INTRODUCTION TO OBJECTS AND INPUT/OUTPUT - Java Programming: From Problem Analysis to Program Design - page 118

Java variables

Primitive type
variables

Reference
variables

FIGURE 3-6 Java variables

Primitive type variables store data directly into their own memory spaces. Reference
variables store the address of the object containing the data. An object is an instance of
a class and the operator new is used to instantiate an object. In some languages, such as
C++, reference variables are called pointers.
Before discussing the class String , we first discuss how to use predefined methods in a
program.

Using Predefined Classes and Methods
in a Program
Recall that a method is a collection of instructions. When a method executes, it
accomplishes something. The method main , which you used in Chapter 2, executes
automatically when you run a Java program. Other methods execute only when they are
activated—that is, called. Java comes with a wealth of classes called predefined classes.
In addition, every predefined class contains many predefined methods, which accom-
plish useful results. In this section, you do not learn how to write your own methods, but
you do learn how to use some of the predefined classes and methods that accompany Java.
Recall from Chapter 2 that predefined classes are organized as a collection of packages,
called class libraries. A particular package can contain several classes and each class can
contain several methods. Therefore, to use a predefined class and/or a method, you need
to know the name of the package , the name of the class , and the name of the method.
To use a method, you also need to know a few other things, which are described shortly.
There are two types of methods in a class : static and non- static .A static
method can be used, that is, called, using the name of the class containing the method.
(Chapter 8 describes these methods in detail. At this point, you need to know only how
to use predefined methods, which may be static or non- static .)
The Java system contains the class Math , which in turn contains powerful and useful
mathematical functions. The class Math is contained in the package java.lang .
Every method of the class Math is a static method. Therefore, you can use every
method of the class Math using the name of the class , which is Math .