Before you can use a variable in a Java program, you must create the variable by declaring its name and the type of information it will store. The type of information is listed first, followed by the name of the variable. The following are all examples of variable declarations:

int loanLength;

String message;

boolean gameOver;

Local variables can be declared at any place inside a method, just like any other Java statement, but they must be declared before they can be used. The normal place for variable declarations is immediately after the statement that names and identifies the method.

In the following example, three variables are declared at the top of a program’s main() method:

public static void main(String[] arguments) {
    int total;
    String reportTitle;
    boolean active;
}

If you are creating several variables of the same type, you can declare all of them in the same statement by separating the variable names with commas. The following statement creates three String variables named street, city, and state:

String street, city, state;

Variables can be assigned a value when they are created by using an equal sign (=) followed by the value. The following statements create new variables and give them initial values:

int zipCode = 90210;

int box = 350;

boolean pbs = true;

String name = "Zoom", city = "Boston", state = "MA";

As the last statement indicates, you can assign values to multiple variables of the same type by using commas to separate them.

You must give values to local variables before you use them in a program or the program won’t compile successfully. For this reason, it is good practice to give initial values to all local variables.

Instance and class variable definitions are given an initial value depending on the type of information they hold, as in the following:

Variable names in Java must start with a letter, an underscore character (“_”), or a dollar sign (“$”). They cannot start with a number. After the first character, variable names can include any combination of letters or numbers.

When naming a variable and using it in a program, it’s important to remember that Java is case sensitive—the capitalization of letters must be consistent. Because of this, a program can have a variable named X and another named x (and Rose is not a rose is not a ROSE).

In programs in this book and elsewhere, Java variables are given meaningful names that include several words joined together. To make it easier to spot the words, the following rule of thumb is used:

The following variable declarations follow this rule of naming:

Button loadFile;

int localAreaCode;

boolean quitGame;

In addition to a name, a variable declaration must include the data type of information being stored. The type can be any of the following:

You learn how to declare and use array variables on Day 4. Today’s lesson focuses on the other variable types.

String lastName = "Hopper";

Color hair;

VolcanoRobot vr;

After a variable has been declared, a value can be assigned to it with the assignment operator, which is an equal sign (“=”). The following are examples of assignment statements:

idCode = 8675309;

accountOverdrawn = false;

Variables are useful when you need to store information that can be changed as a program runs.

If the value should never change during a program’s runtime, you can use a type of variable called a constant. A constant, which also is called a constant variable, is a variable with a value that never changes. This might seem like an oxymoron, given the meaning of the word “variable.”

Constants are useful in defining shared values for the use of all methods of an object. In Java, you can create constants for all kinds of variables: instance, class, and local.

To declare a constant, use the final keyword before the variable declaration and include an initial value for that variable, as in the following:

final float PI = 3.141592;

final boolean DEBUG = false;

final int PENALTY = 25;

In the preceding statements, the names of the constants are capitalized: PI, DEBUG, and PENALTY. This is a convention adopted by many Java programmers that makes it clear you’re using a constant instead of a variable.

Constants can be handy for naming various states of an object and then testing for those states. Suppose you have a program that takes directional input from the numeric keypad on the keyboard—push 8 to go up, 4 to go left, and so on. You can define those values as constant integers:

final int LEFT = 4;
final int RIGHT = 6;
final int UP = 8;
final int DOWN = 2;

Constants often make a program easier to understand. To illustrate this point, consider which of the following two statements is more informative as to its function:

guide.direction = 4;

guide.direction = LEFT;

Today’s first project is a Java application that creates several variables, assigns them initial values, and displays two of them as output. The full source code is in Listing 2.1.

 1: public class Variables {
 2:
 3:     public static void main(String[] arguments) {
 4:         final char UP = 'U';
 5:         byte initialLevel = 12;
 6:         short location = 13250;
 7:         int score = 3500100;
 8:         boolean newGame = true;
 9:
10:         System.out.println("Level: " + initialLevel);
11:         System.out.println("Up: " + UP);
12:     }
13: }

Compile this application and run the class file Variables.class. This program produces the following output:

Level: 12
Up: U

This class uses four local variables and one constant, making use of System.out.println() in lines 10–11 to produce output.

System.out.println() is a method called to display strings and other information to the standard output device, which usually is the screen.

System.out.println() takes a single argument within its parentheses: a string. To present more than one variable or literal as the argument to println(), you can use the “+” operator to combine these elements into a single string, which will be described later today.

There’s also a System.out.print() method, which displays a string without terminating it with a newline character. You can call print() instead of println() to display several strings on the same line.

Java has several integer literals. The number 4, for example, is an integer literal of the int variable type. It also can be assigned to byte and short variables because the number is small enough to fit into those integer types. An integer literal larger than an int can hold is automatically considered to be of the type long. You also can indicate that a literal should be a long integer by adding the letter L (upper- or lowercase) to the number. For example, the following statement treats the value 4 as a long integer:

pennyTotal = pennyTotal + 4L;

To represent a negative number as a literal, prepend a minus sign (“-”) to the literal—for example, -45.

Floating-point literals use a period character (“.”) for the decimal point, as you would expect. The following statement uses a literal to set up a double variable:

double myGPA = 2.25;

All floating-point literals are considered to be of the double variable type instead of float. To specify a literal of float, add the letter F (upper- or lowercase) to the literal, as in the following example:

float piValue = 3.1415927F;

You can use exponents in floating-point literals by using the letter e or E followed by the exponent, which can be a negative number. The following statements use exponential notation:

double x = 12e22;

double y = 19E-95;

The Boolean literals true and false are the only two values you can use when assigning a value to a boolean variable type or using a Boolean in a statement.

The following statement sets a boolean variable:

boolean chosen = true;

If you have programmed in other languages, you might expect that a value of 1 is equivalent to true and 0 is equivalent to false. This isn’t the case in Java; you must use the values true or false to represent Boolean values.

Note that the literal true does not have quotation marks around it. If it did, the Java compiler would assume that it was a string.

Character literals are expressed by a single character surrounded by single quotation marks, such as ‘a’, ‘#’, and ‘3’. You might be familiar with the ASCII character set, which includes 128 characters, including letters, numerals, punctuation, and other characters useful in computing. Java supports thousands of additional characters through the 16-bit Unicode standard.

Some character literals represent characters that are not readily printable or accessible through a keyboard. Table 2.2 lists the special codes that can represent these special characters as well as characters from the Unicode character set. In Table 2.2, the letter d in the octal, hex, and Unicode escape codes represents a number or a hexadecimal digit (a–f or A–F).

The final literal that you can use in a Java program represents strings of characters. A string in Java is an object rather than a primitive data type. Strings are not stored in arrays as they are in languages such as C.

Because string objects are real objects in Java, methods are available to combine strings, modify strings, and determine whether two strings have the same value.

String literals consist of a series of characters inside double quotation marks, as in the following statements:

String quitMsg = "Are you sure you want to quit?";

String password = "swordfish";

Strings can include the character escape codes listed previously in Table 2.2, as shown here:

String example = "Socrates asked, "Hemlock is poison?"";

System.out.println("Sincerely,
Millard Fillmore
");

String title = "Sams Teach Yourself Ruby on Rails While You Sleepu2122"

In the last of the preceding examples, the Unicode code sequence u2122 produces a ^™ symbol on systems that have been configured to support Unicode.

Although string literals are used in a manner similar to other literals in a program, they are handled differently behind the scenes.

With a string literal, Java stores that value as a String object. You don’t have to explicitly create a new object, as you must when working with other objects, so they are as easy to work with as primitive data types. Strings are unusual in this respect—none of the basic types are stored as an object when used. You learn more about strings and the String class later today and tomorrow.

Five operators are used to accomplish basic arithmetic in Java, as shown in Table 2.3.

Each operator takes two operands, one on either side of the operator. The subtraction operator also can be used to negate a single operand, which is equivalent to multiplying that operand by 1.

One thing to be mindful of when performing division is the kind of numbers being used. If you store a division operation into an integer, the result will be truncated to the next lower whole number because the int data type can’t handle floating-point numbers. As an example, the expression 31 / 9 results in 3 if stored as an integer.

Modulus division, which uses the % operator, produces the remainder of a division operation. The expression 31 % 9 results in 4 because 31 divided by 9, with the whole number result of 3, leaves a remainder of 4.

Note that many arithmetic operations involving integers produce an int regardless of the original type of the operands. If you’re working with other numbers, such as floating-point numbers or long integers, you should make sure that the operands have the same type you’re trying to end up with.

Listing 2.2 contains a class that demonstrates simple arithmetic in Java.

 1: public class Weather {
 2:     public static void main(String[] arguments) {
 3:         float fah = 86;
 4:         System.out.println(fah + " degrees Fahrenheit is ...");
 5:         // To convert Fahrenheit into Celsius
 6:         // Begin by subtracting 32
 7:         fah = fah - 32;
 8:         // Divide the answer by 9
 9:         fah = fah / 9;
10:         // Multiply that answer by 5
11:         fah = fah * 5;
12:         System.out.println(fah + " degrees Celsius
");
13:
14:         float cel = 33;
15:         System.out.println(cel + " degrees Celsius is ...");
16:         // To convert Fahrenheit into Celsius
17:         // Begin by subtracting 9
18:         cel = cel * 9;
19:         // Divide the answer by 5
20:         cel = cel / 5;
21:         // Add 32 to the answer
22:         cel = cel + 32;
23:         System.out.println(cel + " degrees Fahrenheit");
24:     }
25: }

When you compile and run this Java application, it produces the following output:

86.0 degrees Fahrenheit is ...
30.0 degrees Celsius

33.0 degrees Celsius is ...
91.4 degrees Fahrenheit

In lines 3–12 of this Java application, a temperature in Fahrenheit is converted to Celsius using the arithmetic operators:

A similar thing happens in lines 14–23 but in the reverse direction—a temperature in Celsius is converted to Fahrenheit.

Assigning a value to a variable is an expression because it produces a value. Because of this feature, you can string assignment statements together the following way:

x = y = z = 7;

In this statement, all three variables end up with the value of 7.

The right side of an assignment expression always is calculated before the assignment takes place. This makes it possible to use an expression statement as in the following code:

int x = 5;
x = x + 2;

In the expression x = x + 2, the first thing that happens is that x + 2 is calculated. The result of this calculation, 7, is then assigned to x.

Using an expression to change a variable’s value is a common task in programming. Several operators are used strictly in these cases.

Table 2.4 shows these assignment operators and the expressions they are functionally equivalent to.

x = x / y + 5;

x /= y + 5;

Another common task required in programming is to add or subtract 1 from an integer variable. There are special operators for these expressions, which are called increment and decrement operations. Incrementing a variable means to add 1 to its value, and decrementing a variable means to subtract 1 from its value.

The increment operator is ++, and the decrement operator is --. These operators are placed immediately after or immediately before a variable name, as in the following code example:

int x = 7;
x = x++;

In this example, the statement x = x++ increments the x variable from 7 to 8.

These increment and decrement operators can be placed before or after a variable name, and this affects the value of expressions that involve these operators.

Increment and decrement operators are called prefix operators if listed before a variable name and postfix operators if listed after a name.

In a simple expression such as standards--;, using a prefix or postfix operator produces the same result, making the operators interchangeable. When increment and decrement operations are part of a larger expression, however, the choice between prefix and postfix operators is important.

Consider the following code:

int x, y, z;
x = 42;
y = x++;
z = ++x;

The three expressions in this code yield very different results because of the difference between prefix and postfix operations.

When you use postfix operators, as in y = x++, y receives the value of x before it is incremented by one. When using prefix operators, as in z = ++x, x is incremented by one before the value is assigned to z. The end result of this example is that y equals 42, z equals 44, and x equals 44.

If you’re still having some trouble figuring this out, here’s the example again with comments describing each step:

int x, y, z; // x, y, and z are all declared
x = 42;      // x is given the value of 42
y = x++;     // y is given x's value (42) before it is incremented
             // and x is then incremented to 43
z = ++x;     // x is incremented to 44, and z is given x's value

Java has several operators for making comparisons among variables, variables and literals, or other types of information in a program.

These operators are used in expressions that return Boolean values of true or false, depending on whether the comparison being made is true or not. Table 2.5 shows the comparison operators.

The following example shows a comparison operator in use:

boolean hip;
int age = 36;
hip = age < 25;

The expression age < 25 produces a result of either true or false, depending on the value of the integer age. Because age is 36 in this example (which is not less than 25), hip is given the Boolean value false.

Expressions that result in Boolean values, such as comparison operations, can be combined to form more complex expressions. This is handled through logical operators, which are used for the logical combinations AND, OR, XOR, and logical NOT.

For AND combinations, the & or && logical operators are used. When two Boolean expressions are linked by the & or && operators, the combined expression returns a true value only if both Boolean expressions are true.

Consider this example:

boolean extraLife = (score > 75000) & (playerLives < 10);

This expression combines two comparison expressions: score > 75000 and playerLives < 10. If both of these expressions are true, the value true is assigned to the variable extraLife. In any other circumstance, the value false is assigned to the variable.

The difference between “&” and “&&” lies in how much work Java does on the combined expression. If “&” is used, the expressions on either side of the “&” are evaluated no matter what. If “&&” is used and the left side of the “&&” is false, the expression on the right side of the “&&” never is evaluated.

For OR combinations, the “|” or “||” logical operators are used. These combined expressions return a true value if either Boolean expression is true.

Consider this example:

boolean extralife = (score > 75000) || (playerLevel == 0);

This expression combines two comparison expressions: score > 75000 and playerLevel = 0. If either of these expressions is true, the value true is assigned to the variable extraLife. Only if both of these expressions are false will the value false be assigned to extraLife.

Note the use of “||” instead of “|”. Because of this usage, if score > 75000 is true, extraLife is set to true, and the second expression is never evaluated.

The XOR combination has one logical operator, “^”. This results in a true value only if both Boolean expressions it combines have opposite values. If both are true or both are false, the “^” operator produces a false value.

The NOT combination uses the “!” logical operator followed by a single expression. It reverses the value of a Boolean expression the same way that a minus symbol reverses the positive or negative sign on a number.

For example, if age < 30 returns a true value, !(age < 30) returns a false value.

The logical operators can seem completely illogical when encountered for the first time. You get plenty of chances to work with them for the rest of this week, especially on Day 5, “Creating Classes and Methods.”

When more than one operator is used in an expression, Java has an established precedence hierarchy to determine the order in which operators are evaluated. In many cases, this precedence determines the overall value of the expression.

For example, consider the following expression:

y = 6 + 4 / 2;

The y variable receives the value 5 or the value 8, depending on which arithmetic operation is handled first. If the 6 + 4 expression comes first, y has the value of 5. Otherwise, y equals 8.

In general, the order of evaluation from first to last is the following:

If two operations have the same precedence, the one on the left in the actual expression is handled before the one on the right. Table 2.6 shows the specific precedence of the various operators in Java. Operators farther up the table are evaluated first.

Returning to the expression y = 6 + 4 / 2, Table 2.6 shows that division is evaluated before addition, so the value of y will be 8.

To change the order in which expressions are evaluated, place parentheses around the expressions that should be evaluated first. You can nest one set of parentheses inside another to make sure that expressions are evaluated in the desired order; the innermost parenthetic expression is evaluated first.

The following expression results in a value of 5:

y = (6 + 4) / 2

The value of 5 is the result because 6 + 4 is calculated first, and then the result, 10, is divided by 2.

Parentheses also can improve the readability of an expression. If the precedence of an expression isn’t immediately clear to you, adding parentheses to impose the desired precedence can make the statement easier to understand.

The following question is the kind of thing you could expect to be asked on a Java programming certification test. Answer it without looking at today’s material.

The answer is available on the book’s website at http://www.java21days.com. Visit the Day 2 page and click the Certification Practice link.