementation: Interfaces can only contain abstract methods, while abstract classes can contain both abstract and concrete methods. This means that abstract classes can provide some common implementation and leave specific details to be implemented by subclasses.
• Instantiation: Abstract classes cannot be instantiated directly, while interfaces cannot be instantiated at all. To use an abstract class, you must create a subclass that extends it and provides implementations for abstract methods. To use an interface, you must create a class that implements it and provides implementations for all its methods.

5. When to Use Interfaces

Use interfaces in Java when:

• You want to define a contract that multiple unrelated classes must follow.
• You need to support multiple inheritance since a class can implement multiple interfaces.
• You want to achieve a high level of abstraction without providing any implementation details.

6. When to Use Abstract Classes

Use abstract classes in Java when:

• You want to provide a common base class with some default implementations for its subclasses.
• You want to define a contract and some shared behavior, but leave specific implementations to the subclasses.
• You don’t need to support multiple inheritance since a class can only extend one abstract class.

7. Conclusion

In Java, both interfaces and abstract classes provide mechanisms for achieving abstraction and defining contracts for classes. Interfaces define a contract with only abstract methods, while abstract classes can contain a mix of abstract and concrete methods. The choice between using an interface or an abstract class depends on the specific requirements and design goals of your application.

Interfaces are more flexible in terms of multiple inheritance and defining contracts for

unrelated classes, while abstract classes allow you to provide common behavior and shared implementations for their subclasses. Understanding the differences between interfaces and abstract classes will help you make informed design decisions and create more maintainable and extensible Java applications.