10.1: Declaring Variables

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One of the most powerful features of a programming language is the ability to define and manipulate variables. A variable is a named location that stores a value. Values may be numbers, text, images, sounds, and other types of data. To store a value, you first have to declare a variable.

String message;

This statement is a declaration, because it declares that the variable named message has the type String. Each variable has a type that determines what kind of values it can store. For example, the int type can store integers, and the char type can store characters.

Some types begin with a capital letter and some with lowercase. We will learn the significance of this distinction later, but for now you should take care to get it right. There is no such type as Int or string.

To declare an integer variable named x, you simply type:

int x;

Note that x is an arbitrary name for the variable. In general, you should use names that indicate what the variables mean. For example, if you saw these declarations, you could probably guess what values would be stored:

String firstName;
String lastName;
int hour, minute;

This example declares two variables with type String and two with type int. When a variable name contains more than one word, like firstName, it is conventional to capitalize the first letter of each word except the first. Variable names are case-sensitive, so firstName is not the same as firstname or FirstName.

This example also demonstrates the syntax for declaring multiple variables with the same type on one line: hour and minute are both integers. Note that each declaration statement ends with a semicolon.

You can use any name you want for a variable. But there are about 50 reserved words, called keywords, that you are not allowed to use as variable names. These words include public, class, static, void, and int, which are used by the compiler to analyze the structure of the program.

You can find the complete list of keywords at http://docs.oracle.com/javase/tutorial/java/nutsandbolts/_keywords.html, but you don’t have to memorize them. Most programming editors provide “syntax highlighting”, which makes different parts of the program appear in different colors.