Inheritance
Inheritance in Java is a mechanism in which one object
acquires all the properties and behaviors of a parent object. It is an
important part of OOPs (Object Oriented programming system).
The idea behind inheritance in Java is that we can create
new classes that are built upon existing classes. When we inherit methods from
an existing class, we can reuse methods and fields of the parent class.
However, we can add new methods and fields in your current class also.
What is Inheritance?
Inheritance in Java enables a class to inherit properties
and actions from another class, called a superclass or parent class. A class
derived from a superclass is called a subclass or child group. Through
inheritance, a subclass can access members of its superclass (fields and
methods), enforce reuse rules, and encourage hierarchy.
Inheritance represents the IS-A relationship which is also
known as a parent-child relationship.
The IS-A relationship in Java refers to the relationship
between a subclass (child class) and its superclass (parent class) when
inheritance is used. It indicates that the subclass is a specialized form of
the superclass. Essentially, IS-A means "is a type of," which is a
way of expressing the relationship in real-world or object-oriented terms.
Example in Real Life:
·
A Dog IS-A Animal.
·
A Car IS-A Vehicle.
·
A Manager IS-A Employee.
Why use inheritance in Java?
·
For Method Overriding (so runtime polymorphism
can be achieved).
·
For Code Reusability.
Terms used in Inheritance
·
Class: A class is a group of objects which have
common properties. It is a template or blueprint from which objects are
created.
·
Sub Class/Child Class: Subclass is a class which
inherits the other class. It is also called a derived class, extended class, or
child class.
·
Super Class/Parent Class: Superclass is the
class from where a subclass inherits the features. It is also called a base
class or a parent class.
·
Reusability: As the name specifies, reusability
is a mechanism which facilitates you to reuse the fields and methods of the
existing class when you create a new class. You can use the same fields and
methods already defined in the previous class.
The syntax of Java Inheritance: -
class Subclass-name extends Superclass-name
{
//methods
and fields
}
The extends keyword indicates that we 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 that is inherited is
called a parent or superclass, and the new class is called child or subclass.
Java Inheritance
Example
As displayed in the above figure, Programmer is the subclass
and Employee is the superclass. The relationship between the two classes is
Programmer IS-A Employee. It means that Programmer is a type of Employee.
class Employee{
float salary=40000;
}
class Programmer extends Employee{
int bonus=10000;
public static void main(String args[]){
Programmer p=new Programmer();
System.out.println("Programmer salary is:"+p.salary);
System.out.println("Bonus of Programmer is:"+p.bonus);
}
}
In the above example, Programmer object can access the field
of own class as well as of Employee class i.e. code reusability.
Types of Inheritance in Java
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 one class inherits multiple classes, it is known as
multiple inheritance. For Example:
Single Inheritance Example
When a class inherits another class, it is known as a single
inheritance. In the example given below, Dog class inherits the Animal class,
so there is the single inheritance.
class Animal
{
void eat()
{
System.out.println("eating...");
}
}
class Dog extends Animal
{
void bark()
{
System.out.println("barking...");
}
}
class TestInheritance
{
public static void main(String args[]){
Dog d=new Dog();
d.bark();
d.eat();
}
}
Output:
barking...
eating...
Multilevel Inheritance Example
When there is a chain of inheritance, it is known as
multilevel inheritance. As you can see in the example given below, BabyDog
class inherits the Dog class which again inherits the Animal class, so there is
a multilevel inheritance.
class Animal
{
void eat()
{
System.out.println("eating...");
}
}
class Dog extends Animal
{
void bark()
{
System.out.println("barking...");
}
}
class BabyDog extends Dog
{
void weep()
{
System.out.println("weeping...");
}
}
class TestInheritance2
{
public static void main(String args[])
{
BabyDog d=new BabyDog();
d.weep();
d.bark();
d.eat();
}
}
Output:
weeping...
barking...
eating...
Hierarchical Inheritance Example
When two or more classes inherits a single class, it is
known as hierarchical inheritance. In the example given below, Dog and Cat
classes inherits the Animal class, so there is hierarchical inheritance.
class Animal
{
void eat()
{
System.out.println("eating...");
}
}
class Dog extends Animal
{
void bark()
{
System.out.println("barking...");
}
}
class Cat extends Animal
{
void meow()
{
System.out.println("meowing...");
}
}
class TestInheritance3
{
public static void main(String args[])
{
Cat c=new Cat();
c.meow();
c.eat();
//c.bark();//C.T.Error
}
}
Why multiple inheritance is not supported in Java?
To reduce the complexity and simplify the language, multiple
inheritance is not supported in java.
Suppose there are three classes A, B, and C. The C class
inherits A and B classes. If A and B classes have the same method and we call
it from child class object, there will be ambiguity to call the method of A or
B class.
class A
{
void msg()
{
System.out.println("Hello");
}
}
class B
{
void msg()
{
System.out.println("Welcome");
}
}
class C extends A,B
{//suppose if it were
public static void main(String args[])
{
C obj=new C();
obj.msg();//Now which msg() method would be invoked?
}
}