Inheritance in java

Inheritance in java is the most used and most important topic of OOP. Inheritance is defined as a process where one object acquires the properties of another object. With the help of inheritance, information can be made manageable in a hierarchical order.

Extends and Implements are the two keywords that we come across when we are dealing with inheritance. By using these keywords one object can acquire properties of another object. Inheritance is an IS-A type of relationship.

Types of inheritance –

Inheritance is broadly classified into 4 categories:

            ·         Single Inheritance.

            ·         Multiple Inheritance.

            ·         Multi-level Inheritance.

            ·         Hierarchical Inheritance.

Single Inheritance –

When a class inherits the properties of another class, it is an example of Single Inheritance. In the below example Dog class inherits the properties of Animal class.               

Multiple-level Inheritance –

It is the concept of object oriented programming where one base class inherits the superclass and the same base class acts as Superclass for another base class. In the below example, Animal is the base class, Mammal is extending the Animal class and Mammal class is the base class for Cat class.                     

Hierarchical Inheritance –

It is the concept of Object Oriented programming where one superclass is inherited by multiple base classes or in other words, multiple base classes inherit the single superclass. In the below example, Animal class is extended by Cat, Dog and Tiger class.

Multiple Inheritance –

It is the concept of Object Oriented programming where one class can extend properties of one or more base class. Java does not support Multiple Inheritance. In the below example, cat class is extending both Animal as well as Mammal class.

 

IS-A relation:

Child object IS-A type of parent type object. Let’s see how the “extend” keyword is used to maintain inheritance.

 

Now, based on the above example, the following are true in object oriented words:

• Animal is a superclass of mammal class.

• Animals are the superclass of the reptile class.

• Mammal and reptile are subclasses of the animal class.

• Dog is a subclass of both mammal and animal classes.

Now, if we consider the IS-A relation, we can say:

• Mammal IS-A Animals

• The reptile is an IS-A animal

• Dog IS-A Mammal.

• Hence: Dog IS-A Animal as well.

Except for the private properties of the superclass, the sub-class will be able to inherit all the properties with the use of extends keyword.

 

The instanceOf keyword-

The instanceOf variable is used to determine whether the sub-class is the instance of the superclass. Consider the following example:

 

The output will be:

true

true

true

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