Inheritance in Java with Example

Inheritance is one of the strong features of object-oriented programming (OOP) that allows a class to access (or inherit) properties, such as variables and methods of another class. This technique promotes code reusability and facilitates the creation of hierarchical class structures that represent real-world relationships.

  class Parent {
    // variables
    // methods
  }
  class Child extends Parent {
    // variables
    // methods
  }

'extends' keyword in Java:

The extends keyword is used in places where one class has to inherit the properties (variables and methods) of another class. In the case of interfaces, we use the extends keyword if one interface wants to inherit the properties of another interface.

Terms used in Java Inheritance

Class: A class is a user-defined data type in Java from which objects are created. It represents the set of properties, methods, etc. that are common to all objects of one type. A single Java class can have one or more objects.

Super Class: A class from which the properties are inherited is called the Super Class, Parent Class, or Base Class in Java.

Sub Class: A class that inherits the properties of another class is called a subclass, child class, extended class, or derived class in Java.

Reusability: Inheritance promotes code reusability in Java, which means already developed Java classes and their members (variables and methods) can be used in other classes. This will save our development and testing time and efforts.

Advantages of Inheritance in Java

Inheritance provides a couple of benefits. Let's understand how inheritance provides advantages in the Java programming language.

Disadvantages of Inheritance in Java

Along with the benefits, there are some points where we have to pay the costs of using inheritance in Java.

IS-A Relationship in Java

Inheritance establishes an "IS-A" relationship between classes, where one class (subclass) inherits the features of another class (superclass). This mechanism promotes code reusability, enhances maintainability, and encourages modularity in software design.

In Java, we use inheritance only if there exists an "IS-A" relationship between two classes.

Here, a car can inherit from a vehicle, a mango can inherit from fruit, a lion can inherit from an animal, and so on.

Types of Inheritance:

Note: Multiple inheritance and hybrid inheritance are only supported through the interface in Java; we cannot achieve them through Java classes.

Let's go over each type of inheritance in Java one by one, using definitions and examples.

1. Single Inheritance in Java

In single inheritance, a single class (known as the child class) inherits the properties (variables and methods) of another single class (known as the parent class). Only two classes are required to implement single inheritance in Java.

  class Father {
    String father_money = "20 Lakhs";
    
    void fatherHouse() {
      System.out.println("It's a Father's House");
    }
  }
  class Son extends Father {
    String son_money = "10 Lakhs";
    
    void sonHouse() {
      System.out.println("It's a Son's House");
    }
  }
  public class SingleInheritanceExample {
    public static void main(String[] args) {
      Son sonObject = new Son();
      System.out.println("Father's Money : " + sonObject.father_money);
      System.out.println("Son's Money : " + sonObject.son_money);
      
      sonObject.fatherHouse();
      sonObject.sonHouse();
    }
  }

Father's Money : 20 Lakhs
Son's Money : 10 Lakhs
It's a Father's House
It's a Son's House

2. Multilevel Inheritance in Java

In multilevel inheritance, a child class will inherit a parent class, and the child class also acts as the parent class for another class.

  class Father {
    String father_money = "20 Lakhs";
    
    void fatherHouse() {
      System.out.println("It's a Father's House");
    }
  }
  class Son extends Father {
    String son_money = "10 Lakhs";
    
    void sonHouse() {
      System.out.println("It's a Son's House");
    }
  }
  class GrandSon extends Son {
    String grand_son_money = "5 Lakhs";
    
    void grandSonHouse() {
      System.out.println("It's a Gand Son's House");
    }
  }
  public class SingleInheritanceExample {
    public static void main(String[] args) {
      GrandSon grandSonObject = new GrandSon();
      
      System.out.println("Father's Money : " + grandSonObject.father_money);
      System.out.println("Son's Money : " + grandSonObject.son_money);
      System.out.println("Grand Son's Money : " + grandSonObject.grand_son_money);
      
      grandSonObject.fatherHouse();
      grandSonObject.sonHouse();
      grandSonObject.grandSonHouse();
    }
  }

Father's Money : 20 Lakhs
Son's Money : 10 Lakhs
Grand Son's Money : 5 Lakhs
It's a Father's House
It's a Son's House
It's a Gand Son's House

3. Hierarchical Inheritance in Java

In hierarchical inheritance, one superclass has multiple subclasses, each inheriting the superclass's same properties (variables and methods).

Let's understand hierarchical inheritance through the diagram and example below.

  class Father {
    String father_money = "20 Lakhs";
    
    void fatherHouse() {
      System.out.println("It's a Father's House");
    }
  }
  class FirstSon extends Father {
    String first_son_money = "10 Lakhs";
    
    void firstSonHouse() {
      System.out.println("It's a First Son's House");
    }
  }
  class SecondSon extends Father {
    String second_son_money = "8 Lakhs";
    
    void secondSonHouse() {
      System.out.println("It's a Second Son's House");
    }
  }
  class ThirdSon extends Father {
    String third_son_money = "12 Lakhs";
    
    void thirdSonHouse() {
      System.out.println("It's a Third Son's House");
    }
  }
  public class HierarchicalInheritanceExample {
    public static void main(String[] args) {
      // FirstSon Class Object Creation
      FirstSon firstSonObject = new FirstSon();
      
      System.out.println("Father's Money : " + firstSonObject.father_money);
      System.out.println("First Son's Money : " + firstSonObject.first_son_money);
      
      firstSonObject.fatherHouse();
      firstSonObject.firstSonHouse();
      
      System.out.println(); 			// Line Break
      
      // SecondSon Class Object Creation
      SecondSon secondSonObject = new SecondSon();
      
      System.out.println("Father's Money : " + secondSonObject.father_money);
      System.out.println("Second Son's Money : " + secondSonObject.second_son_money);
      
      secondSonObject.fatherHouse();
      secondSonObject.secondSonHouse();
      
      System.out.println(); 			// Line Break
      
      // ThirdSon Class Object Creation
      ThirdSon thirdSonObject = new ThirdSon();
      
      System.out.println("Father's Money : " + thirdSonObject.father_money);
      System.out.println("Third Son's Money : " + thirdSonObject.third_son_money);
      
      thirdSonObject.fatherHouse();
      thirdSonObject.thirdSonHouse();
    }
  }

Father's Money : 20 Lakhs
First Son's Money : 10 Lakhs
It's a Father's House
It's a First Son's House

Father's Money : 20 Lakhs
Second Son's Money : 8 Lakhs
It's a Father's House
It's a Second Son's House

Father's Money : 20 Lakhs
Third Son's Money : 12 Lakhs
It's a Father's House
It's a Third Son's House

4. Multiple Inheritance in Java

In multiple inheritance, a single child class extends from multiple parent classes, which means one child class will have more than one parent class, but this implementation is not supported by Java; we can achieve this only using interfaces.

5. Hybrid Inheritance in Java

Hybrid inheritance is a combination of two or more types of inheritance in Java. Same as Multiple Inheritance, Hybrid Inheritance is also not supported by using Java classes; however, we can achieve this type of implementation using the Java Interface.

We use the extends keyword for the same data type, like one class can extend another class, same as one interface can extend another interface, but in Java, one class can extend only one class, whereas one interface can extend one or more interfaces.

implements keyword use with different data types, like one class can implement another interface. Note that one class can implement one or more than one interface, but an interface can never implement any class in Java.

✅	With the help of inheritance, we can reuse existing class properties, such as variables and methods, in another class.
✅	Inheritance saves our development, testing, bug-fixing, and maintenance time, effort, and cost as well.
✅	Inheritance provides a clear model structure that is easy to understand and implement.
✅	Inheritance can be used for method overriding in Java.

✅	If we modify or remove the properties of the parent class, that will directly affect the behavior of the child class too.
✅	The subclass becomes dependent on the parent class's implementation.
✅	Inheritance bit impacts the execution speed because inheritance execution takes time and effort.
✅	Inheritance also promotes a tight coupling. Both the parent class and child class get tightly bound to each other, and a developer cannot use these classes independently of each other.
✅	The overuse of inheritance makes Java applications more complex.

Inheritance in Java

'extends' keyword in Java:

Terms used in Java Inheritance

Advantages of Inheritance in Java

Disadvantages of Inheritance in Java

IS-A Relationship in Java

Types of Inheritance:

1. Single Inheritance in Java

2. Multilevel Inheritance in Java

3. Hierarchical Inheritance in Java

4. Multiple Inheritance in Java

5. Hybrid Inheritance in Java

Inheritance - Interview Questions and Answers

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