Java - Variable Types
Java - Variable Types
A variable provides us with named storage that
our programs can manipulate. Each variable in Java has a specific type, which
determines the size and layout of the variable's memory; the range of values
that can be stored within that memory; and the set of operations that can be
applied to the variable.
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 datatypes
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
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
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
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 name ClassName.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
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