Inheritance in Java

Inheritance in Java is a core OOP concept that allows a class to acquire properties and behaviors from another class. It helps in creating a new class from an existing class, promoting code reusability and better organization.

• A subclass can reuse the fields and methods of the parent class without rewriting the code
• A subclass can add its own fields and methods or modify existing ones to extend functionality.

Dog barks
Cat meows
Cow moos

Explanation:

• Animal is the base class.
• Dog, Cat and Cow are derived classes that extend Animal class and provide specific implementations of the sound() method.
• The Geeks class is the driver class that creates objects and demonstrates runtime polymorphism using method overriding.

Syntax:

class Parent {
// fields and methods
}
class Child extends Parent {
// additional fields and methods
}

Note: In Java, inheritance is implemented using the extends keyword.

Key Terminologies in Java Inheritance

• Class : Blueprint from which objects are created
• Superclass : Class whose properties are inherited
• Subclass : Class that inherits another class
• extends : Keyword used to inherit a class

Types of Inheritance in Java

It defines the different ways a class can inherit properties and behavior from one or more classes.

Below are the different types of inheritance which are supported by Java.

1. Single Inheritance

In single inheritance, a sub-class is derived from only one super class. It inherits the properties and behavior of a single-parent class. Sometimes, it is also known as simple inheritance.

This is a Vehicle
This Vehicle is Car

2. Multilevel Inheritance

In Multilevel Inheritance, a derived class will be inheriting a base class and as well as the derived class also acts as the base class for other classes.

This is a Vehicle
4 Wheeler Vehicles
This 4 Wheeler Vehicle is a Car

3. Hierarchical Inheritance

In hierarchical inheritance, more than one subclass is inherited from a single base class. i.e. more than one derived class is created from a single base class. For example, cars and buses both are vehicle

This is a Vehicle
This Vehicle is Car
This is a Vehicle
This Vehicle is Bus

4. Multiple Inheritance (Through Interfaces)

In Multiple inheritances, one class can have more than one superclass and inherit features from all parent classes.

Note: that Java does not support multiple inheritances with classes. In Java, we can achieve multiple inheritances only through Interfaces.

This is an AmphibiousVehicle
This is a WaterVehicle
This is a LandVehicle

5. Hybrid Inheritance

It is a mix of two or more of the above types of inheritance. In Java, we can achieve hybrid inheritance only through Interfaces if we want to involve multiple inheritance to implement Hybrid inheritance.

This is a Vehicle
This is a Car
This is a Vehicle
This is a Bus
Fare information

Explanation:

• class Car extends Vehicle->Single Inheritance
• class Bus extends Vehicle and class Bus implements Interface Fare->Hybrid Inheritance (since Bus inherits from two sources, forming a combination of single + multiple inheritance).

Java IS-A type of Relationship

IS-A represents an inheritance relationship in Java, meaning this object is a type of that object.

Now, based on the above example, in Object-Oriented terms, the following are true:

• SolarSystem is the superclass of Earth class.
• SolarSystem is the superclass of Mars class.
• Earth and Mars are subclasses of SolarSystem class.
• Moon is the subclass of both Earth and SolarSystem classes.

true
true
true

What Can Be Done in a Subclass?

In sub-classes we can inherit members as is, replace them, hide them or supplement them with new members: 

• The inherited fields can be used directly, just like any other fields.
• We can declare new fields in the subclass that are not in the superclass.
• The inherited methods can be used directly as they are.
• We can write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it (as in the example above, toString() method is overridden).
• We can write a new static method in the subclass that has the same signature as the one in the superclass, thus hiding it.
• We can declare new methods in the subclass that are not in the superclass.
• We can write a subclass constructor that invokes the constructor of the superclass, either implicitly or by using the keyword super.

 Advantages of Inheritance in Java

• Code Reusability: Inheritance allows for code reuse and reduces the amount of code that needs to be written. The subclass can reuse the properties and methods of the superclass, reducing duplication of code.
• Abstraction: Inheritance allows for the creation of abstract classes that define a common interface for a group of related classes. This promotes abstraction and encapsulation, making the code easier to maintain and extend.
• Class Hierarchy: Inheritance allows for the creation of a class hierarchy, which can be used to model real-world objects and their relationships.
• Polymorphism: Inheritance allows for polymorphism, which is the ability of an object to take on multiple forms. Subclasses can override the methods of the superclass, which allows them to change their behavior in different ways.

Limitations of Inheritance in Java

• Complexity -> Deep inheritance hierarchies can make code hard to understand . Correction -> keep hierarchy simple or use composition instead
• Tight Coupling -> Changes in superclass can affect subclasses . Correction -> use loose coupling (interfaces or composition)