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Sunday, 22 February 2015

Java Variable Declaration Scope Lifetime and Type Conversion

                           Java Variable Declaration

The variable is the basic unit of storage in a Java program. A variable is defined  by the combination of an identifier, a type and an optional initializer. All variables have a scope, which defines their visibility and lifrtime.

Declaring a Variable:

In Java, all variables must be declared before they can be used. The basic form of a variable declaration is shown here:

type identifier [ = value], [identifier[ = value]...];

The type is one of Java's atomic types, or the name of a class or interface. The identifier is the name of the variable. You can initialize the variable by specifying an equal sign and a value.

int a, b, c; //declare three int a , b and c.
int d = 4, e, f = 5; //declared three ints, initializing d and f.
byte g = 7;  //declares g and initialize.
double h = 8.465 //declarers a double a.
char i = 'i' //the variable i has the value of 'i'.

Dynamic Initialization:

In Java variables can be initialized dynamically.

Scope and Lifetime of Variables.

Variable are created when their scope is entered, and destroyed when their scope is left. This means that a variable will not hold its value once it has gone out of scope Therefore, variables declared within a method will not hold their values between calls to that method. Also a variable declared within a block will lose its value when the block is left. Thus, the lifetime of a variable is confined to its scope.
If a variable declaration includes an initializer, then that variable will be reinitialized each time the block in which it is declared is entered.

Type Conversion and Casting:

If two variables are compatible, then java will perform the conversion automatically.

Java's Automatic Conversions:

When two type of data is assigned to another type of variable, and automatic type conversion will take place  if the following two conditions are met:
  • The two types are compatible.
  • The destination type is larger than the source type.

Casting Incompatible Types:

To create a conversion between two incompatible types, you must use a cast. A cast is simply an explicit type conversion. It has this general form:

(target-type) value.

Here, target-type specifies the desired type to convert the specified value to.
A different type of conversion will occur when a floating-point value is assigned to an integer type, the fractional part is lost. This is called truncation.

Automatic Type Promotion in Expression:

Automatic conversion is also used in expressions when the value exceed the range of their operand.
In this bytes and shorts are elevated to int then to long.

Written By: Asad Hussain

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