Inheritance in Java
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.
Why use inheritance in Java?
**#For Method Overriding (so runtime polymorphism can be achieved).(TBD)
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.
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.
File Name: Programmer.java
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);
}
}
Types of Inheritance in Java
Parent – Single Child --> Single Inheritance
Parent – Multiple Children --> Hierarchical Inheritance
Parent – Child – GrandChild --> Multilevel Inheritance
On the basis of class, there can be three types of inheritance in java: single, multilevel and hierarchical.
Note: Multiple inheritance is not supported in Java through class.
When one class inherits multiple classes, it is known as multiple inheritance. For Example:Multiple inheritance in Java
*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.
File: TestInheritance.java
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();
}}
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.
File: TestInheritance2.java
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();
}}
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.
File: TestInheritance3.java
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?(TBD)
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.
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.
it support through interface
File Name: C.java
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?
}
}
Reference:https://www.javatpoint.com/inheritance-in-java
Method Overriding in Java
If subclass (child class) has the same method as declared in the parent class, it is known as method overriding in Java.
In other words, If a subclass provides the specific implementation of the method that has been declared by one of its parent class, it is known as method overriding.
Method Overloading:
Same method name with different number of arguments or with different types of arguments, in same class.
Method Overriding:
Same Method Name, Same Number of arguments but in two different classes. Those two different classes should maintain Parent – Child relationship.
Reference:https://www.javatpoint.com/method-overriding-in-java
Why final keyword used in java?
No overridding,if parent class want no overridding keyword FINAL is used
If a class is declared as final, It cannot be extended. i.e., We cannot create Sub classes for it.
If a class is declared as final, we cannot use this class in Inheritance.
Final field: cannot be re-assigned.
Final method: cannot be overridden.
final class: cannot be extended.
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