Key differences between extends and implements keywords in Java

extends keywords in Java

In Java, the extends keyword is used to establish an inheritance relationship between two classes. It signifies that a new class (known as the subclass or derived class) is inheriting the attributes and methods from an existing class (known as the superclass or base class). By using extends, the subclass automatically acquires all the non-private properties and behavior of the superclass, allowing for the reuse of existing code. This mechanism is a fundamental part of Java’s object-oriented programming model, facilitating polymorphism and code maintenance. Additionally, the subclass can override inherited methods to provide specific implementations and can extend the superclass’s functionality by adding new methods or fields. However, Java supports single inheritance only, meaning a class can only extend one superclass.

Functions of extends keywords in Java:

• Code Reusability:

By using extends, a subclass can inherit accessible properties and methods from the superclass, allowing developers to reuse existing code instead of rewriting it. This facilitates more efficient coding and reduces the likelihood of errors.

• Method Overriding:

Subclasses can override methods from the superclass, providing specific implementations for inherited methods. This is crucial for tailoring inherited behavior to the specific needs of the subclass.

• Forming Hierarchies:

The extends keyword helps in forming a class hierarchy that represents is-a relationships between general classes and more specific classes. This hierarchical arrangement is key to organizing complex systems.

• Support for Polymorphism:

Inheritance supported by extends allows for polymorphism, where a subclass object can be treated as an instance of the superclass. This enables general programming techniques where methods can operate on groups of related objects.

• Base Class Initialization:

Through inheritance, constructors of the superclass can be invoked in the subclass, ensuring proper initialization of inherited fields according to the rules defined in the superclass.

• Abstract Class Implementation:

If a superclass is abstract, using extends makes it mandatory for the subclass to implement all the abstract methods defined in the superclass unless the subclass is also declared abstract.

• Interface Implementation:

When a class extends another class that already implements certain interfaces, the subclass automatically becomes an implementer of those interfaces, thereby inheriting the contract to fulfill the methods of the interface.

• Enhanced Maintainability and Extensibility:

The hierarchical organization of classes using extends makes it easier to maintain and extend the software. Changes at higher levels in the hierarchy can propagate benefits across all subclasses, while keeping modifications localized and manageable.

Example of extends keywords in Java:

In this example, we will create a basic class hierarchy involving animals. We will have a generic Animal class that will be extended by a more specific Dog class.

// Base class

class Animal {

    // Field for the animal’s name

    String name;

    // Constructor to initialize the Animal

    public Animal(String name) {

        this.name = name;

    }

    // Method to make the animal make a sound

    public void makeSound() {

        System.out.println(this.name + ” makes a sound.”);

    }

}

// Derived class

class Dog extends Animal {

    // Constructor for Dog class

    public Dog(String name) {

        super(name);  // Call the superclass (Animal) constructor

    }

    // Override the makeSound method for the Dog class

    @Override

    public void makeSound() {

        System.out.println(this.name + ” barks.”);

    }

}

// Main class to run the program

public class Main {

    public static void main(String[] args) {

        Animal myAnimal = new Animal(“Generic Animal”);

        Dog myDog = new Dog(“Rex”);

        // Calls method from Animal class

        myAnimal.makeSound();

        // Calls overridden method in Dog class

        myDog.makeSound();

    }

}

Explanation

• Animal Class:

This is the base class or superclass. It has a field name and a method makeSound() that prints a generic sound.

• Dog Class:

This is the derived class or subclass that extends Animal. It uses the extends keyword to inherit from Animal. It includes its own constructor that calls the constructor of Animal using super(name). It also overrides the makeSound() method to provide behavior specific to dogs (i.e., barking).

• Main Class:

This class contains the main method to run the program. It creates an instance of Animal and an instance of Dog, then calls the makeSound method on both, demonstrating polymorphic behavior where Dog has a specific implementation of a method from Animal.

implements keywords in Java

In Java, the implements keyword is used when a class wants to adhere to a specific interface. An interface in Java is an abstract type that is used to specify a set of methods that a class must implement, without providing the implementation itself. By using the implements keyword, a class commits to providing concrete implementations of all the abstract methods declared in the interface or interfaces it agrees to follow. This mechanism enables Java to support multiple inheritances of type, as a class can implement multiple interfaces, unlike class inheritance where a class can only extend one other class. The implements keyword helps enforce a formal contract between the class and the outside world, ensuring that certain behaviors are guaranteed to be present in the implementing class, enhancing both reliability and flexibility in application development.

Functions of implements keywords in Java:

• Interface Implementation:

The primary function of implements is to declare that a class will implement the methods defined in an interface. This establishes a formal contract that the class must fulfill all the abstract methods declared by the interface.

• Multiple Inheritance of Type:

While Java does not support multiple inheritance of state (i.e., inheriting from multiple classes), it allows multiple inheritance of type through interfaces. A class can implement multiple interfaces, thus inheriting the type definitions and obligations of multiple interfaces at once.

• Behavior Specification:

implements allows a class to commit to specific behaviors (defined as methods in an interface) without dictating how those behaviors are executed. This separation of “what” from “how” is fundamental in abstraction.

• Method Overriding:

When a class implements an interface, it must provide concrete implementations of the interface’s methods, effectively overriding the abstract methods defined in the interface.

• Decoupling Code:

implements facilitates a low coupling design by decoupling the interface (contract) from its implementation (actual code in the class). This makes the code more modular, easier to maintain, and scalable.

• Enhancing Flexibility:

Because a class can implement any number of interfaces, this increases the flexibility of the class in terms of functionality and interaction with other classes. Classes can dynamically interact with each other based on shared interfaces without caring about their specific implementations.

• Polymorphism:

Classes that implement interfaces can be referenced through interface types, which is key for achieving polymorphism. This allows objects of implementing classes to be treated as objects of the interface type, leading to flexible and dynamic code.

• Ensuring Consistency:

By using implements, developers are forced to adhere to a defined template (the interface), ensuring that all necessary methods are implemented, thus maintaining consistency across different implementations.

• Support for Dependency Injection:

Interfaces are crucial for effective dependency injection, a technique where objects receive other objects that they depend on. By defining dependencies via interfaces and using implements, systems become more testable and flexible.

• Legal Contracts for API Design:

In large software systems, particularly those that require interaction between different subsystems or components, implements serves as a legal contract for API design. This ensures that all teams develop components that can interoperate seamlessly based on the agreed interfaces.

Example of implements keywords in Java:

Here is a straightforward example to demonstrate the use of the implements keyword in Java. We will define an interface Animal that specifies a behavior, and then create a class Dog that implements this interface.

Java Example Using implements

// Define an interface with a method

interface Animal {

    void makeSound();  // Interface method (does not have a body)

}

// Create a class that implements the interface

class Dog implements Animal {

    // Implement the method defined in the interface

    @Override

    public void makeSound() {

        System.out.println(“The dog barks.”);

    }

}

// Main class to run the program

public class Main {

    public static void main(String[] args) {

        Dog myDog = new Dog();  // Create an instance of Dog

        myDog.makeSound();  // Call the implemented method

    }

}

Explanation

• Animal Interface:

This interface defines a single method makeSound() but does not provide any implementation. The purpose of an interface is to specify a set of methods that implementing classes must provide.

• Dog Class:

This class uses the implements keyword to agree to adhere to the Animal interface. It then provides a concrete implementation of the makeSound() method. The @Override annotation is used to indicate that this method is intended to override a method in a superclass or implement a method from an interface.

• Main Class:

In the Main class, we create an instance of Dog and call its makeSound() method. Since Dog implements Animal, it must provide the makeSound() method, which outputs a message to the console.

Key differences between extends and implements keywords in Java

Key Similarities between extends and implements keywords in Java

• Object-Oriented Programming Features:

Both extends and implements are fundamental to Java’s object-oriented programming approach, allowing classes to gain functionality from other classes or interfaces.

• Type Hierarchy:

Both keywords contribute to the creation of a type hierarchy. extends is used to form a hierarchy among classes, while implements is used to integrate classes into an interface-based hierarchy.

• Enforcement of Behavior:

Both keywords enforce a form of contract. With extends, a subclass inherits behavior and attributes from the superclass and can override or extend these behaviors. With implements, a class agrees to adhere to the behavioral contract defined by the interface by implementing its methods.

• Code Reusability:

Both mechanisms promote code reusability. Inheritance (extends) allows subclasses to reuse code from the superclass. Similarly, implements allows different classes to reuse the interface’s method signatures to support common functionality, ensuring consistency across different implementations.

• Polymorphism:

Both extends and implements support polymorphism, where objects of a subclass or an implementing class can be treated as objects of the superclass or interface. This allows for general coding practices where methods can accept objects of a superclass or interface type.

• Compilation-Time Checking:

Java compiler performs checks to ensure a class correctly extends a superclass or implements an interface. This includes checking that all abstract methods in the superclass or interface are implemented, providing compile-time type safety.

• Method Overriding and Overloading:

Both keywords allow for method overriding (redefining methods in subclasses or implementing classes) and method overloading (methods with the same name but different parameters within the same class hierarchy).

• Access to Higher-Level Members:

Both extends and implements provide the subclass or implementing class access to public and protected members of the superclass or interface. This includes methods and, in the case of extends, also includes fields.