The ABCs of Programming - Java 6 - page 35

NOTE

There are actually nine primitive data types in Java if you count
void , which represents nothing. It's used in a method to indicate
that it does not return a value.

Class Types
In addition to the primitive data types, a variable can have a class as its type, as in the
following examples:

2

String lastName = “Hopper”;

Color hair;

VolcanoRobot vr;

When a variable has a class as its type, the variable refers to an object of that class or
one of its subclasses.

The last statement in the preceding list— VolcanoRobot vr; —creates a variable named
vr that is reserved for a VolcanoRobot object. You'll learn more tomorrow about how to
associate objects with variables.

Referring to a superclass as a variable type is useful when the variable might be one of
several different subclasses. For example, consider a class hierarchy with a
CommandButton superclass and three subclasses: RadioButton , CheckboxButton , and
ClickButton . If you create a CommandButton variable called widget , it could refer to a
RadioButton , CheckboxButton , or ClickButton object.

Declaring a variable of type Object means that it can be associated with any kind of
object.

Assigning Values to Variables
After a variable has been declared, a value can be assigned to it with the assignment
operator, which is an equal sign (“=”). The following are examples of assignment state-
ments:

idCode = 8675309;

accountOverdrawn = false;

Constants
Variables are useful when you need to store information that can be changed as a pro-
gram runs.