CS401

Java: Variables

Inside a class you can define variables. Unlike C++ many data types are now treated as instances of Classes (e.g. see String below) rather than as primitive types. The two kinds of variables are listed in the tables below.

Why would you want to use the Class type instead of the primitive type? One reason is that you may want to take advantage of the methods belonging to a Class. Another is that you can not tell they type of a variable if it is defined as a primitive type (you can ofcourse typecast it). However, you can tell the type of a variable if it is defined as a Class type using the instanceof operator.
Primitive Types

byte	Example: byte word; Size = 8 bits.
short	Example: short num; Size = 16 bits.
int	Example: int Year; Size = 32 bits.
long	Example: long word; Size = 64 bits.
float	Example: float time; Size = 32 bits.
double	Example: double time; Size = 64 bits.
char	Example: char symbol; Size = 16 bits.
boolean	Example: boolean answer; value= true or false (not a number as in C++).

Class Types (see book or documentation for details)

String
Example of Declarations:

String Name;
Name = "Lynne Grewe";

OR
String Name = new String("Lynne Grewe");

OR
String Name = "Lynne Grewe";

Boolean

Double

Float

Short

Byte

Long

Variables can be declared with a protection type or status that restricts who can access them. See the page on Protection Types for more information.
Exercise

Static Variables

If a variable is declared static it means that the value of this variable will be the same for any object instatiated from its class. For example,

class Document {
  static int version=10;
  .....
}

This means that any object instatiated from Document will have the same value of version. Whenevery any Document object changes the value of version it is changed for all instances of this object. This is somewhat like the concept of a global variable.

Note; You can access a static variable through the class name or an instance of the class:

Document.version++;
Document d = new Document();
d.version = 7;

Exercise

Final Variables

This is akin to a constant in that it is set once and can never be changed. The value is set at declaration time as shown in the example below.

class Document {

   final int year 1997;
 .....
}