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: http://mainline.brynmawr.edu/Courses/cs11
0/fall2003/Applets/HiLo/Hi.html Hangman Game: http://www.bodo.com/Applets/Hangman/in
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 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.
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
ConfigureOptions 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 FileNewFile 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.INFORMATION_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