A. Introduction to Java Applications

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Fig. A.1 | Text-printing program.

The comment in line 1

begins with //, indicating that it is an end-of-line comment—it terminates at the end of the line on which the // appears. Line 2 is a comment that describes the purpose of the program.

Java also has traditional comments, which can be spread over several lines as in

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These begin and end with delimiters, /* and */. The compiler ignores all text between the delimiters.

In Appendices A–E, every class we define begins with the keyword public. For our application, the file name is Welcome1.java.

A left brace (as in line 5), {, begins the body of every class declaration. A corresponding right brace, }, must end each class declaration.

The left brace in line 8 begins the body of the method declaration. A corresponding right brace must end it (line 10).

The System.out object is known as the standard output object. It allows a Java applications to display information in the command window from which it executes. In recent versions of Microsoft Windows, the command window is the Command Prompt. In UNIX/Linux/Mac OS X, the command window is called a terminal window or a shell. Many programmers call it simply the command line.

Method System.out.println displays (or prints) a line of text in the command window. The string in the parentheses in line 9 is the argument to the method. When System.out.println completes its task, it positions the cursor (the location where the next character will be displayed) at the beginning of the next line in the command window.

The entire line 9, including System.out.println, the argument "Welcome to Java Programming!" in the parentheses and the semicolon (;), is called a statement. Most statements end with a semicolon. When the statement in line 9 executes, it displays Welcome to Java Programming! in the command window.

To prepare to compile the program, open a command window and change to the directory where the program is stored. Many operating systems use the command cd to change directories. On Windows, for example,

changes to the figA_01 directory. On UNIX/Linux/Max OS X, the command

changes to the figA_01 directory.

To compile the program, type

If the program contains no syntax errors, this command creates a new file called Welcome1.class (known as the class file for Welcome1) containing the platform-independent Java bytecodes that represent our application. When we use the java command to execute the application on a given platform, the JVM will translate these bytecodes into instructions that are understood by the underlying operating system and hardware.

Figure A.2 shows the program of Fig. A.1 executing in a Microsoft^® Windows^® 7 Command Prompt window. To execute the program, type java Welcome1. This command launches the JVM, which loads the .class file for class Welcome1. The command omits the .class file-name extension; otherwise, the JVM will not execute the program. The JVM calls method main. Next, the statement at line 9 of main displays "Welcome to Java Programming!"

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Fig. A.3 | Printing a line of text with multiple statements.

The program is similar to Fig. A.1, so we discuss only the changes here. Line 2 is a comment stating the purpose of the program. Line 4 begins the Welcome2 class declaration. Lines 9–10 of method main display one line of text. The first statement uses System.out’s method print to display a string. Each print or println statement resumes displaying characters from where the last print or println statement stopped displaying characters. Unlike println, after displaying its argument, print does not position the output cursor at the beginning of the next line in the command window—the next character the program displays will appear immediately after the last character that print displays. Thus, line 10 positions the first character in its argument (the letter “J”) immediately after the last character that line 9 displays (the space character before the string’s closing double-quote character).

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Fig. A.4 | Printing multiple lines of text with a single statement.

Line 2 is a comment stating the program’s purpose. Line 4 begins the Welcome3 class declaration. Line 9 displays four separate lines of text in the command window. Normally, the characters in a string are displayed exactly as they appear in the double quotes. Note, however, that the paired characters and n (repeated three times in the statement) do not appear on the screen. The backslash () is an escape character. which has special meaning to System.out’s print and println methods. When a backslash appears in a string, Java combines it with the next character to form an escape sequence. The escape sequence represents the newline character. When a newline character appears in a string being output with System.out, the newline character causes the screen’s output cursor to move to the beginning of the next line in the command window.

Figure A.5 lists several common escape sequences and describes how they affect the display of characters in the command window.

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Fig. A.6 | Displaying multiple lines with method System.out.printf.

Lines 9–10 represent only one statement. Java allows large statements to be split over many lines. We indent line 10 to indicate that it’s a continuation of line 9.

Method printf’s first argument is a format string that may consist of fixed text and format specifiers. Fixed text is output by printf just as it would be by print or println. Each format specifier is a placeholder for a value and specifies the type of data to output. Format specifiers also may include optional formatting information.

Format specifiers begin with a percent sign (%) followed by a character that represents the data type. For example, the format specifier %s is a placeholder for a string. The format string in line 9 specifies that printf should output two strings, each followed by a newline character. At the first format specifier’s position, printf substitutes the value of the first argument after the format string. At each subsequent format specifier’s position, printf substitutes the value of the next argument. So this example substitutes "Welcome to" for the first %s and "Java Programming!" for the second %s. The output shows that two lines of text are displayed.

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Fig. A.7 | Addition program that displays the sum of two numbers.

The application begins execution with the main method (lines 8–26). The left brace (line 9) marks the beginning of method main’s body, and the corresponding right brace (line 26) marks its end. Method main is indented one level in the body of class Addition, and the code in the body of main is indented another level for readability.

Line 11 is a variable declaration statement that specifies the name (input) and type (Scanner) of a variable that’s used in this program. A Scanner enables a program to read data (e.g., numbers and strings) for use in a program. The data can come from many sources, such as the user at the keyboard or a file on disk. Before using a Scanner, you must create it and specify the source of the data.

The = in line 11 indicates that Scanner variable input should be initialized (i.e., prepared for use in the program) in its declaration with the result of the expression to the right of the equals sign—new Scanner(System.in). This expression uses the new keyword to create a Scanner object that reads characters typed by the user at the keyboard. The standard input object, System.in, enables applications to read bytes of information typed by the user. The Scanner translates these bytes into types (like ints) that can be used in a program.

Other data types include float and double, for holding real numbers (such as 3.4, 0.0 and —11.19), and char, for holding character data. Variables of type char represent individual characters, such as an uppercase letter (e.g., A), a digit (e.g., 7), a special character (e.g., * or %) or an escape sequence (e.g., the newline character, ). The types int, float, double and char are called primitive types. Primitive-type names are keywords and must appear in all lowercase letters. Appendix L summarizes the characteristics of the eight primitive types (boolean, byte, char, short, int, long, float and double).

Our program assumes that the user enters a valid integer value. If not, a runtime logic error will occur and the program will terminate. Appendix H discusses how to make your programs more robust by enabling them to handle such errors—this makes your program more fault tolerant.

In line 18, we place the result of the call to method nextInt (an int value) in variable number1 by using the assignment operator, =. The statement is read as “number1 gets the value of input.nextInt().” Operator = is called a binary operator, because it has two operands—number1 and the result of the method call input.nextInt(). This statement is called an assignment statement, because it assigns a value to a variable. Everything to the right of the assignment operator, =, is always evaluated before the assignment is performed.

Calculations can also be performed inside printf statements. We could have combined the statements at lines 23 and 25 into the statement

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The parentheses around the expression number1 + number2 are not required—they’re included to emphasize that the value of the entire expression is output in the position of the %d format specifier.

In the addition program of Fig. A.7, when the following statement (line 18) executes:

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the number typed by the user is placed into a memory location corresponding to the name number1. Suppose that the user enters 45. The computer places that integer value into number1 (Fig. A.8), replacing the previous value (if any) in that location. The previous value is lost.

When the statement (line 21)

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executes, suppose that the user enters 72. The computer places that integer value into location number2. The memory now appears as shown in Fig. A.9.

After the program of Fig. A.7 obtains values for number1 and number2, it adds the values and places the total into variable sum. The statement (line 23)

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performs the addition, then replaces any previous value in sum. After sum has been calculated, memory appears as in Fig. A.10. number1 and number2 contain the values that were used in the calculation of sum. These values were used, but not destroyed, as the calculation was performed. When a value is read from a memory location, the process is nondestructive.

Integer division yields an integer quotient. For example, the expression 7 / 4 evaluates to 1, and the expression 17 / 5 evaluates to 3. Any fractional part in integer division is simply discarded (i.e., truncated)—no rounding occurs. Java provides the remainder operator, %, which yields the remainder after division. The expression x % y yields the remainder after x is divided by y. Thus, 7 % 4 yields 3, and 17 % 5 yields 2. This operator is most commonly used with integer operands but can also be used with other arithmetic types.

If an expression contains nested parentheses, such as

the expression in the innermost set of parentheses (a + b in this case) is evaluated first.

1. Multiplication, division and remainder operations are applied first. If an expression contains several such operations, they’re applied from left to right. Multiplication, division and remainder operators have the same level of precedence.

2. Addition and subtraction operations are applied next. If an expression contains several such operations, the operators are applied from left to right. Addition and subtraction operators have the same level of precedence.

These rules enable Java to apply operators in the correct order.¹ When we say that operators are applied from left to right, we’re referring to their associativity. Some operators associate from right to left. Figure A.12 summarizes these rules of operator precedence. A complete precedence chart is included in Appendix K.

1. We use simple examples to explain the order of evaluation of expressions. Subtle issues occur in the more complex expressions you’ll encounter. For more information on order of evaluation, see Chapter 15 of The Java^™ Language Specification (https://docs.oracle.com/javase/specs/jls/se7/jls7.pdf).

The parentheses are required because division has higher precedence than addition. The entire quantity (a + b + c + d + e) is to be divided by 5. If the parentheses are erroneously omitted, we obtain a + b + c + d + e / 5, which evaluates as

Here’s an example of the equation of a straight line:

No parentheses are required. The multiplication operator is applied first because multiplication has a higher precedence than addition. The assignment occurs last because it has a lower precedence than multiplication or addition.

The following example contains remainder (%), multiplication, division, addition and subtraction operations:

The circled numbers under the statement indicate the order in which Java applies the operators. The *, % and / operations are evaluated first in left-to-right order (i.e., they associate from left to right), because they have higher precedence than + and -. The + and - operations are evaluated next. These operations are also applied from left to right. The assignment (=) operation is evaluated last.

The multiplication operations are evaluated first in left-to-right order (i.e., they associate from left to right), because they have higher precedence than addition. (Java has no arithmetic operator for exponentiation in Java, so x² is represented as x * x. Section C.16 shows an alternative for performing exponentiation.) The addition operations are evaluated next from left to right. Suppose that a, b, c and x are initialized (given values) as follows: a = 2, b = 3, c = 7 and x = 5. Figure A.13 illustrates the order in which the operators are applied.

Conditions in if statements can be formed by using the equality operators (== and !=) and relational operators (>, <, >= and <=) summarized in Fig. A.14. Both equality operators have the same level of precedence, which is lower than that of the relational operators. The equality operators associate from left to right. The relational operators all have the same level of precedence and also associate from left to right.

Figure A.15 uses six if statements to compare two integers input by the user. If the condition in any of these if statements is true, the statement associated with that if statement executes; otherwise, the statement is skipped. We use a Scanner to input the integers from the user and store them in variables number1 and number2. The program compares the numbers and displays the results of the comparisons that are true.

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Fig. A.15 | Compare integers using if statements, relational operators and equality operators.

The declaration of class Comparison begins at line 6. The class’s main method (lines 9–40) begins the execution of the program. Line 12 declares Scanner variable input and assigns it a Scanner that inputs data from the standard input (i.e., the keyboard).

Lines 14–15 declare the int variables used to store the values input from the user.

Lines 17–18 prompt the user to enter the first integer and input the value, respectively. The input value is stored in variable number1.

Lines 20–21 prompt the user to enter the second integer and input the value, respectively. The input value is stored in variable number2.

Lines 23–24 compare the values of number1 and number2 to determine whether they’re equal. An if statement always begins with keyword if, followed by a condition in parentheses. An if statement expects one statement in its body, but may contain multiple statements if they’re enclosed in a set of braces ({}). The indentation of the body statement shown here is not required, but it improves the program’s readability by emphasizing that the statement in line 24 is part of the if statement that begins at line 23. Line 24 executes only if the numbers stored in variables number1 and number2 are equal (i.e., the condition is true). The if statements in lines 26–27, 29–30, 32–33, 35–36 and 38–39 compare number1 and number2 using the operators !=, <, >, <= and >=, respectively. If the condition in one or more of the if statements is true, the corresponding body statement executes.

There’s no semicolon (;) at the end of the first line of each if statement. Such a semicolon would result in a logic error at execution time. For example,

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would actually be interpreted by Java as

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where the semicolon on the line by itself—called the empty statement—is the statement to execute if the condition in the if statement is true. When the empty statement executes, no task is performed. The program then continues with the output statement, which always executes, regardless of whether the condition is true or false, because the output statement is not part of the if statement.

Note the use of white space in Fig. A.15. Recall that the compiler normally ignores white space. So, statements may be split over several lines and may be spaced according to your preferences without affecting a program’s meaning. It’s incorrect to split identifiers and strings. Ideally, statements should be kept small, but this is not always possible.

Figure A.16 shows the operators discussed so far in decreasing order of precedence. All but the assignment operator, =, associate from left to right. The assignment operator, =, associates from right to left, so an expression like x = y = 0 is evaluated as if it had been written as x = (y = 0), which first assigns the value 0 to variable y, then assigns the result of that assignment, 0, to x.

a) A(n) __________ begins the body of every method, and a(n) __________ ends the body of every method.

b) The __________ statement is used to make decisions.

c) __________ begins an end-of-line comment.

d) __________, __________ and __________ are called white space.

e) __________ are reserved for use by Java.

f) Java applications begin execution at method __________.

g) Methods __________, __________ and __________ display information in a command window.

A.2 State whether each of the following is true or false. If false, explain why.

a) Comments cause the computer to print the text after the // on the screen when the program executes.

b) All variables must be given a type when they’re declared.

c) Java considers the variables number and NuMbEr to be identical.

d) The remainder operator (%) can be used only with integer operands.

e) The arithmetic operators *, /, %, + and - all have the same level of precedence.

A.3 Write statements to accomplish each of the following tasks:

a) Declare variables c, thisIsAVariable, q76354 and number to be of type int.

b) Prompt the user to enter an integer.

c) Input an integer and assign the result to int variable value. Assume Scanner variable input can be used to read a value from the keyboard.

d) Print "This is a Java program" on one line in the command window. Use method System.out.println.

e) Print "This is a Java program" on two lines in the command window. The first line should end with Java. Use method System.out.println.

f) Print "This is a Java program" on two lines in the command window. The first line should end with Java. Use method System.out.printf and two %s format specifiers.

g) If the variable number is not equal to 7, display "The variable number is not equal to 7".

A.4 Identify and correct the errors in each of the following statements:

a)

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if ( c < 7 );
    System.out.println( "c is less than 7" );

b)

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if ( c => 7 )
    System.out.println( "c is equal to or greater than 7" );

A.5 Write declarations, statements or comments that accomplish each of the following tasks:

a) State that a program will calculate the product of three integers.

b) Create a Scanner called input that reads values from the standard input.

c) Declare the variables x, y, z and result to be of type int.

d) Prompt the user to enter the first integer.

e) Read the first integer from the user and store it in the variable x.

f) Prompt the user to enter the second integer.

g) Read the second integer from the user and store it in the variable y.

h) Prompt the user to enter the third integer.

i) Read the third integer from the user and store it in the variable z.

j) Compute the product of the three integers contained in variables x, y and z, and assign the result to the variable result.

k) Display the message "Product is" followed by the value of the variable result.

A.6 Using the statements you wrote in Exercise A.5, write a complete program that calculates and prints the product of three integers.

a) left brace ({), right brace (}).

b) if.

c) //.

d) Space characters, newlines and tabs.

e) Keywords.

f) main.

g) System.out.print, System.out.println and System.out.printf.

A.2

a) False. Comments do not cause any action to be performed when the program executes. They’re used to document programs and improve their readability.

b) True.

c) False. Java is case sensitive, so these variables are distinct.

d) False. The remainder operator can also be used with noninteger operands in Java.

e) False. The operators *, / and % are higher precedence than operators + and -.

A.3

a)

int c, thisIsAVariable, q76354, number;

or

int c;
int thisIsAVariable;
int q76354;
int number;

b) System.out.print( "Enter an integer: " );

c) value = input.nextInt();

d) System.out.println( "This is a Java program" );

e) System.out.println( "This is a Java program" );

f) System.out.printf( "%s %s ", "This is a Java", "program" );

g)

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if ( number != 7 )
   System.out.println( "The variable number is not equal to 7" );

A.4

a) Error: Semicolon after the right parenthesis of the condition ( c < 7 ) in the if. Correction: Remove the semicolon after the right parenthesis. [Note: As a result, the output statement will execute regardless of whether the condition in the if is true.]

b) Error: The relational operator => is incorrect. Correction: Change => to >=.

A.5

a) // Calculate the product of three integers

b) Scanner input = new Scanner( System.in );

c)

int x, y, z, result;

or

int x;
int y;
int z;
int result;

d) System.out.print( "Enter first integer: " );

e) x = input.nextInt();

f) System.out.print( "Enter second integer: " );

g) y = input.nextInt();

h) System.out.print( "Enter third integer: " );

i) z = input.nextInt();

j) result = x * y * z;

k) System.out.printf( "Product is %d ", result );

A.6 The solution to Self-Review Exercise A.6 is as follows:

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a) __________ are used to document a program and improve its readability.

b) A decision can be made in a Java program with a(n) __________.

c) Calculations are normally performed by __________ statements.

d) The arithmetic operators with the same precedence as multiplication are and __________.

e) When parentheses in an arithmetic expression are nested, the __________ set of parentheses is evaluated first.

f) A location in the computer’s memory that may contain different values at various times throughout the execution of a program is called a(n) __________.

A.8 Write Java statements that accomplish each of the following tasks:

a) Display the message "Enter an integer: ", leaving the cursor on the same line.

b) Assign the product of variables b and c to variable a.

c) Use a comment to state that a program performs a sample payroll calculation.

A.9 State whether each of the following is true or false. If false, explain why.

a) Java operators are evaluated from left to right.

b) The following are all valid variable names: _under_bar_, m928134, t5, j7, her_sales$, his_$account_total, a, b$, c, z and z2.

c) A valid Java arithmetic expression with no parentheses is evaluated from left to right.

d) The following are all invalid variable names: 3g, 87, 67h2, h22 and 2h.

A.10 Assuming that x = 2 and y = 3, what does each of the following statements display?

a) System.out.printf( "x = %d ", x );

b) System.out.printf( "Value of %d + %d is %d ", x, x, ( x + x ) );

c) System.out.printf( "x =" );

d) System.out.printf( "%d = %d ", ( x + y ), ( y + x ) );

A.11 (Arithmetic, Smallest and Largest) Write an application that inputs three integers from the user and displays the sum, average, product, smallest and largest of the numbers. Use the techniques shown in Fig. A.15. [Note: The calculation of the average in this exercise should result in an integer representation of the average. So, if the sum of the values is 7, the average should be 2, not 2.3333....]

A.12 What does the following code print?

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System.out.printf( "%s %s %s ", "*", "***", "*****" );

A.13 (Largest and Smallest Integers) Write an application that reads five integers and determines and prints the largest and smallest integers in the group. Use only the programming techniques you learned in this appendix.

A.14 (Odd or Even) Write an application that reads an integer and determines and prints whether it’s odd or even. [Hint: Use the remainder operator. An even number is a multiple of 2. Any multiple of 2 leaves a remainder of 0 when divided by 2.]

A.15 (Multiples) Write an application that reads two integers, determines whether the first is a multiple of the second and prints the result. [Hint: Use the remainder operator.]

A.16 (Diameter, Circumference and Area of a Circle) Here’s a peek ahead. In this appendix, you learned about integers and the type int. Java can also represent floating-point numbers that contain decimal points, such as 3.14159. Write an application that inputs from the user the radius of a circle as an integer and prints the circle’s diameter, circumference and area using the floating-point value 3.14159 for π. Use the techniques shown in Fig. A.7. [Note: You may also use the predefined constant Math.PI for the value of π. This constant is more precise than the value 3.14159. Class Math is defined in package java.lang. Classes in that package are imported automatically, so you do not need to import class Math to use it.] Use the following formulas (r is the radius):

diameter = 2r

circumference = 2πr

area = πr²

Do not store the results of each calculation in a variable. Rather, specify each calculation as the value that will be output in a System.out.printf statement. The values produced by the circumference and area calculations are floating-point numbers. Such values can be output with the format specifier %f in a System.out.printf statement. You’ll learn more about floating-point numbers in Appendix B.

A.17 (Separating the Digits in an Integer) Write an application that inputs one number consisting of five digits from the user, separates the number into its individual digits and prints the digits separated from one another by three spaces each. For example, if the user types in the number 42339, the program should print

Assume that the user enters the correct number of digits. What happens when you execute the program and type a number with more than five digits? What happens when you execute the program and type a number with fewer than five digits? [Hint: It’s possible to do this exercise with the techniques you learned in this appendix. You’ll need to use both division and remainder operations to “pick off” each digit.]

A.18 (Table of Squares and Cubes) Using only the programming techniques you learned in this appendix, write an application that calculates the squares and cubes of the numbers from 0 to 10 and prints the resulting values in table format, as shown below. [Note: This program does not require any input from the user.]