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Type Casting / Type Conversion

Converting the values of one type to another for the evaluation of an expression is called type casting or type conversion.

Type casting is of two types:-

1) Upcasting – Conversion of lower types to upper types is called upcasting. Upcasting is implicitly performed.

Upcasting is performed according to the following graph:-

Explanation -

a. Let n be an integer and n = 2ⁿ then to store n in memory in integer representation k bits are required.

For e.g

32768 = 2¹⁵

0 111111111111111

15 bits are required to store 32768.

b. In floating point representation number is not stored rather its exponent is stored.

i.e

32768 = 2¹⁵

Then only 15 need to be stored.

n=2^k and k = 2^l where l<k then to store n only l bits are required.

2) Downcasting – Conversion of higher types to lower type is called downcasting. Downcasting need to be explicitly performed.

Syntax –

(target type) expression;

Example- float f = 12.45; ==========> float f = (float)12.45; OR float f = 12.45f;
By this statement compiler

PROGRAM – Rectangle.java
public class Rectangle
{
private int l, b;
public void Rectangle(int x, int y)
{
l = x;
b = y;
}
public void display()
{
System.out.println("Length = " + l);
System.out.println("Breadth = " + b);
}
public int area()
{
return l * b;
}
public void setDimension(int x, int y)
{
l = x;
b = y;
}
}
Syntax of creating object of a class-
className ReferenceVariable = new className();
OR
className ReferenceVariable;
ReferenceVariable = new className();

Reference Variable – is an implicit pointer that contains the reference of a class object. In Java, objects are dynamically created. Hence, they don’t have names. They are referred using the name of their reference variable.

In C++ :-
Rectangle r;

Traditionally in languages when we write statement
int a=5, b=6, c=10;
a table is created during compilation.

Variable Name	Type	Address	Value	Scope
a	int	0 + 1000	5	main
B	int	2 + 1000	6	main
c	int	4 + 1000	10	main

int * p;

 Address & Reference denotes two different things.
o Address represents actual memory location whereas Reference represents a mechanism of finding out objects.
 Different JRE provides different implementations of references.
o In Microsft JRE, References are implemented as pointers that is they actually holds the address of object.

o In Sun Microsystem JRE, Reference Variables are implemented as pointer to pointer.

In this implementation, Reference and Address are different.

Now we will write a program RectTest.java to test the program Rectangle.java written above.

public class RectTest
{
public static void main(String[] args)
{
Rectangle r = new Rectangle();
r.l = 5;
r.b = 7;
r.setDimension(5, 4);
System.out.println("Dimension of rectangle r ");
r.display();
System.out.println("Area of r = " + r.area());
}
}

Output –

Since variables l and b are declared private so we cannot access it from outside.

Now the same program RectTest.java again.

PROGRAM RectTest.java
public class RectTest
{
public static void main(String[] args)
{
Rectangle r = new Rectangle();
//r.l = 5; // Invalid statement
//r.b = 7; // Invalid statement
r.setDimension(5, 4);
System.out.println("Dimension of rectangle r ");
r.display();
System.out.println("Area of r = " + r.area());
}
}