Using Java on Linux

Laboratory Exercise 3

CMP 420: Database Systems
Lehman College, City University of New York
27 February 2003

Unix Commands

Each lab begins with several Unix commands that will be useful for completing today's lab and future labs and assignments. Try the following commands:

printenv
When you log-on on the lab machines, a working environment is set up for you. Your environment includes your username, your hostname, the operating system type, the default language (used by help files and the keyboard), etc. To see your current environment, type:
```
 
    printenv
```
grep pattern [filename]
The grep command lets you search for a pattern in standard input or a file. For example,
```
    printenv | grep PATH
```
searches the output from the printenv command for any lines containing the word "PATH." If you don't specify exactly where a program is that you would like to run, the computer first checks in all the directories listed in your path.
which commandname
When you type name of a program, the computer checks all the directories in your path for that program to run. Often, you need to know which version of the desired program the computer will choose to use. To do this, use the which. For example, if you would like to know which java compiler is used when you type javac, you type:
```
    which javac
```
locate filename
The locate command searches the entire file system for the filename. For example,
```
    locate javac
```
gives the names of all files that contain "javac" in their complete name. Sometimes, this can be a lot of files and will scroll off the screen. For example, if you type:
```
    locate java
```
you will see the 1900 files that contain "java" in their name.

Using Java

Today's lab focuses on using Java on the Linux systems. Installed on the lab machines is the Java Developer's Toolkit (JDK 1.2.2), a standard release from Sun Microsystems.

Create a directory (or folder) for this lab.
mkdir lab3
(This command creates a new directory called ``lab3''.)
Use this directory when creating any files for the exercise. To move to that directory, type:
```
    cd lab3
```
(This ``changes directories'' to p1.) When you create files in the text editor, save them to your subdirectory, lab3. (To return to your home or main directory, type: cd .)
Create a simple java program that will print Hello World! as text. Call the class Hello and save the file as Hello.java.
To compile the file, type:
```
    javac Hello.java
```
If there are errors, you will need to edit your program, fix them, and compile again.
To run your program, type:
java Hello
(The program should say "Hello World!")

Applets

Now, make an applet that will print "Hello World" in red letters on a blue background. As in other systems, you will need to create an HTML file for you applet. Compile your file as before and save your HTML file as hello.html. To view your applet, you can load the webpage into a browser (as you did on other systems), or you can use a program designed for viewing applets, called appletviewer. To use appletviewer, you type:

    appletviewer hello.html

Add to your applet, 3 buttons that change the background color.

If you have forgotten some of your java, the API's on the Sun webpages are at: http://java.sun.com/products/jdk/1.2/docs/api/index.html.

Final Comments

Before leaving the lab, you need to log out and shut down your computer. First, you should quit all applications. Then, go to the K menu in the bottom left hand corner and select Log Out. Choose the Shut Down option, and shut off the monitor when you're done.

Remember to pick up any files you sent to the printer and to take any personal belongings. It is difficult to get back into the lab later (since it's locked when not in use), so, it's worth making sure you haven't forgotten anything.