3. Data types and Variables

Data types and VariablesData types and Variables
By
Nilesh Dalvi
Lecturer, Patkar-Varde College.Lecturer, Patkar-Varde College.
http://www.slideshare.net/nileshdalvi01
Java and Data StructuresJava and Data Structures

Data Types
A data type in a programming language is a set of data
with values having pre-defined characteristics.
Nilesh Dalvi, Lecturer@Patkar-Varde College, Goregaon(W).

Data Types
Data Type Default Value Default size
boolean false 1 bit
char 'u0000‘ or 0 2 byte
byte 0 1 byte
short 0 2 byte
int 0 4 byte
long 0L 8 byte
float 0.0f 4 byte
double 0.0d 8 byte

Demonstrating double data type
class Area
{
public static void main(String args [])
{
double pi, r, a;
r = 10.8; // radius of a circle
pi = 3.146; // value of pi
a = pi*r*r; // compute area
System.out.println("Area of circle" +a);
}
}

Demonstrating char data type
class CharDemo
{
public static void main(String args [])
{
char ch = 'x';
System.out.println("Character is" +ch);
ch ++; // increment ch;
System.out.println("Character is" +ch);
}
}

Variables:
• Declaring variables:
type identifier = value;
• Example:
• Types of variables:
There are three types of variables in java
– local variable
– instance variable
– static variable
int a, b, c; // Demonstrating three integers
int d = 3, e, f = 5;
byte z = 22;
double pi = 3.1416;
char x = ‘x’;

Variables:
Dynamic initialization:
class DynInit {
public static void main(String args []){
double a = 3.0, b = 4.0;
//c is dynamically initialized
double c = Math.sqrt(a*a+b*b);
System.out.println("Hypotenuse is" +c);
}
}

Local variable:
• Declared in methods, constructors, or blocks.
• Local variables are created when the method, constructor or
block is entered and the variable will be destroyed once it exits
the method, constructor or block.
• Access modifiers cannot be used for local variables.
• Local variables are visible only within the declared method,
constructor or block.
• There is no default value for local variables so local variables
should be declared and an initial value should be assigned
before the first use.

Local variable:
class Test{
public void pupAge(){
int age;
age = age + 7;
System.out.println("Puppy age is : " + age);
}
public static void main(String args[]){
Test test = new Test();
test.pupAge();
}
}

Local variable:
class Test{
public void pupAge(){
int age = 0;
age = age + 7;
System.out.println("Puppy age is : " + age);
}
Test test = new Test();
test.pupAge();
}
}

Instance variable:
• Declared in a class, but outside a method, constructor or any block.
• When a space is allocated for an object in the heap, a slot for each
instance variable value is created.
• Created when an object is created with the use of the keyword 'new' and
destroyed when the object is destroyed.
• Instance variables can be declared in class level.
• Access modifiers can be given for instance variables.
• They are visible for all methods, constructors and block in the class.
• Instance variables have default values.
• It can be accessed directly by calling the variable name inside the class.
ObjectReference.VariableName.

Instance variable:
class Employee{
// this instance variable is visible for any child class.
public String name;
// salary variable is visible in Employee class only.
private double salary;
// The name variable is assigned in the constructor.
public Employee (String empName){
name = empName;
}
// The salary variable is assigned a value.
public void setSalary(double empSal){
salary = empSal;
}
// This method prints the employee details.
public void printEmp(){
System.out.println("name : " + name );
System.out.println("salary :" + salary);
}
Employee empOne = new Employee("Ram");
empOne.setSalary(1000);
empOne.printEmp();
}
}

Class/ Static variable:
• Declared with the static keyword in a class, but outside a
method, constructor or a block.
• There would only be one copy of each class variable per class,
regardless of how many objects are created from it.
• Static variables are created when the program starts and
destroyed when the program stops.
• Visibility is similar to instance variables.
• Default values are same as instance variables.
• Static variables can be accessed by calling with the class name
ClassName.variableName

Class/ Static variable:
class Employee{
// salary variable is a private static variable
private static double salary;
// DEPARTMENT is a constant
public static final String DEPARTMENT = "Development ";
salary = 1000;
System.out.println(DEPARTMENT+"average salary:"+salary);
}
}

Type Conversion and Casting
Java’s Automatic Conversions(Implicit):
• When one type of data is assigned to another type of
variable, an automatic type conversion will take place if the
following conditions satisfied:
– Two types are compatible.
– Destination type is larger than the source type.
– e.g.
int a = byte b;

Casting Incompatible types(Explicit):
• To create a conversion between two incompatible types, you
must use a cast.
• A cast is simply an explicit type conversion.
• It has the general form:
– (target-type) value;
– target-type – specifies the desired type to convert the specified value
to.
– e.g. int a;
byte b;
// ...
b = (byte) a;

Demonstrating Casting
class Conversion
{
public static void main(String args[])
{
byte b;
int i = 257;
double d = 323.142;
System.out.println("nConversion of int to byte.");
b = (byte) i;
System.out.println("i and b " + i + " " + b);
System.out.println("nConversion of double to int.");
i = (int) d;
System.out.println("d and i " + d + " " + i);
System.out.println("nConversion of double to byte.");
b = (byte) d;
System.out.println("d and b " + d + " " + b);
}
}

Demonstrating Casting
(323)10 = (1 0 1 0 0 0 0 1 1 )2
= 0 x 27
+ 1 x 26 + 0 x 25+ 0 x 24+
0 x 23+ 0 x 22+ 1 x 21+ 1 x 20
= (67)10
Output:
Conversion of int to byte.
i and b 257 -> 1
Conversion of double to int.
d and i 323.142 -> 323
Conversion of double to byte.
d and b 323.142 -> 67
2 323
2 161 1
2 80 1
2 40 0
2 20 0
2 10 0
2 5 0
2 2 1
2 1 0
2 0 1
Byte is 8-bit

Java type casting is classified in two types:

Arrays

3. Data types and Variables

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