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Variables

Textbook: Section 2.4

Definition

A variable is just a place in memory. From the programmer's point of view, a Java variable has three things associated with it.

• The name of the variable, to be used to refer to the memory location within the program code.

  Names of variables (and classes and methods) can be any sequence of letters, digits, and underscores '_', but the name must not begin with a digit.

  The typical Java convention is to name variables will all lower-case letters, with underscores separating different words. Some typical variable names are i, width, and grid_height. Java programmers use this convention for variables, but they use different conventions for class names and method names. By doing this, they can immediately recognize the purpose of an identifier.

  For example, the naming convention for classes is to include no underscores and capitalize each word: DrawableFrame. For method names, the convention is the same, except the first letter is lower-case: runProgram().

• The value of the variable, which is held in the memory location during program execution.

• The type of the variable, which is used by the compiler to check your code for correctness. There are many types in Java, but for the moment let's just keep ourselves down to four.
  • int for holding integer values
  • double for holding floating-point values
  • boolean for holding Boolean (true/false) values
  • char for holding character values

Declaring a variable

Before you use a variable, you first have to tell Java that it exists so that the compiler knows about it. This is a declaration. To declare a variable in Java, type the type name followed by the variable name followed by a semicolon.

        int i;

        int i, num_students, total_grade;

The scope of a variable is the area of the program where you can use it. In Java, a variable's scope begins at the point of the variable's declaration and proceeds to the end of the innermost brace pair in which the declaration lies.

import csbsju.cs160.*;

public class Example {
    public static void run() {
        IO.println("hello world!");
        ... {
            // i isn't usable here
            int i;
            // now it is
            ... {
                // we can still use i within this brace pair
            }
            // and we're still within the brace pair
        }
        // but now we're out of the brace pair; i is unavailable
    }
}

Assigning a value

To assign a value to a variable, type the variable name, followed by an equal sign, followed by the value, followed by a semicolon.

        i = 2;

It's often convenient to combine the declaration and the first assignment.

        int i = 2;

What do you think the testI() method will print?

import csbsju.cs160.*;

public class Example {
    public static void testI() {
        int i;
        i = 2;
        IO.println("i");
        IO.println(i);
        i = 5;
        IO.println(i);
    }
}

Constants like 2 have an immediately recognizable type. In the case of integers, a constant is just a sequence of digits. For double values, the constant contains a decimal point (or an exponent, as in 3e8, which represents 3×10⁸). For boolean constants, the only permissible values are true and false. And for characters, you type the character in a set of single quotes: 'A' (the capital A character) or ' ' (the space character). It's important that the character be in quotes; otherwise, the compiler will understand it as a variable name (or whitespace, or whatever else is appropriate).

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