2. Variables¶

Note

One of the most powerful features of a programming language is the ability to define and manipulate variables. A variable is a named location in memory 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 called 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.

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.

String firstName;
String lastName;
int hour, minute;

This example declares two variables with type String and two with type int. The last line shows how to declare multiple variables with the same type: hour and minute are both integers. Note that each declaration statement ends with a semicolon ;.

Variable names usually begin with a lowercase letter, in contrast to class names (like Hello) that start with a capital letter. When a variable name contains more than one word (like firstName), it is conventional to capitalize the first letter of each subsequent word. Variable names are case-sensitive, so firstName, firstname, and FirstName are all different variable names.

Note

Rules for an identifier

• An identifier in Java must be a sequence of letters (a-z, A-Z), underscore (_), dollar signs ($), and digits (0-9)
• Must start with a letter, underscore, or dollar sign