Introduction to Java A lab course by Dr. Junaid Ahmed Zubairi SUNY Fredonia

Introduction to Java  Java Applications and Applets  Java Development kit  JCreator  Console Programs in Java  Applet Development  Graphics

Java Applications and Applets  In Java, you can develop applications and applets  Applications can be executed by themselves  Applets are executed in a controlled environment, usually within web browsers  Let us see the example of an applet embedded in a web page

Some Applets  Hilo Game:  0/fall2003/Applets/HiLo/Hi.html 0/fall2003/Applets/HiLo/Hi.html  Hangman Game:  dex.html dex.html

Applet Mechanism  Java applets are programs that are stored with a web page  When a user requests that web page, the applet embedded with it is sent from the server to the user’s computer  Then the applet is executed in a “sandbox” preventing it from corrupting the user’s computer

JDK (Java Development Kit)  JDK or SDK packages are released by Sun Microsystems  A recent version Java2 SDK v is available through the URL:  it225/csit225.html it225/csit225.html  On this web page, you can also find the JCreator v 3.

JCreator  JCreator is a nice free package available through the web  We are using its older version (2.5) on lab machines. However, I have made version 3.0 available on my homepage  JCreator is an IDE (Integrated Development Environment) for developing Java applications and applets

On Your Home Machine  Download and install Java 2 SDK v1.4.3  (Optional) Download and install Java Documentation  Download and install JCreator version 3  Now you are all set to start using Java  If Java compiler does not run, Use Configure  Options and check JDK profile

Console Programs in Java  You can start by typing a simple program that will work through the console window  Open JCreator, Choose File  New  File and choose “Java File”. Give it the name “myfirst.java”  Type the following program into the editor window, then save, compile and run the program

First Program  Public class myfirst {  Public static void main(String[] args)  {  System.out.println(“Hi!! Nice Coffee”);  }

Java Rules!!  Each source file can contain only one public class  The name of the public class must match the name of the file in which it is stored  Each Java application must have a method named main()  main() starts running first so make sure that you put the initial code in it

Exercises  Try replacing the single statement in the first program by the following statements one by one and note the output:  System.out.println(3+4);  System.out.print(3+4);  System.out.println(3+4);  System.out.println("\"Hi!! Nice Coffee\"");

Opening A Little Window  Now is the time to open a small colorful window in our application  We would like to read the name of the user and generate a greeting for the user  Reading from the keyboard in Java requires adding an import line on top of the program  “import javax.swing.JOptionPane”  Java is case sensitive!!

Using Input Window  Please add the following line to capture and store the keyboard input into a string variable  String username = JOptionPane.showInputDialog("What is your name?");  You can display the stored name by the following line:  System.out.print(username);  Please complete the program by adding a greeting

Showing the Results in a Window  In order to get rid of the black and white console completely, we learn how to show the results in a colored window  We use another method from the goodies bag!!  This method is named showMessageDialog  It takes four parameters

Showing Results  Leave the first parameter as null  After comma, write the text string that you want to be displayed  The third parameter is the title of the window  The fourth one defines the type of message. You may choose EROR_MESSAGE to show errors and INFORMATION_MESSAGE to display normal results

Showing Results  Try this line  JOptionPane.showMessageDialog(null,"Go od Morning "+username,"Greeting Window",JOptionPane.INFORMATION_ MESSAGE);

Exercises  Change the output window to error message type window  Swap the greeting to occur after the name  Add “How are you” to the greeting. Strings can be added to other strings with + sign  Change the title to your own  Give a demo

Reading Numbers from the Keyboard  Numbers cannot be directly read from the keyboard because the methods used by us read everything as text  If the user has typed a number, we have to extract it from the string  Assume the user has entered a number in the string username  Type the following to extract and store the number into variable “age”  int age = Integer.parseInt(username);

Programming Exercise  Use the comparison statements to display the appropriate message  For example, use the following statement to display the message that the user can drive  if (age>=16)  JOptionPane.ShowMessageDialog(null, “You can drive”,”Info”,JOptionPane.INFORMATIO N_MESSAGE);

Programming Exercise  Add a statement that displays the fact that the user’s age is less than 16 and thus the user cannot drive  Add another pair of statements showing the decision if the user can or cannot vote, given the voting age is 18