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.
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