Inheritance in java is a mechanism in which one object acquires all the properties and behaviors of parent object.
The idea behind inheritance in java is that you can create new classes that are built upon existing classes. When you inherit from an existing class, you can reuse methods and fields of parent class, and you can add new methods and fields also.
Inheritance represents the IS-A relationship, also known as parent-child relationship.
The extends keyword indicates that you are making a new class that derives from an existing class. The meaning of "extends" is to increase the functionality.
In the terminology of Java, a class which is inherited is called parent or super class and the new class is called child or subclass.
As displayed in the above figure, Programmer is the subclass and Employee is the superclass. Relationship between two classes is Programmer IS-A Employee.It means that Programmer is a type of Employee.
In the above example, Programmer object can access the field of own class as well as of Employee class i.e. code reusability.
On the basis of class, there can be three types of inheritance in java: single, multilevel and hierarchical.
In java programming, multiple and hybrid inheritance is supported through interface only. We will learn about interfaces later.
Note: Multiple inheritance is not supported in java through class.
When a class extends multiple classes i.e. known as multiple inheritance. For Example:
To reduce the complexity and simplify the language, multiple inheritance is not supported in java.
Consider a scenario where A, B and C are three classes. The C class inherits A and B classes. If A and B classes have same method and you call it from child class object, there will be ambiguity to call method of A or B class.
Since compile time errors are better than runtime errors, java renders compile time error if you inherit 2 classes. So whether you have same method or different, there will be compile time error now.