2: Chapter Writing Java Programs
Chapter 2
Writing Java Programs
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2.1 A Simple Java Program
The following program displays the message Java rules! on the screen. JavaRules.java
// Displays the message "Java rules!" public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); } }
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Java Programs in General
Building blocks of a Java program:
Classes. A class is a collection of related variables and/ or methods (usually both). A Java program consists of one or more classes. Methods. A method is a series of statements. Each class may contain any number of methods. Statements. A statement is a single command. Each method may contain any number of statements.
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2.2 Executing a Java Program
Steps involved in executing a Java program:
Enter the program Compile the program Run the program
With an integrated development environment, all three steps can be performed within the environment itself.
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Integrated Development Environments
An integrated development environment (IDE) is an integrated collection of software tools for developing and testing programs. A typical IDE includes at least an editor, a compiler, and a debugger. A programmer can write a program, compile it, and execute it, all without leaving the IDE.
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Entering a Java Program
Any editor or word processor can be used to enter a program, including Notepad or WordPad. If you use a word processor, be careful to save the file as a Text Document or as Text Only. You can also enter the program from a DOS window using the DOS edit program.
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Compiling a Java Program
Before compiling a program under Windows, open a DOS window and use the cd (change directory) command to select the directory that contains the program:
cd c:\myfiles\java\programs
To compile the program, use the javac command:
javac JavaRules.java
The file name must match the program name, and the .java extension must be included.
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Running a Java Program
If the compiler doesnt find any errors in the program, it creates a class file containing bytecode instructions. This file will have the same name as the program but with .class as the extension. To run a program, use the java command, which executes the Java interpreter. java requires that the name of a class file be specified (without the .class extension):
java JavaRules
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Using Multiple Windows
If you use edit to create and modify programs, it's a good idea to have two DOS windows open simultaneously. You can run edit in one window and use the other window to compile and test the program. Its also a good idea to enable the doskey utility when you go into a DOS window. doskey remembers previous commands that have been entered in that window.
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2.3 Program Layout
The JavaRules program raises a couple of issues:
Why do we put comments into programs, and what are the rules for doing so? How should we lay out a program? Does it matter where we put spaces and blank lines? Where should the curly braces go?
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Comments
Comments are an important part of every program. They provide information thats useful for anyone who will need to read the program in the future. Typical uses of comments:
To document who wrote the program, when it was written, what changes have been made to it, and so on. To describe the behavior or purpose of a particular part of the program, such as a variable or method. To describe how a particular task was accomplished, which algorithms were used, or what tricks were employed to get the program to work.
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Types of Comments
Single-line comments:
// Comment style 1
Multiline comments:
/* Comment style 2 */
Doc comments:
/** Comment style 3 */
Doc comments are designed to be extracted by a special program, javadoc.
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Problems with Multiline Comments
Forgetting to terminate a multiline comment may cause the compiler to ignore part of a program:
System.out.print("My "); /* forgot to close this comment... System.out.print("cat "); System.out.print("has "); /* so it ends here */ System.out.println("fleas");
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Single-line Comments
Many programmers prefer // comments to /* */ comments, for several reasons:
Ease of use Safety Program readability Ability to comment out portions of a program
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Tokens
A Java compiler groups the characters in a program into tokens. The compiler then puts the tokens into larger groups (such as statements, methods, and classes). Tokens in the JavaRules program:
public class JavaRules { public String [ ] args ) { System . "Java rules!" ) ; } } static out . void main println ( (
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Avoiding Problems with Tokens
Always leave at least one space between tokens that would otherwise merge together:
publicclassJavaRules {
Dont put part of a token on one line and the other part on the next line:
pub lic class JavaRules {
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Indentation
Programmers use indentation to indicate nesting. An increase in the amount of indentation indicates an additional level of nesting. The JavaRules program consists of a statement nested inside a method nested inside a class:
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How Much Indentation?
Common amounts of indentation:
2 spaces: the bare minimum 3 spaces: the optimal amount 4 spaces: what many programmers use 8 spaces: probably too much
The JavaRules program with an indentation of four spaces:
public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); } } Java Programming
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Brace Placement
Brace placement is another important issue. One technique is to put each left curly brace at the end of a line. The matching right curly brace is lined up with the first character on that line:
public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); } }
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Brace Placement
Some programmers prefer to put left curly braces on separate lines:
public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); } }
This makes it easier to verify that left and right braces match up properly. However, program files become longer because of the additional lines.
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Brace Placement
To avoid extra lines, the line containing the left curly brace can be combined with the following line:
public class JavaRules { public static void main(String[] args) { System.out.println("Java rules!"); } }
In a commercial environment, issues such as indentation and brace placement are often resolved by a coding standard, which provides a uniform set of style rules for all programmers to follow.
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2.4 Using Variables
In Java, every variable must be declared before it can be used. Declaring a variable means informing the compiler of the variables name and its properties, including its type. int is one of Javas types. Variables of type int can store integers (whole numbers).
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Declaring Variables
Form of a variable declaration:
The type of the variable The name of the variable A semicolon
Example:
int i; // Declares i to be an int variable
Several variables can be declared at a time:
int i, j, k;
Its often better to declare variables individually.
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Initializing Variables
A variable is given a value by using =, the assignment operator:
i = 0;
Initializing a variable means to assign a value to the variable for the first time. Variables always need to be initialized before the first time their value is used. The Java compiler checks that variables declared in methods are initialized prior to their first use.
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Initializers
Variables can be initialized at the time theyre declared:
int i = 0;
0 is said to be the initializer for i. If several variables are declared at the same time, each variable can have its own initializer:
int i = 0, j, k = 1;
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Changing the Value of a Variable
The assignment operator can be used both to initialize a variable and to change the value of the variable later in the program:
i = 1; i = 2; // Value of i is now 1 // Value of i is now 2
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Program: Printing a Lottery Number
Lottery.java
// Displays the winning lottery number public class Lottery { public static void main(String[] args) { int winningNumber = 973; System.out.print("The winning number "); System.out.print("in today's lottery is "); System.out.println(winningNumber); } } Java Programming
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2.5 Types
A partial list of Java types:
int An integer double A floating-point number boolean Either true or false char A character
Declarations of double, boolean, and char variables:
double x, y; boolean b; char ch;
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Literals
A literal is a token that represents a particular number or other value. Examples of int literals:
0 297 30303 48. 4.8e1 4.8e+1 .48e2 480e-1
Examples of double literals:
48.0
The only boolean literals are true and false. char literals are enclosed within single quotes:
'a' 'z' 'A' 'Z' '0'
31
'9'
'%'
'.'
' '
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Using Literals as Initializers
Literals are often used as initializers:
double x = 0.0, y = 1.0; boolean b = true; char ch = 'f';
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2.6 Identifiers
Identifiers (names chosen by the programmer) are subject to the following rules:
Identifiers may contain letters (both uppercase and lowercase), digits, and underscores (_). Identifiers begin with a letter or underscore. Theres no limit on the length of an identifier. Lowercase letters are not equivalent to uppercase letters. (A language in which the case of letters matters is said to be case-sensitive.)
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Multiword Identifiers
When an identifier consists of multiple words, its important to mark the boundaries between words. One way to break up long identifiers is to use underscores between words:
last_index_of
Another technique is to capitalize the first letter of each word after the first:
lastIndexOf
This technique is the one commonly used in Java.
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Conventions
A rule that we agree to follow, even though its not required by the language, is said to be a convention. A common Java convention is beginning a class name with an uppercase letter:
Color FontMetrics String
Names of variables and methods, by convention, never start with an uppercase letter.
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Keywords
The following keywords cant be used as identifiers because Java has already given them a meaning:
abstract boolean break byte case catch char class const continue default do double else extends final finally float for goto if implements import instanceof int interface long native new package private protected public return short static super switch synchronized this throw throws transient try void volatile while
null, true, and false are also reserved.
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2.7 Performing Calculations
In general, the right side of an assignment can be an expression. A literal is an expression, and so is a variable. More complicated expressions are built out of operators and operands. In the expression 5 / 9, the operands are 5 and 9, and the operator is /. The operands in an expression can be variables, literals, or other expressions.
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Operators
Javas arithmetic operators:
+ * / % Addition Subtraction Multiplication Division Remainder 8 4 12 3
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Examples:
6+2 6-2 6*2 6/2
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Integer Division
If the result of dividing two integers has a fractional part, Java throws it away (we say that it truncates the result). Examples:
1/2 0 5/3 1
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double Operands
+, -, *, and / accept double operands:
6.1 6.1 6.1 6.1 6.1 6.1 6.1 6.1 + * / + * / 2.5 2.5 2.5 2.5 2 2 2 2 8.6 3.6 15.25 2.44
int and double operands can be mixed:
8.1 4.1 12.2 3.05
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Binary Operators
The +, -, *, and / operators are said to be binary operators, because they require two operands. Theres one other binary arithmetic operator: % (remainder). The % operator produces the remainder when the left operand is divided by the right operand:
13 % 3 1
% is normally used with integer operands. Its a good idea to put a space before and after each binary operator.
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Unary Operators
Java also has two unary arithmetic operators:
+ Plus Minus
Unary operators require just one operand. The unary + and - operators are often used in conjunction with literals (-3, for example).
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Round-Off Errors
Calculations involving floating-point numbers can sometimes produce surprising results. If d is declared as follows, its value will be 0.09999999999999987 rather than 0.1:
double d = 1.2 - 1.1;
Round-off errors such as this occur because some numbers (1.2 and 1.1, for example) cant be stored in double form with complete accuracy.
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Operator Precedence
Whats the value of 6 + 2 * 3?
(6 + 2) * 3, which yields 24? 6 + (2 * 3), which yields 12?
Operator precedence resolves issues such as this. *, /, and % take precedence over + and -. Examples:
5+2/2 8*3-5 6-1*7 9/4+6 6+2%3 5 + (2 / 2) (8 * 3) - 5 6 - (1 * 7) (9 / 4) + 6 6 + (2 % 3)
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6 19 1 8 8
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Associativity
Precedence rules are of no help when it comes to determining the value of 1 - 2 - 3. Associativity rules come into play when precedence rules alone arent enough. The binary +, -, *, /, and % operators are all left associative:
2 + 3 - 4 (2 + 3) - 4 1 2 * 3 / 4 (2 * 3) / 4 1
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Parentheses in Expressions
Parentheses can be used to override normal precedence and associativity rules. Parentheses in the expression (6 + 2) * 3 force the addition to occur before the multiplication. Its often a good idea to use parentheses even when theyre not strictly necessary:
(x * x) + (2 * x) - 1
However, dont use too many parentheses:
((x) * (x)) + ((2) * (x)) - (1)
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Assignment Operators
The assignment operator (=) is used to save the result of a calculation in a variable:
area = height * width;
The type of the expression on the right side of an assignment must be appropriate for the type of the variable on the left side of the assignment. Assigning a double value to an int variable is not legal. Assigning an int value to a double variable is OK, however.
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Using Assignment to Modify a Variable
Assignments often use the old value of a variable as part of the expression that computes the new value. The following statement adds 1 to the variable i:
i = i + 1;
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Compound Assignment Operators
The compound assignment operators make it easier to modify the value of a variable. A partial list of compound assignment operators:
+= -= *= /= %= Combines addition and assignment Combines subtraction and assignment Combines multiplication and assignment Combines division and assignment Combines remainder and assignment
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Compound Assignment Operators
Examples:
i i i i i += -= *= /= %= 2; 2; 2; 2; 2; // // // // // Same Same Same Same Same as as as as as i i i i i = = = = = i i i i i + * / % 2; 2; 2; 2; 2;
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Program: Converting from Fahrenheit to Celsius
FtoC.java
// Converts a Fahrenheit temperature to Celsius public class FtoC { public static void main(String[] args) { double fahrenheit = 98.6; double celsius = (fahrenheit - 32.0) * (5.0 / 9.0); System.out.print("Celsius equivalent: "); System.out.println(celsius); } } Java Programming
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2.8 Constants
A constant is a value that doesnt change during the execution of a program. Constants can be named by assigning them to variables:
double freezingPoint = 32.0; double degreeRatio = 5.0 / 9.0;
To prevent a constant from being changed, the word final can be added to its declaration:
final double freezingPoint = 32.0; final double degreeRatio = 5.0 / 9.0;
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Naming Constants
The names of constants are often written entirely in uppercase letters, with underscores used to indicate boundaries between words:
final double FREEZING_POINT = 32.0; final double DEGREE_RATIO = 5.0 / 9.0;
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Adding Constants to the FtoC Program
FtoC2.java
// Converts a Fahrenheit temperature to Celsius public class FtoC2 { public static void main(String[] args) { final double FREEZING_POINT = 32.0; final double DEGREE_RATIO = 5.0 / 9.0; double fahrenheit = 98.6; double celsius = (fahrenheit - FREEZING_POINT) * DEGREE_RATIO; System.out.print("Celsius equivalent: "); System.out.println(celsius); } } Java Programming
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Advantages of Naming Constants
Advantages of naming constants:
Programs easier are to read. The alternative is a program full of magic numbers. Programs are easier to modify. Inconsistencies and typographical errors are less likely.
Always create meaningful names for constants. Theres no point in defining a constant whose name signifies its value:
final int TWELVE = 12;
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2.9 Methods
A method is a series of statements that can be executed as a unit. A method does nothing until it is activated, or called. To call a method, we write the name of the method, followed by a pair of parentheses. The methods arguments (if any) go inside the parentheses. A call of the println method:
System.out.println("Java rules!");
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Methods in the Math Class
The Math class contains a number of methods for performing mathematical calculations. These methods are called by writing Math.name, where name is the name of the method. The methods in the Math class return a value when they have completed execution.
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The pow and sqrt Methods
The pow method raises a number to a power:
Math.pow(2.0, 3.0) 8.0 Math.pow(-2.0, 3.0) 8.0 Math.pow(2.0, -1.0) 0.5
The sqrt method computes the square root of a number:
Math.sqrt(2.0) 1.4142135623730951 Math.sqrt(4.0) 2.0
Both pow and sqrt return values of type double.
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The abs and max Methods
The abs method computes the absolute value of a number:
Math.abs(2.0) 2.0 Math.abs(-2.0) 2.0 Math.abs(2) 2 Math.abs(-2) 2
The max method finds the larger of two numbers:
Math.max(3.0, 5.5) 5.5 Math.max(10.0, -2.0) 10.0 Math.max(12, -23) 12 Math.max(-5, -2) 2
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The min Method
The min method finds the smaller of two numbers:
Math.min(3.0, 5.5) 3.0 Math.min(10.0, -2.0) 2.0 Math.min(12, -23) 23 Math.min(-5, -2) 5
The value returned by abs, max, and min depends on the type of the argument:
If the argument is an int, the methods return an int. If the argument is a double, the methods return a double.
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The round Method
The round method rounds a double value to the nearest integer:
Math.round(4.1) 4 Math.round(4.5) 5 Math.round(4.9) 5 Math.round(5.5) 6 Math.round(-4.1) 4 Math.round(-4.5) 4 Math.round(-4.9) 5 Math.round(-5.5) 5
round returns a long value rather than an int value.
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Using the Result of a Method Call
The value returned by a method can be saved in a variable for later use:
double y = Math.abs(x);
Another option is to use the result returned by a method directly, without first saving it in a variable. For example, the statements
double y = Math.abs(x); double z = Math.sqrt(y);
can be combined into a single statement:
double z = Math.sqrt(Math.abs(x));
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Using the Result of a Method Call
Values returned by methods can also be used as operands in expressions. Example (finding the roots of a quadratic equation):
double (-b double (-b root1 = + Math.sqrt(b * b - 4 * a * c)) / (2 * a); root2 = - Math.sqrt(b * b - 4 * a * c)) / (2 * a);
Because the square root of b2 4ac is used twice, it would be more efficient to save it in a variable:
double discriminant = Math.sqrt(b * b - 4 * a * c); double root1 = (-b + discriminant) / (2 * a); double root2 = (-b - discriminant) / (2 * a);
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Using the Result of a Method Call
The value returned by a method can be printed without first being saved in a variable:
System.out.println(Math.sqrt(2.0));
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2.10 Input and Output
Most programs require both input and output. Input is any information fed into the program from an outside source. Output is any data produced by the program and made available outside the program.
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Displaying Output on the Screen
Properties of System.out.print and System.out.println:
Can display any single value, regardless of type. The argument can be any expression, including a variable, literal, or value returned by a method. println always advances to the next line after displaying its argument; print does not.
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Displaying a Blank Line
One way to display a blank line is to leave the parentheses empty when calling println:
System.out.println("Hey Joe"); System.out.println(); // Write a blank line
The other is to insert \n into a string thats being displayed by print or println:
System.out.println("A hop,\na skip,\n\nand a jump");
Each occurrence of \n causes the output to begin on a new line.
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Escape Sequences
The backslash character combines with the character after it to form an escape sequence: a combination of characters that represents a single character. The backslash character followed by n forms \n, the new-line character.
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Escape Sequences
Another common escape sequence is \", which represents " (double quote):
System.out.println("He yelled \"Stop!\" and we stopped.");
In order to print a backslash character as part of a string, the string will need to contain two backslash characters:
System.out.println("APL\\360");
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Printing Multiple Items
The + operator can be used to combine multiple items into a single string for printing purposes:
System.out.println("Celsius equivalent: " + celsius);
At least one of the two operands for the + operator must be a string.
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Obtaining Input from the User
The SimpleIO class (not part of standard Java) simplifies the task of obtaining input. The SimpleIO.prompt method is first used to prompt the user:
SimpleIO.prompt("Enter Fahrenheit temperature: ");
Next, the SimpleIO.readLine method is used to read the users input:
String userInput = SimpleIO.readLine();
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Obtaining Input from the User
If the users input represents a number, it will need to be converted to numeric form. The Convert class (not part of standard Java) provides a toDouble method that converts a string to a double value:
double fahrenheit = Convert.toDouble(userInput);
To convert a string to an integer, the Integer.parseInt method is used instead:
int n = Integer.parseInt(userInput);
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Packages
Java allows classes to be grouped into larger units known as packages. The package that contains SimpleIO and Convert is named jpb (Java Programming: From the Beginning). Java comes with a large number of standard packages.
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Import Declarations
Accessing the classes that belong to a package is done by using an import declaration:
import package-name . * ;
Import declarations go at the beginning of a program. A program that needs SimpleIO or Convert would begin with the line
import jpb.*;
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Application Programming Interfaces
The packages that come with Java belong to the Java Application Programming Interface (API). In general, an API consists of code that someone else has written but that we can use in our programs. Typically, an API allows an application programmer to access a lower level of software. In particular, an API often provides access to the capabilities of a particular operating system or windowing system.
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Program: Converting from Fahrenheit to Celsius (Revisited)
FtoC3.java
// Converts a Fahrenheit temperature entered by the user to // Celsius import jpb.*; public class FtoC3 { public static void main(String[] args) { final double FREEZING_POINT = 32.0; final double DEGREE_RATIO = 5.0 / 9.0; SimpleIO.prompt("Enter Fahrenheit temperature: "); String userInput = SimpleIO.readLine(); double fahrenheit = Convert.toDouble(userInput); double celsius = (fahrenheit - FREEZING_POINT) * DEGREE_RATIO; System.out.println("Celsius equivalent: " + celsius);
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}
}
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2.11 Case Study: Computing a Course Average
The CourseAverage program will calculate a class average, using the following percentages:
Programs Quizzes Test 1 Test 2 Final exam 30% 10% 15% 15% 30%
The user will enter the grade for each program (0 20), the score on each quiz (010), and the grades on the two tests and the final (0100). There will be eight programs and five quizzes.
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Output of the CourseAverage Program
Welcome to the CSc 2310 average calculation program. Enter Enter Enter Enter Enter Enter Enter Enter Enter Enter Enter Enter Enter Program Program Program Program Program Program Program Program Quiz Quiz Quiz Quiz Quiz 1 2 3 4 5 1 2 3 4 5 6 7 8 score: score: score: score: score: score: score: score: 20 19 15 18.5 20 20 18 20
score: score: score: score: score:
9 10 5.5 8 9.5
Enter Test 1 score: 78 Enter Test 2 score: 92 Enter Final Exam score: 85 Course average: 88
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Design of the CourseAverage Program
1. Print the introductory message ("Welcome to the CSc 2310 average calculation program"). 2. Prompt the user to enter eight program scores. 3. Compute the program average from the eight scores. 4. Prompt the user to enter five quiz scores. 5. Compute the quiz average from the five scores. 6. Prompt the user to enter scores on the tests and final exam. 7. Compute the course average from the program average, quiz average, test scores, and final exam score. 8. Round the course average to the nearest integer and display it.
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Design of the CourseAverage Program
double variables can be used to store scores and averages. Computing the course average involves scaling the program average and quiz average so that they lie between 0 and 100:
courseAverage = .30 programAverage 5 + .10 quizAverage 10 + .15 test1 + .15 test2 + .30 finalExam
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CourseAverage.java
// // // // // // // // // // // // // // // // // // Program name: CourseAverage Author: K. N. King Written: 1998-04-05 Modified: 1999-01-27 Prompts the user to enter eight program scores (0-20), five quiz scores (0-10), two test scores (0-100), and a final exam score (0-100). Scores may contain digits after the decimal point. Input is not checked for validity. Displays the course average, computed using the following formula: Programs Quizzes Test 1 Test 2 Final exam 30% 10% 15% 15% 30%
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The course average is rounded to the nearest integer.
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Chapter 2: Writing Java Programs import jpb.*; public class CourseAverage { public static void main(String[] args) { // Print the introductory message System.out.println("Welcome to the CSc 2310 average " + "calculation program.\n"); // Prompt the user to enter eight program scores SimpleIO.prompt("Enter Program 1 score: "); String userInput = SimpleIO.readLine(); double program1 = Convert.toDouble(userInput); ...