Variables in Java Programming.

Variables:
The variable is the basic unit of storage in a java program.
A variable is an identifier that denotes a storage location used to store a data value.
Unlike constants that remain unchanged during the execution of a program ,a variable may take different values at different times during the execution of program.
A variable name can be chosen by the programmer in a meaningful way so as to reflect what it represents in the program .Some examples of variable names are:
(a)average
(b)height
(c)total
(d)price
Variable names may consist of alphabets ,digits,the underscore(_)and dollar characters,subjects to the following conditions:
1.They must not begin with a digit
2.Uppercase and lowercase are distinct.This means that the variable Total is not the same as total or TOTAL.
3.It should not be a keyword.
4.White space is not allowed.
5.Variable names can be of any length.

Types of Variable :

There are three types of variables in java ·         local variable ·         instance variable ·         static variable

Local Variable :

    A variable that is declared inside the method is called local variable

Instance Variable :

A variable that is declared inside the class but outside the method is called instance variable . It is not declared as static. • 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 and different class ( when instance variables are given accessibility) should be called using the fully qualified name . ObjectReference.VariableName.

Static variable :

A variable that is declared as static is called static variable. It cannot be local. It is also known as class variable. • 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 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 variables names (constants) are all in upper case.