Ivor Horton's Beginning Java™ 2, JDK™ 5th Edition

2.1. Data and Variables

A variable is a named piece of memory that you use to store information in your Java program—a piece of data of some description. Each named piece of memory that you define in your program is able to store data only of one particular type. If you define a variable to store integers, for example, you can't use it to store a value that is a decimal fraction, such as 0.75. If you've defined a variable that you use to refer to a Hat object, you can only use it to reference an object of type Hat (or any of its subclasses, as you'll see in Chapter 6). Since the type of data that each variable can store is fixed, the compiler can verify that each variable you define in your program is not used in a manner or a context that is inappropriate to its type. If a method in your program is supposed to process integers, the compiler will be able to detect when you inadvertently try to use the method with some other kind of data—for example, a string or a numerical value that is not integral.

Explicit data values that appear in your program are called literals. Each literal will also be of a particular type: 25, for example, is an integer literal of type int. I will go into the characteristics of the various types of literals that you can use as I discuss each variable type.

Before you can use a variable you must specify its name and type in a declaration statement. Before I describe how you write a declaration for a variable, let's consider what flexibility you have in ...