Implements vs. Extends: When to Use Which in Object-Oriented Programming?

Implements vs. Extends: A Comprehensive Guide

In object-oriented programming, understanding the distinction between "implements" and "extends" is crucial.

Implements

"Implements" is used when a class fulfills the contract specified by an interface. An interface declares a set of abstract methods (methods without implementation) that a class must implement. By implementing an interface, a class agrees to provide concrete implementations of all its methods.

For instance:

interface MyInterface {
    void doAction();
    int doThis(int number);
}

class MyClass implements MyInterface {
    @Override
    public void doAction() {
        // Implement the method
    }

    @Override
    public int doThis(int number) {
        // Implement the method
    }
}

Extends

"Extends" is used when a class inherits from another class. The child class (also known as a subclass) gains access to the parent class's (also known as a superclass) fields and methods. Subclasses can also override or extend the parent class's behavior.

For instance:

class SuperClass {
    private int num;

    public int getNum() {
        return num;
    }
}

class SubClass extends SuperClass {
    @Override
    public int getNum() {
        return num + 1; // Overriding the parent's implementation
    }
}

Key Differences

• Interface vs. Class: "Implements" is used with interfaces, which only declare methods but do not implement them. "Extends" is used with classes, which can have both methods and fields.
• Multiple Inheritance: Java does not support multiple inheritance for classes. However, it allows multiple interfaces to be implemented, providing a way to achieve similar functionality.
• Polymorphism: Implemented interfaces provide a level of polymorphism, allowing objects of different classes that implement the same interface to be treated as if they were of the same type.
• @Override Annotation: The @Override annotation is used in overridden method implementations in subclasses but not in methods implementing an interface.

When to Use

• Implements: When a class fulfills the requirements of an existing interface and provides concrete implementations for its abstract methods.
• Extends: When a class inherits from an existing class and wants to reuse its functionality, possibly overriding or extending its behavior.

Understanding these concepts is essential for effective object-oriented design and code reusability.

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