Chapter 2 - Introduction to Java Applications Outline 2.1Introduction 2.2A Simple Program: Printing a Line of Text 2.3Another Java Application: Adding

Chapter 2 - Introduction to Java Applications

Outline2.1 Introduction2.2 A Simple Program: Printing a Line of Text2.3 Another Java Application: Adding Integers2.4 Memory Concepts2.5 Arithmetic2.6 Decision Making: Equality and Relational

Operators

2.1 Introduction

• In this chapter– Introduce examples to illustrate features of Java

– Two program styles - applications and applets

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

Java program

Program Output

1 // Fig. 2.1: Welcome1.java

2 // A first program in Java

3

4 public class Welcome1 {

5 public static void main( String args[] )

6 {

7 System.out.println( "Welcome to Java Programming!" );

8 }

Welcome to Java Programming!

9 }


– // 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



– 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



– 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



– 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




• 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


6 {


– 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 ;



– 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


• 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

Java program

Program Output



3



6 {


8 }

9 }


• Other methods– System.out.println

• Positions cursor on new line after displaying argument

– System.out.print• Keeps cursor on same line after displaying argument

Line numbers

1-2: Comments3: Blank4: Begin class Welcome2

5: Method main

6: Begin main body7: Method System.out.print

8: Method System.out.println

9: end main10: end Welcome2

Program Output


2 // Printing a line with multiple statements

3



6 {

77 System.out.print( "Welcome to " );

8 System.out.println( "Java Programming!" );

9 }

10 }

Welcome to Java Programming!

System.out.print keeps the cursor on the same line, so System.out.println continues on the same line.


• 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!" );

Class Welcome1

1. main

2. System.out.println (uses \n for newline)

Program Output


2 // Printing multiple lines with a single statement

3



6 {

77 System.out.println( "Welcome\nto\nJava\nProgramming!" );

8 }

9 }

WelcometoJavaProgramming!

Notice how a new line is output for each \n escape sequence.


• 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


• Upcoming program– Application that uses dialog boxes

– Explanation will come afterwards

Java program using dialog box

Program Output


2 // Printing multiple lines in a dialog box

3 import javax.swing.JOptionPane; // import class JOptionPane

4



7 {

8 JOptionPane.showMessageDialog(

9 null, "Welcome\nto\nJava\nProgramming!" );

10

11 System.exit( 0 ); // terminate the program

12 }

13 }


– 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


4



7 {


– 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




– 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




– 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


1. import statement

2. Class Welcome4

2.1 main

2.2 showMessageDialog

2.3 System.exit

Program Output


2 // Printing multiple lines in a dialog box


4



7 {



10


12 }

13 }

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

Java Program using input dialogs

1 // Fig. 2.8: Addition.java2 // An addition program 34 import javax.swing.JOptionPane; // import class JOptionPane56 public class Addition {

7 public static void main( String args[] )8 {9 String firstNumber, // first string entered by user10 secondNumber; // second string entered by user11 int number1, // first number to add12 number2, // second number to add13 sum; // sum of number1 and number21415 // read in first number from user as a string16 firstNumber =17 JOptionPane.showInputDialog( "Enter first integer" );1819 // read in second number from user as a string20 secondNumber =21 JOptionPane.showInputDialog( "Enter second integer" );2223 // convert numbers from type String to type int24 number1 = Integer.parseInt( firstNumber ); 25 number2 = Integer.parseInt( secondNumber );2627 // add the numbers28 sum = number1 + number2;2930 // display the results

Program Output


32 null, "The sum is " + sum , "Results",

33 JOptionPane.PLAIN_MESSAGE );

34


36 }

37 }


– 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


6 public class Addition {

9 String firstNumber, // first string entered by user

10 secondNumber; // second 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


10 secondNumber; // second string entered by user


– 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

11 int number1, // first number to add

12 number2, // second number to add

13 sum; // sum of number1 and number2


– 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

15 // read in first number from user as a string16 firstNumber =17 JOptionPane.showInputDialog( "Enter first integer" );


– 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" )

15 // read in first number from user as a string16 firstNumber =17 JOptionPane.showInputDialog( "Enter first integer" );


– 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

19 // read in second number from user as a string20 secondNumber =21 JOptionPane.showInputDialog( "Enter second integer" );

23 // convert numbers from type String to type int24 number1 = Integer.parseInt( firstNumber ); 25 number2 = Integer.parseInt( secondNumber );


– 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

27 // add the numbers

28 sum = number1 + number2;


– 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


32 null, "The sum is " + sum, "Results",



– 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




1. import

2. class Addition

2.1 Declare variables (name and data type)

3. showInputDialog

4. parseInt

5. Add numbers, put result in sum

1 // Fig. 2.8: Addition.java2 // An addition program 34 import javax.swing.JOptionPane; // import class JOptionPane56 public class Addition {

7 public static void main( String args[] )8 {9 String firstNumber, // first string entered by user10 secondNumber; // second string entered by user11 int number1, // first number to add12 number2, // second number to add13 sum; // sum of number1 and number21415 // read in first number from user as a string16 firstNumber =17 JOptionPane.showInputDialog( "Enter first integer" );1819 // read in second number from user as a string20 secondNumber =21 JOptionPane.showInputDialog( "Enter second integer" );2223 // convert numbers from type String to type int24 number1 = Integer.parseInt( firstNumber ); 25 number2 = Integer.parseInt( secondNumber );2627 // add the numbers28 sum = number1 + number2;2930 // display the results

6. showMessageDialog

7. System.exit

Program Output




34


36 }

37 }

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

2.5 Arithmetic

• Arithmetic calculations are used in most programs– Use * for multiplication and / for division, +, -

• No operator for exponentiation (more in Chapter 5)

– Integer division truncates remainder7 / 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

2.5 Arithmetic

Operator(s) Operation(s) Order of evaluation (precedence)

() Parentheses Evaluated first. If the parentheses are nested, the expression in the innermost pair is evaluated first. If there are several pairs of parentheses “on the same level” (i.e., not nested), they are evaluated left to right.

*, /, or % Multiplication Division Modulus

Evaluated second. If there are several, they are evaluated left to right.

+ or - Addition Subtraction

Evaluated last. If there are several, they are evaluated left to right.

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


• Upcoming program uses if structures– Discussion afterwards

Standard algebraic equality operator or relational operator

Java equality or relational operator

Example of Java condition

Meaning of Java condition

Relational operators

> > x > y x is greater than y

< < x < y x is less than y

>= x >= y x is greater than or equal to y

<= x <= y x is less than or equal to y

Equality operators

= == x == y x is equal to y

!= x != y x is not equal to y

<_

>_

=

1 // Fig. 2.17: Comparison.java2 // Using if statements, relational operators 3 // and equality operators45 import javax.swing.JOptionPane;67 public class Comparison {8 public static void main( String args[] )9 {10 String firstNumber, // first string entered by user11 secondNumber, // second string entered by user12 result; // a string containing the output13 int number1, // first number to compare14 number2; // second number to compare1516 // read first number from user as a string17 firstNumber =18 JOptionPane.showInputDialog( "Enter first integer:" );1920 // read second number from user as a string21 secondNumber =22 JOptionPane.showInputDialog( "Enter second integer:" );23 24 // convert numbers from type String to type int25 number1 = Integer.parseInt( firstNumber );26 number2 = Integer.parseInt( secondNumber );2728 // initialize result to the empty string29 result = "";30

31 if ( number1 == number2 )32 result = result + number1 + " == " + number2;3334 if ( number1 != number2 )35 result = result + number1 + " != " + number2;3637 if ( number1 < number2 )38 result = result + "\n" + number1 + " < " + number2;3940 if ( number1 > number2 )41 result = result + "\n" + number1 + " > " + number2;4243 if ( number1 <= number2 )44 result = result + "\n" + number1 + " <= " + number2;4546 if ( number1 >= number2 )47 result = result + "\n" + number1 + " >= " + number2;4849 // Display results


51 null, result, "Comparison Results",

52 JOptionPane.INFORMATION_MESSAGE );

53

54 System.exit( 0 );

55 }

56 }

Program Output


– Lines 1-9: Comments, import JOptionPane, begin class Comparison and main

– Declare variables

– Input data from user and assign to variables


11 secondNumber, // second string entered by user

12 result; // a string containing the output

13 int number1, // first number to compare

14 number2; // second number to compare

17 firstNumber =

18 JOptionPane.showInputDialog( "Enter first integer:" );

21 secondNumber =

22 JOptionPane.showInputDialog( "Enter second integer:" );


– 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

25 number1 = Integer.parseInt( firstNumber );

26 number2 = Integer.parseInt( secondNumber );

29 result = "";

31 if ( number1 == number2 )

32 result = result + number1 + " == " + number2;


• Other if structures

– Lines 50-52: result displayed in a dialog box using showMessageDialog

34 if ( number1 != number2 )35 result = result + number1 + " != " + number2;3637 if ( number1 < number2 )38 result = result + "\n" + number1 + " < " + number2;3940 if ( number1 > number2 )41 result = result + "\n" + number1 + " > " + number2;4243 if ( number1 <= number2 )44 result = result + "\n" + number1 + " <= " + number2;4546 if ( number1 >= number2 )47 result = result + "\n" + number1 + " >= " + number2;48

1. import

2. Class Comparison

2.1 main

2.2 Declarations

2.3 Input data (showInputDialog)

2.4 parseInt

2.5 Initialize result

1 // Fig. 2.17: Comparison.java2 // Using if statements, relational operators 3 // and equality operators45 import javax.swing.JOptionPane;67 public class Comparison {8 public static void main( String args[] )9 {10 String firstNumber, // first string entered by user11 secondNumber, // second string entered by user12 result; // a string containing the output13 int number1, // first number to compare14 number2; // second number to compare1516 // read first number from user as a string17 firstNumber =18 JOptionPane.showInputDialog( "Enter first integer:" );1920 // read second number from user as a string21 secondNumber =22 JOptionPane.showInputDialog( "Enter second integer:" );23 24 // convert numbers from type String to type int25 number1 = Integer.parseInt( firstNumber );26 number2 = Integer.parseInt( secondNumber );2728 // initialize result to the empty string29 result = "";30

3. if statements

4. showMessageDialog

31 if ( number1 == number2 )32 result = result + number1 + " == " + number2;3334 if ( number1 != number2 )35 result = result + number1 + " != " + number2;3637 if ( number1 < number2 )38 result = result + "\n" + number1 + " < " + number2;3940 if ( number1 > number2 )41 result = result + "\n" + number1 + " > " + number2;4243 if ( number1 <= number2 )44 result = result + "\n" + number1 + " <= " + number2;4546 if ( number1 >= number2 )47 result = result + "\n" + number1 + " >= " + number2;4849 // Display results


51 null, result, "Comparison Results",

5252 JOptionPane.INFORMATION_MESSAGE );

53

54 System.exit( 0 );

55 }

56 }Notice use of JOptionPane.INFORMATION_MESSAGE

Program Output

Homework for chapter 2

1) Compile and run all of the example programs.

2) 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).

3) 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

Documents

Chapter 2 - Introduction to Java Applications Outline 2.1Introduction 2.2A Simple Program: Printing a Line of Text 2.3Another Java Application: Adding