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VARIABLES IN JAVA



The variable is the basic unit of storage in a Java program. A variable is defined by the combination of a type, an identifier and an optional initialize. Type is a data type or the name of a class or interface (class and interface will discussed further).for identify the type of variable identifier is a variable name and optional initializer means users directly initialize the value to the variable if they want.

Declare Variable

Syntax for declaring a variable is...
                        type identifier1=val , identifier2;
Exp.
            int a=100, b;  //initialize a and declare b.
            byte z = 22;   // initialize z.
            double pi = 3.14159;   // declares an approximation of pi.
            char x = 'x';   // the variable x has the value 'x'.

Dynamic Initialization
Java allows variables to be initialized dynamically.
  
The Scope and Lifetime of Variables

Every variable have scope and life time throughout the program. Here we study when the new scope will be create for how long time (Life time). A block is begun with an opening curly brace and ended by a closing curly brace. A block defines a scope. We use so many variables in our program and declared these variables at the at the start of the main( ) method. Most other computer languages define two general categories of scopes: global and local. However, these traditional scopes do not fit well with Java’s strict, object oriented model.


Array:
              It is a bunch or you can say group of variables with same data types.
Java has three reference data types:

Data Type
Description
Array
A collection of several items of the same data type. For example names of student.
Class
A collection of variables and methods.
Interface
An abstract class created to implement multiple inheritance in JAVA.
  
We have one dimensional and multidimensional array in java. It works differently compare to C and C++ language. We declare one char type array and size of that array is 10. So we have 10 different char type variables like ch[0] to ch[9]. Value in the square bracket is called index of the array and index value of any array always start from 0.

Arrays can be declared in three ways:
Description
Syntax
Example
Only
Declaration
Just declares the array.
data type identifier[]
char ch[ ]; declares a character array named ch.
Declaration and creation
Declares and allocates memory for the array elements using the reserved word ‘new’.
data type identifier[] =new data type[size];
char ch[] = new char[10]; declares an array ch to store 10 characters.
Declaration, creation and initialization
Declares the array, allocates memory for it and assigns initial values to its elements.
data type identifier[]  = {value1, value2, ….ValueN};
char ch[] ={‘A’,’B’,’C’,’D’}; declares an array ch to store 4 pre-as signed character values.


Syntax of One dimensional array :
                        Declaration syntax : type var-name[ ];
 Exp.
             int month[ ];
 Here we just declare month variable of type int array. But actually or physically no array exists. To link this array into actual physical array of integers we have to use new keyword.

Syntax:   array-var = new type[size];

Exp.
                        month = new month[12];

So obtaining an array is a two-step process. First, you must declare a variable of the desired array type. Second, you must allocate the memory that will hold the array, using new, and assign it to the array variable. Thus, in Java all arrays are dynamically allocated. Now we have 12 different int type variable month[0] to month[11]. It is possible to combine the declaration of the array variable with the allocation of the array itself like... int month[] = new int[12];
Multidimensional array : 
                        Declaration syntax : type var-name[ ][ ];
Exp.
                        int mat[ ][ ] = new int[4][5];
In Java, multidimensional arrays are actually arrays of arrays. To declare a multidimensional array variable, specify each additional index using another set of square brackets. This allocates a 4 by 5 array and assigns it to mat. Internally this matrix is implemented as an array of arrays of int. conceptually.



Anurag

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