1. Chapter 2 - Introduction to Java Applications
Outline 2.1 Introduction 2.2 A Simple Program: Printing a Line of Text 2.3 Another Java Application: Adding Integers 2.4 Memory Concepts 2.5 Arithmetic 2.6 Decision Making: Equality and Relational Operators

2. 2.1 Introduction In this chapter
Introduce examples to illustrate features of Java
Two program styles - applications and applets

3. 2.2 A Simple Program: Printing a Line of Text
Application
Program that executes using the java interpreter
Sample program
We will show you an program then analyze each line in detail

4. Java program Program Output
1 // Fig. 2.1: Welcome1.java
Java program
Program Output
2 // A first program in Java 3
4 public class Welcome1 {
5 public static void main( String args[] )
6 {
System.out.println( "Welcome to Java Programming!" );
8 }
9 }
Welcome to Java Programming!

5. 2.2 A Simple Program: Printing a Line of Text
// indicates the remainder of the line is a comment
Comments are ignored by the compiler
Use comments to document and describe code
Can also use multiple line comments: /* ... */
/* This is a multiple
line comment. It can
be split over many lines */
Another line of comments that describes the program
Note: line numbers are not part of the program; they are added for our reference
1 // Fig. 2.1: Welcome1.java
2 // A first program in Java

6. 2.2 A Simple Program: Printing a Line of Text
A blank line
Blank lines and spaces make a program more readable
Blank lines, spaces, and tabs are known as whitespace characters, and are ignored by the compiler
Begins a class definition for class Welcome1
Every Java program has at least one user-defined class
class keyword immediately followed by class name
Keyword words reserved for use by Java
Naming classes: capitalize every word
SampleClassName 3
4 public class Welcome1 {

7. 2.2 A Simple Program: Printing a Line of Text
Name of class called identifier
Series of characters consisting of letters, digits, underscores ( _ ) and dollar signs ( $ )
Does not begin with a digit
Contains no spaces
Examples: Welcome1, $value, _value, button7
7button is invalid
Case sensitive (capitalization matters)
a1 and A1 are different
For chapters 2 to 7, use public keyword
Certain details are not important now, full discussions will come later
4 public class Welcome1 {

8. 2.2 A Simple Program: Printing a Line of Text
Saving files
File name is class name and .java extension
Welcome1.java
Left brace
Begins body of every class
Right brace ends definition (line 9)
Part of every Java application
Applications begin executing at main
Parenthesis indicate main is a method
Java applications contain one or more methods
4 public class Welcome1 {
5 public static void main( String args[] )

9. 2.2 A Simple Program: Printing a Line of Text
Exactly one method must be called main
Methods can perform tasks and return information
void means main returns no information
For now, mimic main's first line
Left brace begins body of method definition
Ended by right brace
5 public static void main( String args[] )
6 {

10. 2.2 A Simple Program: Printing a Line of Text
Instructs computer to perform an action
Prints string of characters between double quotes
String - series characters inside double quotes
White spaces in strings are not ignored by compiler
System.out - standard output object
Allows java to print to command window (i.e., MS-DOS prompt)
Method System.out.println displays a line of text
Argument inside parenthesis
Entire line known as a statement
All statements must end with a semicolon ;
System.out.println( "Welcome to Java Programming!" );

11. 2.2 A Simple Program: Printing a Line of Text
Ends method definition
Ends class definition
Some programmers add comments to keep track of ending braces
Lines 8 and 9 could be rewritten as:
Remember that the compiler ignores comments
8 }
9 }
8 } // end of method main()
9 } // end of class Welcome1

12. 2.2 A Simple Program: Printing a Line of Text
Compiling a program
Open a command window, go to directory where program is stored
Type javac Welcome1.java
If there are no errors, file Welcome1.class is created
Contains Java bytecodes that represent application
Bytecodes passed to Java interpreter
Executing a program
Type java Welcome1
Launches interpreter to load .class file for class Welcome1
.class extension omitted from command
Interpreter calls method main

13. Java program Program Output
1 // Fig. 2.1: Welcome1.java
Java program
Program Output
2 // A first program in Java 3
4 public class Welcome1 {
5 public static void main( String args[] )
6 {
System.out.println( "Welcome to Java Programming!" );
8 }
9 }

14. 2.2 A Simple Program: Printing a Line of Text
Other methods
System.out.println
Positions cursor on new line after displaying argument
System.out.print
Keeps cursor on same line after displaying argument

15. 7: Method System.out.print 8: Method System.out.println
1 // Fig. 2.3: Welcome2.java
2 // Printing a line with multiple statements 3
4 public class Welcome2 {
5 public static void main( String args[] )
6 {
System.out.print( "Welcome to " );
System.out.println( "Java Programming!" );
9 }
10 }
Line numbers
1-2: Comments
3: Blank
4: Begin class Welcome2
5: Method main
6: Begin main body
7: Method System.out.print
8: Method System.out.println
9: end main
10: end Welcome2
Program Output
System.out.print keeps the cursor on the same line, so System.out.println continues on the same line.
Welcome to Java Programming!

16. 2.2 A Simple Program: Printing a Line of Text
Escape characters
Backslash ( \ )
Indicates that special characters are to be output
Backslash combined with a character makes an escape sequence
\n - newline
\t - tab
Others in Fig. 2.5
Usage
Can use in System.out.println or System.out.print to create new lines
System.out.println( "Welcome\nto\nJava\nProgramming!" );

17. 2. System.out.println (uses \n for newline)
1 // Fig. 2.4: Welcome3.java
2 // Printing multiple lines with a single statement 3
4 public class Welcome3 {
5 public static void main( String args[] )
6 {
System.out.println( "Welcome\nto\nJava\nProgramming!" );
8 }
9 }
Class Welcome1
1. main
2. System.out.println (uses \n for newline)
Program Output
Notice how a new line is output for each \n escape sequence.
Welcome to
Java
Programming!

18. 2.2 A Simple Program: Printing a Line of Text
Display
Although our first programs executed in the command window, most Java applications use windows or a dialog box
Netscape Communicator and Microsoft Internet Explorer execute in their own windows
Java has class JOptionPane that allows us to use dialog boxes
Packages
Java has a set of predefined classes for us to use
Groups of related classes called packages
Group of all packages known as Java class library or Java applications programming interface (Java API)
JOptionPane is in the javax.swing package

19. 2.2 A Simple Program: Printing a Line of Text
Upcoming program
Application that uses dialog boxes
Explanation will come afterwards

20. Java program using dialog box Program Output
1 // Fig. 2.6: Welcome4.java
2 // Printing multiple lines in a dialog box
Java program using dialog box
Program Output
3 import javax.swing.JOptionPane; // import class JOptionPane 4
5 public class Welcome4 {
6 public static void main( String args[] )
7 {
JOptionPane.showMessageDialog(
null, "Welcome\nto\nJava\nProgramming!" ); 10
System.exit( 0 ); // terminate the program
12 }
13 }

21. 2.2 A Simple Program: Printing a Line of Text
Lines 1-2: comments as before
import statements - locate the classes we intend to use
Tells compiler to load class JOptionPane from javax.swing package
This package contains many Graphical User Interface components
Lines 4-7: Blank line, begin class Welcome4 and main
3 import javax.swing.JOptionPane; // import class JOptionPane 4
5 public class Welcome4 {
6 public static void main( String args[] )
7 {

22. 2.2 A Simple Program: Printing a Line of Text
Call method showMessageDialog of class JOptionPane
Requires two arguments
Multiple arguments separated by commas (,)
For now, first argument always null
Second argument is string to display
showMessageDialog is a static method of class JOptionPane
static methods called by using class name, dot (.) then method name
All statements end with ;
A single statement can therefore span multiple lines
JOptionPane.showMessageDialog(
null, "Welcome\nto\nJava\nProgramming!" );

23. 2.2 A Simple Program: Printing a Line of Text
Executing lines 8 and 9 displays the dialog box shown below
Automatically includes an OK button
Hides or dismisses dialog box
Title bar has string Message
JOptionPane.showMessageDialog(
null, "Welcome\nto\nJava\nProgramming!" );

24. 2.2 A Simple Program: Printing a Line of Text
Calls static method exit of class System
Terminates application
Use with any application displaying a GUI
Because method is static, just needs class name and dot (.)
Identifiers starting with capital letters are usually class names
Argument of 0 means application ended successfully
Non-zero usually means an error occurred
Class System part of package java.lang
No import statement needed
java.lang automatically imported in every Java program
Lines 12-13: End Welcome4 and main
System.exit( 0 ); // terminate the program

25. 1. import statement 2. Class Welcome4 2.1 main 2.2 showMessageDialog
1 // Fig. 2.6: Welcome4.java
2 // Printing multiple lines in a dialog box
1. import statement
2. Class Welcome4
2.1 main
2.2 showMessageDialog
2.3 System.exit
Program Output
3 import javax.swing.JOptionPane; // import class JOptionPane 4
5 public class Welcome4 {
6 public static void main( String args[] )
7 {
JOptionPane.showMessageDialog(
null, "Welcome\nto\nJava\nProgramming!" ); 10
System.exit( 0 ); // terminate the program
12 }
13 }

26. 2.3 Another Java Application: Adding Integers
Upcoming program
Use input dialogs to input two values from user
Use message dialog to display sum of the two values

27. Java Program using input dialogs
1 // Fig. 2.8: Addition.java
2 // An addition program 3
Java Program using input dialogs
4 import javax.swing.JOptionPane; // import class JOptionPane 5
6 public class Addition {
7 public static void main( String args[] )
8 {
String firstNumber, // first string entered by user
secondNumber; // second string entered by user
int number1, // first number to add
number2, // second number to add
sum; // sum of number1 and number2 14
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" ); 18
// read in second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer" ); 22
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber ); 26
// add the numbers
sum = number1 + number2; 29
// display the results

28. Program Output 31 JOptionPane.showMessageDialog(
null, "The sum is " + sum , "Results",
JOptionPane.PLAIN_MESSAGE );
Program Output 34
System.exit( 0 ); // terminate the program
36 }
37 }

29. 2.3 Another Java Application: Adding Integers
Lines 1-2: Comments
Specifies location of JOptionPane for use in the program
Begins public class Addition
Recall that file name must be Addition.java
Lines 7-8: main
Declaration
firstNumber and secondNumber are variables
4 import javax.swing.JOptionPane; // import class JOptionPane
6 public class Addition {
String firstNumber, // first string entered by user
secondNumber; // second string entered by user

30. 2.3 Another Java Application: Adding Integers
String firstNumber, // first string entered by user
Variables: location in memory that can store a value
Must be declared with a name and data type before use
firstNumber and secondNumber are of data type String (java.lang), so they will hold strings
Variable name: any valid identifier
Declarations end with semicolons ;
Can declare multiple variables of the same type at a time
Use a comma separated list
Programmers often add comments to describe purpose of variables
secondNumber; // second string entered by user

31. 2.3 Another Java Application: Adding Integers
int number1, // first number to add
number2, // second number to add
sum; // sum of number1 and number2
Declares variables number1, number2, and sum of type int
int can hold integer values (whole numbers): i.e., 0, -4, 97
Data types float and double can hold decimal numbers
Data type char can hold a single character
Known as primitive data types - more in Chapter 4

32. 2.3 Another Java Application: Adding Integers
Reads a String from the user, representing the first number to be added
Method JOptionPane.showInputDialog displays the following:
Message called a prompt - directs user to perform an action
Argument appears as prompt text
If wrong type of data entered (i.e. non-integer) then error occurs
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" );

33. 2.3 Another Java Application: Adding Integers
Result of call to showInputDialog (a String with the user input) given to firstNumber with the assignment operator =
Assignment statement
= binary operator - takes two operands
Expression on right evaluated and assigned to variable on left
Read as: firstNumber gets value of JOptionPane.showInputDialog( "Enter first integer" )
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" );

34. 2.3 Another Java Application: Adding Integers
Similar to previous statement
Assigns variable secondNumber to second integer input
Method Integer.parseInt
Converts its String argument into an integer (type int)
Class Integer in java.lang
Integer returned by Integer.parseInt is assigned to variable number1 (line 24)
Remember that number1 was declared as type int
Line 25 similar
// read in second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer" );
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );

35. 2.3 Another Java Application: Adding Integers
Assignment statement
First calculates sum of number1 and number2 (right hand side)
Next, uses assignment operator = to assign result to variable sum
Read as: sum gets the value of number1 + number2
// add the numbers
sum = number1 + number2;

36. 2.3 Another Java Application: Adding Integers
Uses showMessageDialog to display results
"The sum is " + sum
Uses the operator + to "add" the string literal "The sum is" and sum
Allows concatenation of a String and another data type
Results in a new string
If sum contains 117, then "The sum is " + sum results in the new string "The sum is 117"
Note the space in "The sum is "
More on strings in Chapter 10
JOptionPane.showMessageDialog(
null, "The sum is " + sum, "Results",
JOptionPane.PLAIN_MESSAGE );

37. 2.3 Another Java Application: Adding Integers
Different version of showMessageDialog
Requires four arguments (instead of two as before)
First argument: null for now
Second: message to display
Third: string to display in title bar
Fourth: value indicating type of message dialog
JOptionPane.PLAIN_MESSAGE indicates no icon
JOptionPane.ERROR_MESSAGE
JOptionPane.INFORMATION_MESSAGE
JOptionPane.WARNING_MESSAGE
JOptionPane.QUESTION_MESSAGE
JOptionPane.showMessageDialog(
null, "The sum is " + sum, "Results",
JOptionPane.PLAIN_MESSAGE );

38. 2.1 Declare variables (name and data type)
1 // Fig. 2.8: Addition.java
2 // An addition program 3
1. import
2. class Addition
2.1 Declare variables (name and data type)
3. showInputDialog
4. parseInt
5. Add numbers, put result in sum
4 import javax.swing.JOptionPane; // import class JOptionPane 5
6 public class Addition {
7 public static void main( String args[] )
8 {
String firstNumber, // first string entered by user
secondNumber; // second string entered by user
int number1, // first number to add
number2, // second number to add
sum; // sum of number1 and number2 14
// read in first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer" ); 18
// read in second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer" ); 22
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber ); 26
// add the numbers
sum = number1 + number2; 29
// display the results

39. 6. showMessageDialog 7. System.exit Program Output
JOptionPane.showMessageDialog(
null, "The sum is " + sum, "Results",
JOptionPane.PLAIN_MESSAGE );
6. showMessageDialog
7. System.exit
Program Output 34
System.exit( 0 ); // terminate the program
36 }
37 }

40. Visual representation
2.4 Memory Concepts
Variables
Variable names correspond to locations in the computer's memory
Every variable has a name, a type, a size and a value
Whenever a new value is placed into a variable it replaces (and destroys) previous value
Reading variables from memory does not change them
Visual representation
number1 45

41. Arithmetic calculations are used in most programs
Use * for multiplication and / for division, +, -
No operator for exponentiation (more in Chapter 5)
Integer division truncates remainder
7 / 5 evaluates to 1
Modulus operator % returns the remainder
7 % 5 evaluates to 2
Operator precedence
Some arithmetic operators act before others (i.e., multiplication before addition)
Use parenthesis when needed
Example: Find the average of three variables a, b and c
Do not use: a + b + c / 3
Use: (a + b + c ) / 3

42. 2.5 Arithmetic

43. 2.6 Decision Making: Equality and Relational Operators
if control structure
Simple version in this section, more detail later
If a condition is true, then the body of the if statement executed
0 interpreted as false, non-zero is true
Control always resumes after the if structure
Conditions for if structures can be formed using equality or relational operators (next slide)
if ( condition )
statement executed if condition true
No semicolon needed after condition

44. 2.6 Decision Making: Equality and Relational Operators
< _
> =
Upcoming program uses if structures
Discussion afterwards

45. 1 // Fig. 2.17: Comparison.java
2 // Using if statements, relational operators
3 // and equality operators 4
5 import javax.swing.JOptionPane; 6
7 public class Comparison {
8 public static void main( String args[] )
9 {
String firstNumber, // first string entered by user
secondNumber, // second string entered by user
result; // a string containing the output
int number1, // first number to compare
number2; // second number to compare 15
// read first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer:" ); 19
// read second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:" ); 23
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber ); 27
// initialize result to the empty string
result = ""; 30

46. if ( number1 == number2 )
result = result + number1 + " == " + number2; 33
if ( number1 != number2 )
result = result + number1 + " != " + number2; 36
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2; 39
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2; 42
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2; 45
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2; 48
// Display results
JOptionPane.showMessageDialog(
null, result, "Comparison Results",
JOptionPane.INFORMATION_MESSAGE ); 53
System.exit( 0 );
55 }
56 }

47. Program Output

48. 2.6 Decision Making: Equality and Relational Operators
Lines 1-9: Comments, import JOptionPane, begin class Comparison and main
Declare variables
Input data from user and assign to variables
String firstNumber, // first string entered by user
secondNumber, // second string entered by user
result; // a string containing the output
int number1, // first number to compare
number2; // second number to compare
firstNumber =
JOptionPane.showInputDialog( "Enter first integer:" );
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:" );

49. 2.6 Decision Making: Equality and Relational Operators
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber );
Convert Strings to ints and assign to variables
Initialize String result with the empty string
if structure to test for equality using (==)
If the variables are equal (condition true)
String result concatenated using + operator
result = result + other strings
Right side evaluated first: result and other strings are concatenated to form a new string
The new string is assigned to result
If variables are not equal, then statement skipped
result = "";
if ( number1 == number2 )
result = result + number1 + " == " + number2;

50. 2.6 Decision Making: Equality and Relational Operators
Other if structures
Lines 50-52: result displayed in a dialog box using showMessageDialog
if ( number1 != number2 )
result = result + number1 + " != " + number2; 36
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2; 39
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2; 42
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2; 45
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2; 48

51. 2.3 Input data (showInputDialog)
1 // Fig. 2.17: Comparison.java
2 // Using if statements, relational operators
3 // and equality operators
1. import
2. Class Comparison
2.1 main
2.2 Declarations
2.3 Input data (showInputDialog)
2.4 parseInt
2.5 Initialize result 4
5 import javax.swing.JOptionPane; 6
7 public class Comparison {
8 public static void main( String args[] )
9 {
String firstNumber, // first string entered by user
secondNumber, // second string entered by user
result; // a string containing the output
int number1, // first number to compare
number2; // second number to compare 15
// read first number from user as a string
firstNumber =
JOptionPane.showInputDialog( "Enter first integer:" ); 19
// read second number from user as a string
secondNumber =
JOptionPane.showInputDialog( "Enter second integer:" ); 23
// convert numbers from type String to type int
number1 = Integer.parseInt( firstNumber );
number2 = Integer.parseInt( secondNumber ); 27
// initialize result to the empty string
result = ""; 30

52. 3. if statements 4. showMessageDialog
if ( number1 == number2 )
result = result + number1 + " == " + number2; 33
3. if statements
4. showMessageDialog
if ( number1 != number2 )
result = result + number1 + " != " + number2; 36
if ( number1 < number2 )
result = result + "\n" + number1 + " < " + number2; 39
if ( number1 > number2 )
result = result + "\n" + number1 + " > " + number2; 42
if ( number1 <= number2 )
result = result + "\n" + number1 + " <= " + number2; 45
if ( number1 >= number2 )
result = result + "\n" + number1 + " >= " + number2; 48
// Display results
JOptionPane.showMessageDialog(
null, result, "Comparison Results",
JOptionPane.INFORMATION_MESSAGE ); 53
System.exit( 0 );
55 }
56 }
Notice use of JOptionPane.INFORMATION_MESSAGE

53. Program Output

54. Homework for chapter 2 Due date: Monday 9/7/01
Compile and run all of the example programs.
Write an application that inputs three integers from the user and display the sum, average, product, smallest and largest of these numbers in an information message dialog (use the GUI techniques) and standard output (using System class).
Write an applet that allows the user to input the four arguments required by method drawRect and then draws an rectangular using four input values.
Due date: Monday 9/7/01