Introduction to Java

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

2. Introduction to Java • Java Applications and Applets • Java Development kit • JCreator • Console Programs in Java • Applet Development • Graphics

3. 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

4. Some Applets • Hilo Game: • http://mainline.brynmawr.edu/Courses/cs110/fall2003/Applets/HiLo/Hi.html • Hangman Game: • http://www.bodo.com/Applets/Hangman/index.html

5. 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

6. JDK (Java Development Kit) • JDK or SDK packages are released by Sun Microsystems • A recent version Java2 SDK v 1.4.3 is available through the URL: • http://www.cs.fredonia.edu/~zubairi/s2k4/csit225/csit225.html • On this web page, you can also find the JCreator v 3.

7. 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

8. 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

9. 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

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

11. 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

12. 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\"");

13. 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!!

14. 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

15. 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

16. 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

17. Showing Results • Try this line • JOptionPane.showMessageDialog(null,"Good Morning "+username,"Greeting Window",JOptionPane.INFORMATION_MESSAGE);

18. 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

19. 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);

20. 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.INFORMATION_MESSAGE);

21. 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