CS 124, Fall 2009
Lab 1: Introduction to Linux and Java

Welcome to CS 124, Introduction to Programming. In this course, you will learn how to write computer programs in the Java programming language. In the labs for this course, you will be using the Linux operating system. For programming outside of lab, you can use your own computer (with almost any operating system that you prefer), or you can continue to use Linux on the computers in the Colleges' computer labs.

This first lab of the semester introduces you to the Linux operating system and to Java. Because of all the new material on using Linux, this lab handout is rather long, and you should read through it in advance.

Getting Into Linux

The computers in the Library Multimedia Lab and in Gulick 208 are ordinarily running Windows. To use Linux on one of these computers, you must first restart the computer. After hitting Control-Alt-Delete, click the "Shutdown" button, choose "Restart" under "What do you want the computer to do?", and click "OK". During the restart process, there is a point where you will have the option of selecting Linux. You have about ten seconds to do so; if you do not change the selection to Linux, the computer will boot into Windows. To boot into Linux, use the down-arrow key to move the selection to "Kubuntu Linux", and press return.

Once the computer has booted into Linux, you will see a login screen. Use your usual user name and the password for Linux that you were given in class. (Your Windows password will not work.) You will log into the KDE 4 Desktop environment. The first time that you log in, it will take a while to set up KDE; subsequent logins won't take as long. (One of the first things you may see the first time you log onto a given computer is a dialog box with the title "Removed Sound Devices". It is safe to check the "Do not ask again..." box, and click "Yes".)

This year, we have installed Version 4 of KDE, which has some annoying features compared to older versions. To get rid of some of the worst of them, I suggest that you do the following:

• Right-click the "K" menu (in the lower left corner of the screen), and select "Switch to Classic Menu Style" from the pop-up menu.
• Point your mouse at the "Desktop Folder" widget. A black control bar will appear on the right. Click the "X" in this control bar to get rid of the widget.
• Right-click anywhere in the Desktop. Select "Appearance Settings" from the pop-up menu. In the dialog box, change the "Desktop Activity" / "Type" to "Folder View". (You can also change the desktop background picture in this dialog box.)

Take some time to look around. Check out some of the many entries in the "K" menu. Try out some of the games (briefly! -- you can play the games on your own time).

Note that KDE is not your only alternative. Another popular desktop, called Gnome is also available. To use Gnome instead of KDE, you must select it in the "Sessions" menu on the Linux login screen, before you log in. You can try out Gnome if you want and ask for help if you need it. Note that the same set of applications is available whether you use KDE or Gnome.

At the end of the lab, before you leave, you will need to reboot the computers into Windows. Just select "Leave" from the "K" menu, and click on "Restart Computer" in the dialog box that pops up.

A Basic Command-Line Environment

This section introduces the command-line environment, where you interact with the computer by typing in commands. Most of the things that you can do on the command line can also be accomplished by menu and mouse. The command line can be more efficient and is essential for a few tasks.

To access the command-line environment, choose the command "Konsole Terminal" from the "System" submenu of the "K" menu. This will open a window where you will enter your commands. When this Terminal window is active and waiting for a command, you will see a blinking cursor next to a prompt. You type certain commands at this prompt, and press return. The computer will attempt to carry out the command. If there is an error, you will usually see an error message and another prompt. If the command is carried out successfully, you might not get any message at all, just the prompt where you can type another command.

Your files are organized into "directories", also called "folders." When working on the command line, you are always working in a "current" directory. Many of the commands that you can enter will apply to the current directory. Here are some common commands:

• ls -- List the contents of the current directory. The contents can include files and more directories, referred to as "sub-directories."
• ls -l -- Same as ls, but gives a "long" listing, with extra information such as the size of the files. In the long listing, directories are marked with a "d" as the first character on the line. The "-l" is called an "option." Many commands have options that can affect what they do. [Note: In case you can't see it, there's a space between "ls" and "-l".]
• cd XXX -- Move into to the directory named XXX. That directory becomes your current directory. Often, XXX is the name of a directory that is inside the current directory, but it can also be a "path" to a directory in another location.
• cd -- When used without a file name, cd moves you back into your main, or "home," directory. Your home directory contains all your files. When you first open a Terminal, the current directory is your home directory.
• mkdir XXX -- Create a new directory in the current directory (or in another location if XXX is a path rather than a simple file name).
• kate XXX -- Edit a text file named XXX. If the file does not exist, it will be created. Kate is the name of a text editor. You can use kate to create and edit Java programs. Note that this command does not complete until you close the editor -- you won't get another command prompt in the Terminal until you do so.
• rm XXX -- Remove (that is, delete) the file named XXX. Be careful with this command. Note that rm XXX will not work if XXX is a directory. For that, you can use rm -r XXX. The "-r" is an option that makes rm remove a directory and everything that it contains; be even more careful with this command!
• cp XXX YYY -- Make a copy of a file named XXX. This command works differently, depending on the nature of YYY. If YYY is an existing directory, then a copy of XXX is placed into YYY. If YYY is an existing file, then YYY is replaced with a copy of XXX, and the previous contents of YYY are lost. (Be careful when using cp!) If YYY doesn't already exist, then a new copy of XXX is created, and the copy is named YYY. (If you want to copy an entire directory named XXX, instead of just a file, use cp -r XXX YYY.)
• kpasswd -- Use this command if you want to change your password. You will be asked to enter your current password, then to enter the new password twice.

You will want to spend some time experimenting with commands on the command line.

You should store all the work that you do for this course in a directory named cs124, inside your home directory. Inside that directory, you make a separate directory for each lab. To get this started for the first lab, type in this sequence of commands on the command line:

cd
mkdir cs124
cd cs124
mkdir lab1
cd lab1
kate Greetings.java

At the end of this, you will have an editor window open where you can type your first Java program. The traditional first program just says "Hello World" -- but let's make it a little more personal. Type the following lines of Java code into kate's edit window, except replace XXX with your own name. (Hint: You could also open this lab handout in a web browser, and use copy-and-paste to copy the code into kate.)

public class Greetings {

   public static void main(String[] args) {
      System.out.println("Hello, XXX!");
      System.out.println("Pleased to meet you.");
   }
   
}

After typing this, save your file and exit from Kate. Back on the command line, you can compile the program with the command

javac Greetings.java

If there are no errors in the program, this command will succeed with no error messages; you will simply get another command prompt. If there are errors in the program, they will be listed. In that case, you have to edit the file to fix the errors and compile the program again. Once the program has been compiled successfully, you will have a new file named Greetings.class. Use the ls command to check that it is there. Greetings.class is the compiled program. You can run the program with the command

java Greetings

The GUI Approach

You are encouraged to learn to use the command line, and you will have to use it for certain things, such as compiling and running programs. However, you might be more comfortable with a Graphical User Interface for many tasks.

You should have a "Home" icon on your desktop. Click this icon to open a file browser window. You can use this folder to view files and folders. (In a GUI context, directories are more commonly called folders.) You can also use the file browser to create, delete, move, copy, and rename files and directories. Specifically,

• To open a folder, to see what's inside, just click the folder icon.
• To create a folder, right-click in the location where you want to create the folder, and select "Create New" / "Folder" from the pop-up menu. You will be asked to enter a name for the folder.
• To create a new text file, such as a Java program, right-click and select "Create New" / "Text File".
• To edit a text file, just click the file icon. The file should open in kate. (If it opens in some other application, right-click the icon and select "Open With" / "Kate" from the pop-up menu.)
• To move or copy an item, just drag the item to its new location, which can be another file browser window or the Desktop. You will be asked whether you want to move the item or copy it. (Alternatively, you can use copy-and-paste or cut-and-paste to copy or move an item.)
• To delete a file or folder, drag it onto the trash can, or right-click and select "Move to Trash." Note that when you move something to the trash, you can get it back -- just open the trash, right-click the item that you want to recover, and select "Restore".
• To rename an item, right-click the icon and select "Rename".

To test this, open a file browser window, navigate to the "lab1" directory that you have already created, and make a new text file in that directory named "Miles.java". Open the new file in kate and enter the following Java program into it:

public class Miles {

   public static void main(String[] args) {
      double miles, feetPerMile, feet;
      miles = 2.5;
      feetPerMile = 5280;
      feet = miles * feetPerMile;
      System.out.print("The number of feet in");
      System.out.print(miles);
      System.out.print(" miles is ");
      System.out.print(feet);
      System.out.println();
   }

}

Now, you want to compile and run this program. The nice thing is, you can do this without even leaving kate. On the bottom of the editor window is a button labeled "Terminal". Click this button, and a small terminal pane opens within the kate window. You can type commands there just like you would in a separate Terminal window. Furthermore, when you first open the terminal pane, it automatically does a cd into the directory that contains the file that you are editing.

Compile and run Miles.java, using the terminal pane in the kate window. Be sure to save the file before compiling it -- you will be compiling the version that is saved on disk, not the version that is in the edit window. The same thing applies if you have to fix errors in the program: Remember to save it before re-compiling it!

Further Assignments

Here are a few other short assignments for you to work on. Remember that you might have to complete these outside of lab.

Make a copy of the program Miles.java, and name the copy Pounds.java. Modify the new copy so that instead of converting 2.5 miles into feet, it will convert 178 ounces into pounds. A pound contains 16 ounces. (Warning: For this program, you have to divide, not multiply. Multiplication in Java is indicated by "*"; division is indicated by "/".)

As a final assignment, write a program to solve the following problem: You are throwing a little party for all of HWS's 2040 students. You want to have 3 glasses of root beer available for each student. A keg of root beer costs $92.95 and serves 180 glasses. How many kegs should you buy, and how much will they cost? Your program should do all the calculations starting from the basic data here, and then print out the answers. In your program, give names to the initial data, and work from there:

people = 2040;
glassesPerPerson = 3;
glassesPerKeg = 180;
costPerKeg = 92.95;

You can name your program anything you want.

Turning in Your Work

After doing all the assignments in this lab, you should have a directory named "lab1" that contains four programs. For each program, there should be a ".java" source code file and a ".class" compiled file. If you don't have all this, read though the lab again, and see what you missed!

To complete the assignment, you must copy your lab1 directory into a homework directory that I have created for you. Your homework directory is in the directory named /classes/f09/cs124/homework, and the name of the directory is the same as your user name. You must copy your lab1 directory into the correct location by the beginning of next week's lab. You can copy it using either the GUI or command-line interface.

Working Outside of Lab

You will need to do a lot of programming work outside of lab. You have many options for working on Java programs on your own.

• You can work on the computers in the Math/CS lab, Lansing 310. These computers are always running Linux and do not have to be rebooted; otherwise, they work the same as the computers in the library lab.
• You can use the computers in the Library Multimedia Lab at any time when it is not in use as a classroom, in the same way that you do in lab.
• You can write, compile, and run Java programs on your own computer, if the JDK (Java Development Kit) has been installed. You will then need to copy your work onto the Math/CS Department's file server; ask for information about how to do this.
• You can use VNC on your own computers to work remotely on Math/CS computers. See http://math.hws.edu/about_linux/vnc.html for details.
• If your computer uses Mac OS or Linux, you can use ssh to connect to the Math/CS lab computers and work on them remotely. Ask for information about this.
• If your computer uses Windows, you can think about installing Linux on it (along with Windows). If you are interested in trying this, ask for more information.

Command-line Tricks, Part 1

To be really comfortable using the command line, whether in a Terminal or inside kate, you need to know some tricks. The more you know, the more you will appreciate the command line. Here are a few of the most basic tricks:

• Use the arrow keys while typing commands. The up-arrow key in particular will take you back through the list of commands that you entered previously. This can be very convenient when you are working through the edit/compile/run cycle while developing a program!
• To avoid having to quit the editor to compile a program (in a Terminal window), run the editor in the background. This allows you to use the edit window and the terminal window at the same time. To do this just add a "&" at the end of the command that you use to run the editor. For example: "kate Greetings.java &". But don't forget to save your changes before you compile! The compiler only sees the saved version, not the version in the edit window.
• Use the tab key for "auto-completion." If you press the tab key while typing a command or file name on the command line, the computer will attempt to complete the word that you are typing. If there is a unique possibility that completes the word, then the rest of the word will be entered automatically. If there are several possibilities, then pressing the tab key twice in a row will list the possible completions.