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Variables

An object stores its state in variables.

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Definition:  A variable is an item of data named by an identifier.

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You must explicitly provide a name and a type for each variable you want to use in your program. The variable's name must be a legal identifier — an unlimited-length sequence of Unicode characters that begins with a letter. You use the variable name to refer to the data that the variable contains. The variable's type determines what values it can hold and what operations can be performed on it. To give a variable a type and a name, you write a variable declaration, which generally looks like this:

type name

In addition to the name and type that you explicitly give a variable, a variable has scope. The section of code where the variable's simple name can be used is the variable's scope. The variable's scope is determined implicitly by the location of the variable declaration, that is, where the declaration appears in relation to other code elements. You'll learn more about scope in the section Scope.

The boldface type in the following program, called MaxVariablesDemo, highlights all the variable declarations in the program.

public class MaxVariablesDemo {
    public static void main(String args[]) {

        //integers
        byte largestByte = Byte.MAX_VALUE;
        short largestShort = Short.MAX_VALUE;
        int largestInteger = Integer.MAX_VALUE;
        long largestLong = Long.MAX_VALUE;

        //real numbers
        float largestFloat = Float.MAX_VALUE;
        double largestDouble = Double.MAX_VALUE;

        //other primitive types
        char aChar = 'S';
        boolean aBoolean = true;

        //display them all
        System.out.println("The largest byte value is "
                           + largestByte);
        System.out.println("The largest short value is "
                           + largestShort);
        System.out.println("The largest integer value is "
                           + largestInteger);
        System.out.println("The largest long value is "
                           + largestLong);

        System.out.println("The largest float value is "
                           + largestFloat);
        System.out.println("The largest double value is "
                           + largestDouble);

        if (Character.isUpperCase(aChar)) {
            System.out.println("The character " + aChar
                               + " is upper case.");
        } else {
            System.out.println("The character " + aChar
                               + " is lower case.");
        }
        System.out.println("The value of aBoolean is "
                           + aBoolean);
    }
}

The output from this program is:

The largest byte value is 127
The largest short value is 32767
The largest integer value is 2147483647
The largest long value is 9223372036854775807
The largest float value is 3.40282e+38
The largest double value is 1.79769e+308
The character S is upper case.
The value of aBoolean is true

The following sections further explore the various aspects of variables:

• Data Types
• Variable Names
• Scope
• Variable Initialization
• Final Variables
• Summary of Variables
• Questions and Exercises: Variables

The MaxVariablesDemo program uses two items with which you might not yet be familiar and are not covered in variable discussion: several constants named MAX_VALUE and an if-else statement. Each MAX_VALUE constant is defined in one of the number classes provided by the Java platform and is the largest value that can be assigned to a variable of that numeric type. These classes are covered in the section Numbers. The if-else statement is covered later in this chapter in the section The if/else Statements.

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