CSCI 201 Introduction to Algorithm Design
home | homework index | labs index 
SPRING 2007  

Downloading a Lab Project

Most CSCI 201 labs begin with a partially completed project. This lab will show you now to download one of these incomplete projects and store it in within your Linux account. You will also be shown how NetBeans guides you in fixing typos in your code, reads input from the "terminal", and "traces" program execution.

Downloading a project

Retrieving starter projects

In spite of all the talk about jar files in the NetBeans Import and Export lab we will be distributing our projects as ZIP files. There are two reasons for this: (1), Most browsers have "out-of-the-box" support for ZIP files; and (2), We ZIPed up most of the projects years ago and are too lazy to unzip and re-jar them.

Unrolling a project

The starter file for your next project is archived in Press the link to start the download.

Soon you should see a window entitled Opening It asks if you wish to use Archive Manager to open Before agreeing, press the checkbox beside Do this automatically for files like this from now on. That way, you won't be asked this question in the future. Then press the OK button.
Firefox opening

After you press OK, the Archive Manager displays a simple window showing you that the ZIP archive contains a single folder called Practice. All CSCI 201 archives will contain only one folder.
Firefox opening

Pay attention to the next part. Start by pressing the Extract icon to bring up the Extract window. At first, the Archive Manager will offer to extract your project into the /tmp directory which is usually full of odd files.
Initial extract window

Within the Extract window you must navigate to your csci/201 directory. Begin this journey by pressing on the Home icon at the upper-right of the window. Then work your way toward your csci/201 directory. Don't go on until your windows looks similar to the one below.
Extract window at csci/201

To speed up the process of connecting to your csci/201 directory in the future, press the Add button. You should now see a short cut icon for your csci/201 directory in the upper right panel. In the future, just press on the 201 short cut, and you'll be taken directly to your csci/201 directory.
Extract window at csci/201

Call your lab instructor over to see the state of your Extract window. If the instructor approves, you can then press the Extract button. The files are extracted without fanfare. You'll return to the window which can now be closed.

Opening the Project

Home at last

At this point the downloaded project is stored in your home directory and you should be able to view it with NetBeans.

NetBeans Open Project dialog

Start NetBeans and use the menu sequence File → Open Project... to bring up a Open Project dialog window. From there navigate to your csci/201 directory where you should find an entry for the Practice project. Click once on Practice and then select Open Project Folder. The most common mistake here is double-clicking on the folder name, in this case Practice. Although that's the natural thing to do, it doesn't open the project. It just gives you a view of the project folder's files. Remember: single-click the folder and mash Open Project Folder.
Open Project dialog

Cleaning up the project panel

At this point, you probably have at least two projects displayed in your project panel. We really don't want old CSCI 201 projects cluttering our view as we move on to new projects; so, for every project other than Practice, right-click on the project's name and then close it. You should end up with only Practice in your list of projects.

Opening the Java file

Click on the expanders (turnstiles) next to Practice then Source Packages and finally practice. Now you should see an entry for Click on that entry to bring up a window displaying the Java code for the Main class of the practice package.
Panels with java code

Show the lab instructor that you have downloaded the Practice project and are displaying the Java source code.

Errors in your program

Syntatic and semantic errors

Not everything that looks like a Java program is a Java program. Java programs must follow some rules. Some rules are syntatic. They are concerned with braces, semicolons, and other such. They are similar to English rules regarding the placement of punctuation. Here are a couple of important Java syntatic rules.

  1. All statements in java must end in a semicolon.
  2. Brackets, such as "{}", are used to break a program into functional blocks which may be nested. Each opening bracket, "{", must have a matching closing bracket, "}".

Java programs must also follow some semantic rules. These are a little harder to explain. The semantic rules govern the meaning of "names" in Java. Java can be very picky about its semantic rules. For example, Java names are case-sensitive: System is the name of an important Java package; system,  and SYSTEM are not.

Finding and correcting errors with NetBeans

If you look at the window containing, you'll see two little red boxes containing cross marks. Each of these boxes mark a Java programming error. If you place your mouse on a box, you'll be rewarded with a message displaying the cause of the error.

For example, the first error occurs because this Java statement does not end with an semicolon.
';' expected
The second error occurs because there is a missing right curly brace.
'}' expected
Go ahead and fix these two syntatic errors.

Yes, as soon as you fix those two errors a third one shows up. This is a semantic error. System is misspelled.
cannot find symbol: variable system
Correct this error and the boxes go away.

Build your project and show the instructor your successful compile.

Running a Project with Input

Giving data to

This Java program reads an integer, computes its square, and prints it to the terminal. We're going to show you how to enter that integer. It's not at all hard.

Providing input to NetBeans

Go ahead and run your program. Notice that an Output panel appears at the bottom on your NetBeans window. You should also see that your program has printed "Give me a number to be squared". At the bottom of the NetBeans window there be a text field labeled Input. Press you mouse within the text field, type a number, and then press the enter key. You program should digest your input number and respond with its square.
Output panel

You don't have to show the lab instructor your success. Squareness is its own reward.

Downloading another project

Practice makes perfect. You're going to do it again, but this time your own your own.

Quick opening of a project

Download a new project stored in and place it in your csci/201 directory. Remember to use the 201 short cut.

In NetBeans, open, build, and run the Syracuse project.


Programs often have "bugs", logic errors in need of correction. One useful way to find bugs is to step through your Java program one statement at a time. To do this, you set a breakpoint in your program. When the breakpoint is reached, NetBeans will allow you to step through the program.

You'll going to use breakpoints to step through a program that has some Java features, such as loops, that you've never seen before. Don't worry about the Java, but do read the directions very carefully.

Setting a break point

Find the main method of your program and place your mouse on the first line of code.

    System.out.println("Please give me a number") ;

Right-click on this line of code and choose Toggle Breakpoint from the pull-down menu.
Setting a breakpoint

Once the breakpoint is set, the line will be displayed on a red background. You'll also see a little red box just to the left of the line.
Set breakpoint

Running to the breakpoint

You can start the debugging process by pressing the Debug Main Project (F5) icon or navaigating the menu selections Run → Debug Main Project. However, easiest is just pressing the F5 key. By the way, if you are debugging on computer running Windows XP, you may receive a "Windows Security Alert" that "Windows Firewall has blocked some features of the program." If so, just click OK and continue.

Your program will very quickly reach the first breakpoint. The bottom of the NetBeans windows will be split into two panels: The leftmost will hold program output and the rightmost will display the values of the variables of your program. The background of the breakpoint statement should be green to indicate that the debugger is stopped at the statement. (Which is a little odd, as green usually means "go".)
At the breakpoint

Stepping over Java code

To step "over" a line of Java code, hit the F8 key to "step over" a statement of Java. Slowly press the F8 key exactly three times and watch it march through your code. Odd, it seems to have stopped on the third F8. Odder, the next line of code doesn't have a green background, but has a very faint yellow background.
At stdin.nextInt()
This statement contains a call to the stdin.nextInt method. Java and NetBeans are waiting for you to type an integer at the Input prompt. Go ahead and enter the number. I recommend 42. But don't type another F8 yet!
At call of syracuse method

Stepping into a Java method

If you typed F8 right now, your program would "step over" the call to the method syracuse. We know methods haven't be covered in class yet; but, if you do exactly what we tell you in the next paragraph, you'll learn a little about methods.

Type F7 for "step into".

This brings you into the syracuse method.
Entering syracuse method

Tiptop through two loops

At this point, we want you to hit the F8 key a few times and see how Java's while causes a program to loop and how Java's if causes a program make choices. Select the Local Variables tab in the lower-right panel and see how the variables N and count change as you step through the Java statements.
local variables

Show your lab instructor that you have gone through at least two iterations of the loop.

Last modified: 01/19/07    

Please Provide Feedback. We will use this information to improve the CSCI 201 Labs.