Java Variables and Data Types
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Variable Declaration Rules in Java
Variable is
an identifier which holds data or another one variable is an identifier whose
value can be changed at the execution time of program. Variable is an
identifier which can be used to identify input data in a program.
You
must declare all variables before they can be used. Following is the basic form
of a variable declaration −
data type variable [ = value][, variable [ = value] ...] ;
Here data
type is one of Java's data types and variable is the
name of the variable. To declare more than one variable of the specified type,
you can use a comma-separated list.
Following
are valid examples of variable declaration and initialization in Java −
Example
int a, b, c;
// Declares three ints, a, b, and c.
int a = 10, b = 10; // Example of
initialization
byte B = 22;
// initializes a byte type variable B.
double pi = 3.14159; // declares and assigns a value of PI.
char a = 'a'; // the char variable a
iis initialized with value 'a'
This
chapter will explain various variable types available in Java Language. There
are three kinds of variables in Java −
- Local variables
- Instance variables
- Class/Static variables
1. Local
Variables
·
Local variables are 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.
·
Local variables are implemented at stack level internally.
·
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.
Example
Here, age is
a local variable. This is defined inside pupAge() method and
its scope is limited to only this method.
public class Test {
public void pupAge() {
int age = 0;
age = age + 7;
System.out.println("Puppy age is :
" + age);
}
public static void main(String args[]) {
Test test = new Test();
test.pupAge();
}
}
This
will produce the following result −
Output
Puppy age is: 7
Example
Following
example uses age without initializing it, so it would give an
error at the time of compilation.
public 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();
}
}
This
will produce the following error while compiling it −
Output
Test.java:4:variable number might not have been initialized
age = age + 7;
^
1 error
2. Instance
Variables
·
Instance variables are 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.
·
Instance variables are created when an object is created with the
use of the keyword 'new' and destroyed when the object is destroyed.
·
Instance variables hold values that must be referenced by more
than one method, constructor or block, or essential parts of an object's state
that must be present throughout the class.
·
Instance variables can be declared in class level before or after
use.
·
Access modifiers can be given for instance variables.
·
The instance variables are visible for all methods, constructors
and block in the class. Normally, it is recommended to make these variables
private (access level). However, visibility for subclasses can be given for
these variables with the use of access modifiers.
·
Instance variables have default values. For numbers, the default
value is 0, for Booleans it is false, and for object references it is null.
Values can be assigned during the declaration or within the constructor.
·
Instance variables can be accessed directly by calling the
variable name inside the class. However, within static methods (when instance variables
are given accessibility), they should be called using the fully qualified name. ObjectReference.VariableName.
Example
import java.io.*;
public 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);
}
public static void main(String args[]) {
Employee empOne = new Employee("Ransika");
empOne.setSalary(1000);
empOne.printEmp();
}
}
This
will produce the following result −
Output
name : Ransika
salary :1000.0
3. Class/Static
Variables
·
Class variables also known as static variables are 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 rarely used other than being declared as
constants. Constants are variables that are declared as public/private, final,
and static. Constant variables never change from their initial value.
·
Static variables are stored in the static memory. It is rare to
use static variables other than declared final and used as either public or
private constants.
·
Static variables are created when the program starts and destroyed
when the program stops.
·
Visibility is similar to instance variables. However, most static
variables are declared public since they must be available for users of the
class.
·
Default values are same as instance variables. For numbers, the
default value is 0; for Booleans, it is false; and for object references, it is
null. Values can be assigned during the declaration or within the constructor.
Additionally, values can be assigned in special static initializer blocks.
·
Static variables can be accessed by calling with the class nameClassName.VariableName.
·
When declaring class variables as public static final, then
variable names (constants) are all in upper case. If the static variables are
not public and final, the naming syntax is the same as instance and local
variables.
Example
import java.io.*;
public class Employee {
// salary variable is a private static variable
private static double salary;
// DEPARTMENT is a
constant
public static final String DEPARTMENT = "Development
";
public static void main(String args[]) {
salary = 1000;
System.out.println(DEPARTMENT + "average
salary:" + salary);
}
}
This
will produce the following result −
Output
Development average salary:1000
Note − If the variables
are accessed from an outside class, the constant should be accessed as
Employee.DEPARTMENT
Java Data Types:
Java Data Types:
Data Type in Java
Data
type is a special keyword used to allocate sufficient memory space for
the each data, in other words Data type is used for representing the data in
main memory (RAM) of the computer.
In general every programming language is containing three
categories of data types. They are
·
Fundamental or primitive data types
·
Derived data types
·
User defined data types.
Primitive data types
Primitive data types are those whose variables allows us to store only one
value but they never allows us to store multiple values of same type. This is a
data type whose variable can hold maximum one value at a time.
Example:
int a; // valid
a=10; // valid
a=10, 20, 30; // invalid
Here
"a" store only one value at a time because it is primitive type
variable.
Derived data types
Derived data types are those whose variables allow us to store multiple
values of same type. But they never allows to store multiple values of
different types. These are the data type whose variable can hold more than one
value of similar type. In general derived data type can be achieve using array.
Example
int a[] = {10,20,30}; // valid
int b[] = {100, 'A', "ABC"}; //
invalid
Here
derived data type store only same type of data at a time not store integer,
character and string at same time.
User defined data types
User
defined data types are those which are developed by programmers by making use
of appropriate features of the language.
User
defined data types related variables allows us to store multiple values either
of same type or different type or both. This is a data type whose variable can
hold more than one value of dissimilar type, in java it is achieved using class
concept.
Note: In java both derived and user defined data type combined name as
reference data type.
In
C language, user defined data types can be developed by using struct, union,
enum etc. In java programming user defined datatype can be developed by using
the features of classes and interfaces.
Example
Student s = new Student();
In
java we have eight data type which are organized in four groups. They are
·
Integer category data types
·
Character category data types
·
Float category data types
·
Boolean category data types
Integer category data types
These
category data types are used for storing integer data in the main memory of
computer by allocating sufficient amount of memory space.
Integer
category data types are divided into four types which are given in following
table
|
Data Type
|
Size
|
Range
|
|
|
1
|
Byte
|
1
|
+ 127 to -128
|
|
2
|
Short
|
2
|
+ 32767 to -32768
|
|
3
|
Int
|
4
|
+ x to - (x+1)
|
|
4
|
Long
|
8
|
+ y to - (y+1)
|
Character category data
types
A
character is an identifier which is enclosed within single quotes. In java to
represent character data, we use a data type called char. This data type takes
two byte since it follows Unicode character set.
|
Data Type
|
Size(Byte)
|
Range
|
|
Char
|
2
|
232767 to -32768
|
Why Java take 2 byte of memory for store
character ?
Java
support more than 18 international languages so java take 2 byte for
characters, because for 18 international language 1 byte of memory is not
sufficient for storing all characters and symbols present in 18 languages. Java
supports Unicode but c support ascii code. In ascii code only English language
are present, so for storing all English latter and symbols 1 byte is
sufficient. Unicode character set is one which contains all the characters
which are available in 18 international languages and it contains 65536
characters
Float category data types
Float
category data type are used for representing float values. This category
contains two data types, they are in the given table
|
Data Type
|
Size
|
Range
|
Number of decimal
places
|
|
Float
|
4
|
+2147483647 to
-2147483648
|
8
|
|
Double
|
8
|
+ 9.223*1018
|
16
|
Boolean category data types
Boolean
category data type is used for representing or storing logical values is true
or false. In java programming to represent Boolean values or logical values, we
use a data type called Boolean.
Why Boolean data types
take zero byte of memory ?
Boolean
data type takes zero bytes of main memory space because Boolean data type of
java implemented by Sun Micro System with a concept of flip - flop. A flip -
flop is a general purpose register which stores one bit of information (one
true and zero false).
Note: In C, C++ (Turbo) Boolean data type is not available for
representing true false values but a true value can be treated as non-zero
value and false values can be represented by zero
|
Data Type
|
Default Value
|
Default size
|
|
boolean
|
false
|
1 bit
|
|
char
|
'\u0000'
|
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
|
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