Introduction To Java - MFC 158 G

Week 3 Lecture notes - Fall 2000

Chapter 2 - Introduction to Java Applications- MFC 158 (1 of 5)

Java Language

- Facilitates a disciplined approach to computer program design

- Applications (typically run locally on a machine / local network using a java interpreter - java)

- Applets (typically run over the internet / with browsers or appletviewer)

// Fig. 2.1: Welcome1.java - pg 36 - A Java Application

// A first program in Java

public class Welcome1 {

public static void main( String args[ ] )

{

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

}

}

(C) Copyright 1999 by Deitel & Associates, Inc. and Prentice Hall.

- Single line comments / multiline comments (c style)

- Javadoc - comments - can create HTML formatted documentation from source code

- Blank lines and space characters are allowed (white-space)

- public class Welcome1 {… - class definition - all Java programs have at least one class defined by the programmer

- the class name must be the same as the filename.java

- keywords (reserved words) are reserved for use by Java ()

- identifiers are names of classes and variables

- statements end with semicolons ( ; )

- every class in chapter 2-7 begins with public - publically accessable class

- braces { } are used to define the beginning and end of classes - to create a 'block' of code

- public static void main( String args[ ]) …a method called main - exists in every Java application

- all applications execute beginning with the main method (interpreter)

- String args[ ] - indicates that method main can accept arguments

- Java Welcome1 argument1 argument2…argumentn

- void indicates that the method main returns NOTHING (void)

- System.out - allows applications to display strings, etc. to the command window

- System.out.println - a method that displays a line of text

- Right brackets indicate end of method main and of the class Welcome1

- The program displays "Welcome to Java Programming!"

Introduction To Java - MFC 158 G

Week 3 Lecture notes - Fall 2000

Chapter 2 - Introduction to Java Applications- MFC 158 (2 of 5)

// Fig. 2.3: Welcome2.java

// Printing a line with multiple statements

public class Welcome2 {

public static void main( String args[] )

{

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

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

}

}

(C) Copyright 1999 by Deitel & Associates, Inc. and Prentice Hall.

System.out.print - prints a string and leaves the cursor at the end of the string

System.out.println - prints a string and puts the cursor at beginning of next line

// Fig. 2.4: Welcome3.java

// Printing multiple lines with a single statement

public class Welcome3 {

public static void main( String args[] )

{

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

}

}

(C) Copyright 1999 by Deitel & Associates, Inc. and Prentice Hall.

A Backslash indicates a 'special' character is to be outputted (I.E. \n - newline, \t - horizontal tab, \\ - print a backslash, \" - print a double quote

Dialog boxes

- windows for displaying messages to users of an application

- Java class JoptionPane - allows you to easily display various dialog boxes

- Gives the application a 'windows' type of interaction

- Can be used for input and output

Introduction To Java - MFC 158 G

Week 3 Lecture notes - Fall 2000

Chapter 2 - Introduction to Java Applications- MFC 158 (3 of 5)

// Fig. 2.6: Welcome4.java

// Printing multiple lines in a dialog box

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

public class Welcome4 {

public static void main( String args[] )

{

JOptionPane.showMessageDialog(

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

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

}

}

(C) Copyright 1999 by Deitel & Associates, Inc. and Prentice Hall.

- the compiler uses import statements to identify and load classes

- import tells the compiler to load class JoptionPane class from the javax.swing package.

- Javax.swing contains several classes for creating GUI applications

- Components such as buttons, menus, text fields and labels can be used.

- ShowMessageDialog

- Accepts two arguments (null for first argument - more on Chapter 13)

- displays the text in a window

- System.exit allows the program to pass a value (zero or other) to a calling program (batch file, script, etc.)

- Class System is part of the package java.lang. No import is necessary as java.lang is automatically imported in all Java programs.

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Concepts to consider for the next program

Variables:

- consist of a name which references a location in computer memory.

- each variable has a name, data type, size and value

Data types: Int / long (integers), float / double (decimal numbers), boolean (true/false), strings (composed of characters and words).

Arithmetic operators	Operator	Algebraic Expression	Java Expression
Addition	+	F + 7	F + 7
Subtraction	-	P - C	P - C
Multiplication	*	BM	B * M
Division	/	X / Y	X / Y
Modulus	%	R mod S	R % S

Introduction To Java - MFC 158 G

Week 3 Lecture notes - Fall 2000

Chapter 2 - Introduction to Java Applications- MFC 158 (4 of 5)

// Fig. 2.8: Addition.java

// An addition program

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

public class Addition {

public static void main( String args[] )

{

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

// read in first number from user as a string

firstNumber =

JOptionPane.showInputDialog( "Enter first integer" );

// 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 );

// add the numbers

sum = number1 + number2;

// display the results

JOptionPane.showMessageDialog(

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

JOptionPane.PLAIN_MESSAGE );

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

}

}

Introduction To Java - MFC 158 G

Week 3 Lecture notes - Fall 2000

Chapter 2 - Introduction to Java Applications- MFC 158 (5 of 5)

Become familiar with the equality, relational operators and the order of evaluation Pg 54-57

// convert numbers from type String to type int

number1 = Integer.parseInt( firstNumber );

number2 = Integer.parseInt( secondNumber );

// initialize result to the empty string

result = "";

if ( number1 == number2 )

result = number1 + " == " + number2;

if ( number1 != number2 )

result = number1 + " != " + number2;

if ( number1 < number2 )

result = result + "\n" + number1 + " < " + number2;

if ( number1 > number2 )

result = result + "\n" + number1 + " > " + number2;

if ( number1 <= number2 )

result = result + "\n" + number1 + " <= " + number2;

if ( number1 >= number2 )

result = result + "\n" + number1 + " >= " + number2;

// Display results

JOptionPane.showMessageDialog(

null, result, "Comparison Results",

JOptionPane.INFORMATION_MESSAGE );

System.exit( 0 );

}