Type Casting / Type Conversion

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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
                                           

2. PROGRAM – Rectangle.java
3. public class Rectangle
4. {
5. private int l, b;
6. public void Rectangle(int x, int y)
7. {
8. l = x;
9. b = y;
10. }
11. public void display()
12. {
13. System.out.println("Length = " + l);
14. System.out.println("Breadth = " + b);
15. }
16. public int area()
17. {
18. return l * b;
19. }
20. public void setDimension(int x, int y)
21. {
22. l = x;
23. b = y;
24. }
25. }
26. Syntax of creating object of a class-
27. className ReferenceVariable = new className();
28. OR
29. className ReferenceVariable;
30. 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.

2. public class RectTest
3. {
4. public static void main(String[] args)
5. {
6. Rectangle r = new Rectangle();
7. r.l = 5;
8. r.b = 7;
9. r.setDimension(5, 4);
10. System.out.println("Dimension of rectangle r ");
11. r.display();
12. System.out.println("Area of r = " + r.area());
13. }
14. }

Output –






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

Now the same program RectTest.java again.

2. PROGRAM RectTest.java
3. public class RectTest
4. {
5. public static void main(String[] args)
6. {
7. Rectangle r = new Rectangle();
8. //r.l = 5; // Invalid statement
9. //r.b = 7; // Invalid statement
10. r.setDimension(5, 4);
11. System.out.println("Dimension of rectangle r ");
12. r.display();
13. System.out.println("Area of r = " + r.area());
14. }
15. }

OUTPUT –

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Question: What is datatype?

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Question: What is datatype?

Answer – Datatype is a set of values that has an associated set of operations and a memory representation for its values.

S. NO.	DATA TYPE	SIGN IN BYTES	DEFAULT VALUES
1.	char	2	\0 (Null Character)
2.	byte	1	0
3.	short	2	0
4.	int	4	0
5.	long	8	0
6.	float	4	0
7.	double	8	0
8.	boolean	Not defined	false

2. class Test1
3. {
4. public static void main(String args[])
5. {
6. int a;
7. System.out.println("a = “ + a); // Compilation error
8. }
9. }

Output -

Explanation-

v  Concept of default value is not applied on local variables of method i.e. local variables of a method need to be explicitly assigned.

v  Default value will be applied in class variables.

Like for example the above program can be rewritten as-

2. PROGRAM Test1.java
3. class Test1
4. {
5. static int a, b;
6. public static void main(String args[])
7. {
8. System.out.println("a & b is : " + a + " " + b); // No compilation error
9. }
10. }

OUTPUT –

2. PROGRAM Test2.java
3. class Test2
4. {
5. public static void main(String args[])
6. {
7. float f = 12.45;
8. System.out.println("Value of f = " + f);
9. }
10. }

What will be the output –

A.     Value of f = 12.45

B.     Value of f = 12

C.     Value of f = 12.00000

D.     Compilation Error  (output)

Now let’s see the output of the above program.

Explanation –

In Java, floating point constants are of double type by default. The value of higher type cannot be directly assigned to a lower type. In such a case type conversion is required.

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