Introduction to Java

To get started, create a directory called introjava. Feel free to go back and peek at the UNIX tutorial if you need to jog your memory on how to do so. Use the cd command to navigate to the introjava directory that you have just created.

Start up jedit, which is the text editor that we will be using to create and modify Java programs. To do so, type jedit & (don't forget the ampersand) in your terminal window. Play around with it a little bit: try to type some text, save it, close jedit, and see if you can open it again.

A first Java program

Look at the program Hello.java. In jedit, type in this program exactly. You might find it easiest to put the two windows side-by-side on your computer screen, or perhaps you might wish to print out Hello.java first and work from the printout. Don't copy and paste the code. You'll learn more if you go through the experience of typing it in and fixing any errors. Save your file under the name Hello.java.

Before we can run your program, we need to compile it (translate it to a language the computer can understand). To compile this program, type:

javac Hello.java

in the terminal. Hopefully, the program should compile without returning any messages. (Any messages you see are errors.) If you do get errors, get help from from the prof, the prefect, or a lab assistant.

Once you've compiled the program successfully, do an ls. You should see a new file in the directory, named Hello.class. This is the "bytecode" file that you will run. To run the program, type:

java Hello

Notice that you do not include the .class extension when you run a Java program.

EXERCISE 1: Modify the program so that it prints out "Welcome to Intro CS, [your name]."

Java input

This time, look at Input.java. Copy and paste the code into jedit (or type it in if you like), and save the file to your introjava directory. Compile the program and run it a couple of times. (Again, get help if you have any error messages.)

EXERCISE 2: You may notice that we sometimes use System.out.print() and sometimes System.out.println(). What's the difference between the two? Play around with both of these in your program (try modifiying it in various ways and rerunning) to see if you can figure it out. Type your answer into a file called introjava.txt that you also store in your introlab directory.

EXERCISE 3: What happens if you enter a number when prompted for your name and a name when prompted for a number? If you get a message, what did the message say? What do you think it means? Put your answers in your introjava.txt file.

EXERCISE 4 (not required, but go for it if you can): Modify the program so that it asks for the current year, and the year you were born. Your program should then print out the difference between the two, which is (approximately) your age. Make sure to put parentheses around your subtraction, like in the example shown.

Submit all of your work using the homework submission program, which is referred to as hsp. (Note: you can submit entire directories using hsp, so it may be easiest to submit your entire introjava directory.) Instructions for using hsp are here. Note that hsp will not work unless you are officially registered for the course in the registrar's systems. If you are working from home, transfer your work to your department Linux account so that you can test it there before submitting.