Upcasting and Downcasting (OOP - JAVA)

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 Upcasting and Downcasting

Upcasting

A object, a variable or a method inside a super class is accessed a object, a variable or a method in a sub class called Upcasting. 

A a = new B(); 

  • super class object reference - A
  • reference variable - a
  • sub class object reference - B

eg: 

int x = 5;

long y = 5;

x = y;    <= wrong way

y = x;    <=correct way

eg: 

double a = 7.88;

float b = 25.98f;

a = b;    <=correct way

b = a;    <=wrong way

Consider the following example.

class A{

}

class B extends A{

}

class Test{

A a = new A();

B b = new B();

b = a;        <=wrong way

a = b;        <=correct way

}

Downcasting

Sub class object which has been assigned to the Super class variable is converting to a sub class object called Downcasting.

consider the following example.

class A{

}

class B extends class A{

}

class Test{

A a = new A();   <=correct

B b = new B();   <=correct

A a = new B();   <=correct

B b = new A();   <=wrong


B b = a;              <=wrong

B b = A a;          <=wrong

B b = (B) a;        <=correct

}






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