Introduction

• Inheritance means acquiring the properties and behaviors of a parent class in a child class.
• It allows a subclass (child class) to inherit fields and methods from a superclass (parent class), promoting code reuse and method overriding.
• Inheritance represents an IS-A relationship, also known as a parent-child relationship. It signifies that a subclass is a type of its superclass.
• For example:
  • A Car IS-A Vehicle.
  • A Dog IS-A Animal.
  • A Surgeon IS-A Doctor.
• How to achieve inheritance in Java?
  • By using the extends keyword for class inheritance.
  • By using the implements keyword for interface inheritance.

• Program 1 (using extends keyword): :

  class Vehicle
  {
      void start()
      {
          System.out.println("Vehicle starts.");
      }
  }
  
  class Car extends Vehicle
  {
      void drive()
      {
          System.out.println("Car drives.");
      }
  }
  
  public class MainApp
  {
      public static void main(String[] args)
      {
          Car myCar = new Car();
  
          myCar.start(); // inherited from Vehicle
          myCar.drive(); // specific to Car
      }
  }

• Program 2 (using implements keyword): :

  interface Animal
  {
      void eat();
  }
  class Dog implements Animal
  {
      public void eat()
      {
          System.out.println("Dog eats.");
      }
  }
  public class MainApp
  {
      public static void main(String[] args)
      {
          Dog myDog = new Dog();
          myDog.eat(); // inherited from Animal
  
          //Animal myAnimal = new Animal();   // error because we cannot create an object of an interface
      }
  }

Advantages of Inheritance

• Code Reusability: Inheritance allows a child class to reuse the code of its parent class.
• Easy Maintenance: Changes made in the parent class automatically propagate to child classes, making maintenance easier.
• Method Overriding: Inheritance enables method overriding, allowing a child class to provide a specific implementation of a method already defined in its parent class.
• Polymorphism: Inheritance supports runtime polymorphism using method overriding.

Disadvantages of Inheritance

• Tight Coupling: Inheritance creates a tight coupling between parent and child classes, if we change the parent class, it may affect all child classes.
• Increased Complexity: Inheritance can lead to complex class hierarchies, making the code harder to understand and maintain.

Types of Inheritance

• There are 5 types of inheritance in Java:
  1. Single Inheritance: One class inherits the properties and behaviors of one parent class.
  2. Multilevel Inheritance: One class inherits the properties and behaviors of a parent class, and that class is inherited by another class.
  3. Hierarchical Inheritance: Multiple classes inherit the properties and behaviors of a single parent class.
  4. Multiple Inheritance: One class inherits the properties and behaviors of multiple classes. (Not supported in Java directly, but can be achieved using interfaces.)
  5. Hybrid Inheritance: A combination of two or more types of inheritance. (Not supported in Java directly, but can be achieved using interfaces.)
• Click Here to learn more about the types of inheritance in Java.

Important Points

• Some important points to remember about inheritance in Java:
  • Java does not support multiple and hybrid inheritance with classes to avoid ambiguity, such as the diamond problem.
  • A class can extend only one class, which is known as single inheritance.
  • Constructors and private members of the parent class are not inherited by the child class.
  • A class can implement multiple interfaces, which is Java's way of achieving multiple inheritance.
  • The super keyword is used to refer to the parent class, such as accessing parent class methods or constructors.
  • The this keyword is used to refer to the current class instance, commonly used to differentiate between instance variables and parameters.