Interface vs Abstract Class — Know the Difference

Interface vs. Abstract Class — Know the Difference

Introduction
When developing object-oriented software, developers often encounter situations where they need to create classes that define a set of common behaviors or functionalities. Two common approaches to achieve this are using interfaces and abstract classes. Both interfaces and abstract classes are used to define contract-like structures that classes must adhere to, but they have distinct differences. This documentation aims to highlight and explain the key differences between interfaces and abstract classes, along with examples to illustrate each concept.

1. Interface

Definition:
An interface in object-oriented programming is a blueprint for a group of related methods that do not have any implementation details. It defines a contract that classes must follow by implementing the methods declared in the interface.

Key Characteristics:

• All methods in an interface are implicitly abstract, meaning they lack implementation and only have method signatures.
• Interfaces can contain constants, which are implicitly public, static, and final.
• A class can implement multiple interfaces.

Example:

java
public interface Animal {
    void makeSound();
    void move();
}

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

2. Abstract Class

Definition:
An abstract class is a class that cannot be instantiated on its own and may contain a mix of both abstract and concrete methods. It serves as a blueprint for other classes and can provide partial implementations of methods.

Key Characteristics:

• Abstract classes can have abstract methods (only method signature) as well as concrete methods (with implementation).
• Abstract classes can have instance variables, constructors, and other typical class features.
• A class can extend only one abstract class (Java doesn't support multiple inheritance for classes).

Example:

java
public abstract class Shape {
    protected int x, y;
    
    public Shape(int x, int y) {
        this.x = x;
        this.y = y;
    }
    
    abstract void draw();
    
    void move(int deltaX, int deltaY) {
        x += deltaX;
        y += deltaY;
    }
}

Explanation:
In this example, we have an abstract class Shape with an abstract method draw() and a concrete method move(). The abstract method draw() is intended to be implemented by the subclasses, while the concrete method move() is already provided with a default implementation.

Comparison:

Feature	Interface	Abstract Class
Instantiation	Cannot be instantiated directly	Cannot be instantiated directly
Method Type	All methods are implicitly abstract	Can have abstract and concrete methods
Method Implementation	No implementation in the interface	Can provide partial implementations
Multiple Inheritance	Supports multiple interfaces	Supports single abstract class inheritance
Variables	Can have only constants	Can have instance variables
Usage	Used for pure abstraction	Used when sharing common functionality

Conclusion:
In conclusion, both interfaces and abstract classes provide a way to define contracts and establish common behaviors in object-oriented programming. Choosing between them depends on the design and requirements of your application. Interfaces are suitable for defining pure abstractions, while abstract classes allow you to provide partial implementations and share common functionalities among related classes.