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

Implements vs Extends: A Comparative Guide

In the realm of object-oriented programming, two concepts often arise: "implements" and "extends." Understanding the subtle distinctions between these two terms is crucial for effective software design.

Implements

Implements establishes a contractual relationship between a class and an interface. An interface outlines a set of methods that the class must provide implementations for. Unlike with classes, interfaces cannot contain any method bodies.

Syntax:

public class ClassName implements InterfaceName {
    // Method implementations
}

Example:

public interface ExampleInterface {
    void doAction();
    String doThis(int number);
}

public class Subclass implements ExampleInterface {
    @Override
    public void doAction() {
        // Implementation provided
    }

    @Override
    public String doThis(int number) {
        // Implementation provided
    }
}

Extends

Extends, on the other hand, establishes an inheritance relationship between a subclass and a superclass. The subclass inherits all the non-private members of the superclass and can override method implementations.

Syntax:

public class SubclassName extends SuperclassName {
    // Additional methods or overridden method implementations
}

Example:

public class Superclass {
    public int getNb() {
        // Implementation provided
    }

    public int getNb2() {
        // Implementation provided
    }
}

public class Subclass extends Superclass {
    @Override
    public int getNb2() {
        // Overridden implementation
    }
}

Differences

In summary, the following key differences exist:

• Implements establishes a contract, while extends establishes an inheritance relationship.
• Interfaces define abstract methods, while classes provide implementations.
• Interfaces can be implemented by multiple classes, while classes can extend at most one other class.

Applications

Understanding the distinction between implements and extends proves essential for effective software design. Implements is ideal for implementing interfaces that define a set of common behaviors among unrelated classes. Extends should be used when a subclass needs to inherit and potentially override functionality from an existing superclass.

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