Extends vs. Implements

Attribute	Extends	Implements
Inheritance	Allows a class to inherit properties and methods from a single parent class.	Allows a class to implement multiple interfaces.
Keyword	extends	implements
Usage	Used to create a subclass that inherits from a single superclass.	Used to implement one or more interfaces in a class.
Relationship	Is-a relationship (subclass to superclass).	Implements-a relationship (class to interface).
Number of Inheritance	Only one class can be extended.	Multiple interfaces can be implemented.
Method Overriding	Allows overriding of superclass methods.	Does not allow method overriding, only method implementation.
Constructor Inheritance	Constructors are not inherited.	Constructors are not inherited.
Interface Implementation	Not applicable, as extends is used for classes.	Not applicable, as implements is used for interfaces.

Introduction

In object-oriented programming, inheritance is a fundamental concept that allows classes to inherit properties and behaviors from other classes. In Java, two keywords,extends andimplements, are used to establish inheritance relationships between classes and interfaces, respectively. While both keywords serve the purpose of inheritance, they have distinct attributes and are used in different contexts. This article aims to explore and compare the attributes ofextends andimplements in Java.

Extends

Theextends keyword is used to establish an inheritance relationship between classes. When a class extends another class, it inherits all the non-private fields and methods of the superclass. This allows the subclass to reuse and extend the functionality of the superclass. In other words, the subclass becomes a specialized version of the superclass, inheriting its attributes and behaviors.

One important attribute ofextends is that a class can only extend a single class. This is known as single inheritance. This limitation ensures a clear and straightforward hierarchy of classes, preventing potential conflicts that may arise from multiple inheritance. However, it also means that a class can only inherit from one superclass, which may limit the flexibility of the class hierarchy.

Another attribute ofextends is that it allows for method overriding. Method overriding occurs when a subclass provides its own implementation of a method that is already defined in the superclass. This enables the subclass to modify or extend the behavior of the inherited method. By using the@Override annotation, the compiler can provide compile-time checks to ensure that the overridden method signature matches the superclass method.

Furthermore, theextends keyword enables polymorphism, which is a key feature of object-oriented programming. Polymorphism allows objects of different classes to be treated as objects of a common superclass. This facilitates code reusability and flexibility, as a method defined in the superclass can be invoked on objects of both the superclass and its subclasses.

In summary, theextends keyword establishes an inheritance relationship between classes, allowing the subclass to inherit and extend the attributes and behaviors of the superclass. It supports single inheritance, method overriding, and enables polymorphism.

Implements

Theimplements keyword is used to establish an inheritance relationship between interfaces and classes. While a class can only extend a single class, it can implement multiple interfaces. This allows a class to inherit the abstract methods defined in the interfaces and provide its own implementation for each method.

One important attribute ofimplements is that it enables interface inheritance. Interfaces define a contract of methods that implementing classes must adhere to. By implementing an interface, a class guarantees that it will provide an implementation for all the methods declared in the interface. This promotes code consistency and ensures that classes with similar behaviors can be used interchangeably.

Another attribute ofimplements is that it allows for multiple inheritance of interfaces. This means that a class can implement multiple interfaces, inheriting the abstract methods defined in each interface. This flexibility enables a class to exhibit behaviors from multiple sources, promoting code reuse and modularity.

Furthermore, theimplements keyword is often used in conjunction with theextends keyword. A class can extend a superclass usingextends and implement one or more interfaces usingimplements. This combination of inheritance relationships allows a class to inherit both the attributes and behaviors of a superclass and the abstract methods defined in interfaces.

In summary, theimplements keyword establishes an inheritance relationship between interfaces and classes, allowing the class to inherit and implement the abstract methods defined in the interfaces. It supports multiple inheritance of interfaces and can be used in conjunction withextends to inherit both superclass attributes and interface behaviors.

Comparison

Now that we have explored the attributes of bothextends andimplements, let's compare them in various aspects:

1. Relationship Type

Theextends keyword establishes an inheritance relationship between classes, while theimplements keyword establishes an inheritance relationship between interfaces and classes. This fundamental difference determines the type of entities that can be extended or implemented.

2. Inheritance Limit

extends supports single inheritance, meaning a class can only extend a single class. On the other hand,implements supports multiple inheritance of interfaces, allowing a class to implement multiple interfaces. This difference provides more flexibility in terms of code reuse and modularity for classes implementing interfaces.

3. Method Implementation

When a class extends another class usingextends, it inherits the methods of the superclass. The subclass can choose to override these methods to provide its own implementation. In contrast, when a class implements an interface usingimplements, it must provide an implementation for all the abstract methods defined in the interface. This ensures that the class adheres to the contract defined by the interface.

4. Code Reusability

Bothextends andimplements promote code reusability. By inheriting attributes and behaviors from a superclass or implementing interfaces, classes can reuse existing code and avoid duplication. However,implements provides more flexibility in terms of reusing code from multiple sources, as a class can implement multiple interfaces.

5. Polymorphism

Polymorphism is a key feature of object-oriented programming that allows objects of different classes to be treated as objects of a common superclass or interface. Bothextends andimplements enable polymorphism, as methods defined in the superclass or interface can be invoked on objects of the subclass or implementing class. This promotes flexibility and code modularity.

6. Class Hierarchy

The use ofextends establishes a class hierarchy, where a subclass extends a superclass. This hierarchy provides a clear and straightforward structure for organizing classes. On the other hand, the use ofimplements does not establish a hierarchy among interfaces. Instead, it allows classes to implement multiple interfaces, each defining a set of abstract methods.

7. Interface Contracts

Interfaces define contracts of methods that implementing classes must adhere to. By implementing an interface, a class guarantees that it will provide an implementation for all the methods declared in the interface. This promotes code consistency and ensures that classes with similar behaviors can be used interchangeably. Theextends keyword does not provide this contract enforcement.

8. Flexibility

Overall, theimplements keyword provides more flexibility compared toextends. It allows for multiple inheritance of interfaces, enabling a class to exhibit behaviors from multiple sources. This flexibility promotes code reuse, modularity, and the ability to adapt to changing requirements.

Conclusion

In conclusion, theextends andimplements keywords in Java serve the purpose of inheritance but have distinct attributes and are used in different contexts.extends establishes an inheritance relationship between classes, supporting single inheritance, method overriding, and enabling polymorphism. On the other hand,implements establishes an inheritance relationship between interfaces and classes, supporting multiple inheritance of interfaces, contract enforcement, and providing more flexibility in terms of code reuse. Understanding the attributes and differences betweenextends andimplements is crucial for designing and implementing object-oriented systems in Java.