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.
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