Overriding

Method Overriding in Java

Whenever same method name is existing in both base class and derived class with same types of parameters or same order of parameters is known as method Overriding. Here we will discuss about Overriding in Java.

Note: Without Inheritance method overriding is not possible.

Advantage of Java Method Overriding

• Method Overriding is used to provide specific implementation of a method that is already provided by its super class.
• Method Overriding is used for Runtime Polymorphism

Rules for Method Overriding

• method must have same name as in the parent class.
• method must have same parameter as in the parent class.
• must be IS-A relationship (inheritance).

Understanding the problem without method overriding

Lets understand the problem that we may face in the program if we do not use method overriding.

Example Method Overriding in Java

class Walking
{  
void walk()
{
System.out.println("Man walking fastly");
}  
}  
class OverridingDemo
{  
public static void main(String args[])
{  
Man obj = new Man();  
obj.walk();  
}
}

Output

Man walking

Problem is that I have to provide a specific implementation of walk() method in subclass that is why we use method overriding.

Example of method overriding in Java

In this example, we have defined the walk method in the subclass as defined in the parent class but it has some specific implementation. The name and parameter of the method is same and there is IS-A relationship between the classes, so there is method overriding.

Example

class Walking
{  
void walk()
{
System.out.println("Man walking fastly");
}  
}  
class Man extends walking
{  
void walk()
{
System.out.println("Man walking slowly");
}
}

class OverridingDemo
{  
public static void main(String args[])
{  
Man obj = new Man();  
obj.walk();  
}
}

Output

Man walking slowly

Note: Whenever we are calling overridden method using derived class object reference the highest priority is given to current class (derived class). We can see in the above example high priority is derived class.

Note: super. (super dot) can be used to call base class overridden method in the derived class.

Accessing properties of base class with respect to derived class object

class A
{
int x;
void f1()
{
x=10;
System.out.println(x);
}
void f4()
{
System.out.println("this is f4()");
System.out.println("-----------------");
}
};
class B extends A
{
int y;
void f1()
{
int y=20;
System.out.println(y);
System.out.println("this is f1()");
System.out.println("------------------");
}
};
class C extends A
{
int z;
void f1()
{
z=10;
System.out.println(z);
System.out.println("this is f1()");
}
};
class Overide
{
public static void main(String[] args)
{
A a1=new B();
a1.f1();
a1.f4();
A c1=new C();
c1.f1();
c1.f4();
}
}

Example of Implement overriding concept

class Person
{
String name;
void sleep(String name)
{
this.name=name;
System.out.println(this.name +"is sleeping+8hr/day");
}
void walk()
{
System.out.println("this is walk()");
System.out.println("-----------------");
}
};
class Student extends Person
{
void writExams()
{
System.out.println("only student write the exam");
}
void sleep(String name)
{
super.name=name;
System.out.println(super.name +"is sleeping 6hr/day");
System.out.println("------------------");
}
};
class Developer extends Person
{
public void designProj()
{
System.out.println("Design the project");
}
void sleep(String name)
{
super.name=name;
System.out.println(super.name +"is sleeping 4hr/day");
System.out.println("------------------");
}
};
class OverideDemo
{
public static void main(String[] args)
{
Student s1=new Student();
s1.writExams();
s1.sleep("student");
s1.walk();
Developer d1=new Developer();
d1.designProj();
d1.sleep("developer");
}
}

Difference between Overloading and Overriding

	Overloading	Overriding
1	Whenever same method or Constructor is existing multiple times within a class either with different number of parameter or with different type of parameter or with different order of parameter is known as Overloading.	Whenever same method name is existing multiple time in both base and derived class with same number of parameter or same type of parameter or same order of parameters is known as Overriding.
2	Arguments of method must be different at least arguments.	Argument of method must be same including order.
3	Method signature must be different.	Method signature must be same.
4	Private, static and final methods can be overloaded.	Private, static and final methods can not be override.
5	Access modifiers point of view no restriction.	Access modifiers point of view not reduced scope of Access modifiers but increased.
6	Also known as compile time polymorphism or static polymorphism or early binding.	Also known as run time polymorphism or dynamic polymorphism or late binding.
7	Overloading can be exhibited both are method and constructor level.	Overriding can be exhibited only at method label.
8	The scope of overloading is within the class.	The scope of Overriding is base class and derived class.
9	Overloading can be done at both static and non-static methods.	Overriding can be done only at non-static method.
10	For overloading methods return type may or may not be same.	For overriding method return type should be same.

Note: In overloading we have to check only methods names (must be same) and arguments types (must be different) except these the remaining like return type access modifiers etc. are not required to check 
But in overriding every things check like method names arguments types return types access modifiers etc.

Method OverLoading

Method Overloading in Java

Whenever same method name is exiting multiple times in the same class with different number of parameter or different order of parameters or different types of parameters is known asmethod overloading.

Why use method Overloading in Java ?

Suppose we have to perform addition of given number but there can be any number of arguments, if we write method such as a(int, int)for two arguments, b(int, int, int) for three arguments then it is very difficult for you and other programmer to understand purpose or behaviors of method they can not identify purpose of method. So we use method overloading to easily figure out the program. For example above two methods we can write sum(int, int) and sum(int, int, int) using method overloading concept.

Syntax

class  class_Name
{
Returntype  method()
{.........}
Returntype  method(datatype1 variable1)
{.........}
Returntype  method(datatype1 variable1, datatype2 variable2)
{.........}
Returntype  method(datatype2 variable2)
{.........}
Returntype  method(datatype2 variable2, datatype1 variable1)
{.........}
}

Different ways to overload the method

There are two ways to overload the method in java

• By changing number of arguments or parameters
• By changing the data type

By changing number of arguments

In this example, we have created two overloaded methods, first sum method performs addition of two numbers and second sum method performs addition of three numbers.

Example Method Overloading in Java

class Addition
{
void sum(int a, int b)
{
System.out.println(a+b);
}
void sum(int a, int b, int c)
{
System.out.println(a+b+c);
}
public static void main(String args[])
{
Addition obj=new Addition();
obj.sum(10, 20);
obj.sum(10, 20, 30);
}
}

Output

30
60

By changing the data type

In this example, we have created two overloaded methods that differs in data type. The first sum method receives two integer arguments and second sum method receives two float arguments.

Example Method Overloading in Java

class Addition
{
void sum(int a, int b)
{
System.out.println(a+b);
}
void sum(float a, float b)
{
System.out.println(a+b);
}
public static void main(String args[])
{
Addition obj=new Addition();
obj.sum(10, 20);
obj.sum(10.05, 15.20);
}
}

Output

30
25.25

Why Method Overloading is not possible by changing the return type of method?

In java, method overloading is not possible by changing the return type of the method because there may occur ambiguity. Let's see how ambiguity may occur:

because there was problem:

Example of Method Overloading

class Addition
{  
  int sum(int a, int b)
  {
  System.out.println(a+b);
  }  
  double sum(int a, int b)
  {
  System.out.println(a+b);
  }  
  public static void main(String args[])
  {  
  Addition obj=new Addition();  
  int result=obj.sum(20,20); //Compile Time Error   
  }
}  

Explanation of Code

Example

int result=obj.sum(20,20);

Here how can java determine which sum() method should be called

Note: The scope of overloading is within the class.

Any object reference of class can call any of overloaded method.

Can we overload main() method ?

Yes, We can overload main() method. A Java class can have any number of main() methods. But run the java program, which class should have main() method with signature as "public static void main(String[] args). If you do any modification to this signature, compilation will be successful. But, not run the java program. we will get the run time error as main method not found.

Example of override main() method

Example

public class mainclass
{
public static void main(String[] args)
{
System.out.println("Execution starts from Main()");
}
void main(int args)
{
System.out.println("Override main()");
}
double main(int i, double d)
{
System.out.println("Override main()");
return d;
}
}

Outpot

Execution starts from Main()

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