CS 102 Lab # 1
An Introduction to Java Programming

Objective: To demonstrate the use of Dr. Java

There should be an icon on your desktop for Dr. Java; Double-click it to run the program. You should see a window much like this:

You are going to create your program by beginning with a skeleton that we have provided for you. In your home directory (labeled by your username), create a CS102 directory. In side the CS102 directory, create a subdirectory lab1. Download the file Robot.java by clicking it while holding the shift key.

In order to begin working with it you must open it with Dr. Java. From the "File" menu, select "Open", or use the Control-O keyboard shortcut.  A dialogue window should appear:

Navigate to the CS102/lab1 folder and double click Robot.java.

Now the Dr. Java window should look like this:

The large area on the right side of the screen displays the contents of the "Robot.java" file. The text of the program as it should initially appear is shown below

import objectdraw.*;

//   CS 102 Laboratory 1 --- practice program
//   Enter your name, lab section and the date here.

public class Robot extends FrameWindowController{

   public void begin()
   {
     // commands placed here are executed when the program begins to execute
   }

   public void onMouseEnter(Location point)
   {
     // commands placed here are executed when the mouse enters the program window
   }

   public void onMouseExit(Location point)
   {
     // commands placed here are executed when the mouse leaves the program window
   }

   public void onMousePress(Location point)
   {
     // commands placed here are executed when the mouse button is depressed
   }

   public void onMouseRelease(Location point)
   {
     // commands placed here are executed when the mouse button is released
   }

}

The text we have placed in "Robot.java" is the skeleton of a complete Java program. It includes the header for the definition of a class Robot that extends FrameWindowController and within the class, headers for the event handling methods you will use. We have not, however, included any Java commands within the bodies of these methods, only a Java comment that reminds you when the Java system will follow any instructions you might add to the method body.

You should follow our lead and always begin writing your programs by typing comments rather than actual Java commands. Near the top of the file we have included a temporary comment telling you to enter your name, lab section time and the current date. Do this now. Dr. Java provides the familiar capabilities of a typical word processor or text editor. You can position the cursor using the mouse or arrow keys. You can enter text and/or cut and paste text using items in the edit menu. Use these features now to replace our comment with your name and other information.

Next, add Java instructions to the body of the begin method that will draw the face of a robot. This is the first method skeleton we have included in "Robot.java". The first thing to do is to replace our comment

     // commands placed here are executed when the program begins to execute

with something that describes what you intend the method to do, like

     // draws the face of a robot when the program begins to execute

You should get in the habit of updating comments to keep them as accurate as possible as you add instructions to your programs. Not only does this mean you will not have to go back and add comments when you're done, it will help you to remember what everything is supposed to do as you are writing your programs.

As a first step, let's add the single instruction needed to draw the rectangle that frames the face. The form of the command you will need to enter is:

new FramedRect(x, y, width, height, canvas);

where x,y,width, and height need to be replaced by the numbers describing the position and size of the rectangle. This line should be placed immediately after the comment line in begin (i.e., just before the line containing the "}" that ends the method body).

Type in the values for the x,y,width, and height of the rectangle. Adjust the values to get a proper size and proportion.

Now you are ready to compile and run your program and see if it works.

• To compile, click the Compile button on the tool bar. Alternatively, you can use Function Key 5 if you like.  The first time you do this, Dr. Java may ask you if you want to save first.  If so, click "Yes".  You can also check the box in the dialogue to automatically save in the future. 
• Important: During this process, Dr. Java may encounter one or more grammatical or typographical errors in your program. If errors are discovered, Dr. Java will abort the process, and display error messages in the bottom section of its window.  The first error and the corresponding line in your program will be highlighted.  Unfortunately, these error message are not always very clear! If you cannot decipher the error messages you receive, ask me for help. You will get better at understanding error messages as the semester progresses. Since Dr. Java really doesn't know how to correct your errors, a single error can leave it sufficiently confused that it reports many bizarre errors. Always start by correcting the first error reported.  If the remaining errors don't seem to make sense, they probably don't. They are just a result of Dr. Java's confusion.  Remember, fix the first error, then compile again.
• Once you get the message "Last compilation completed successfully.", you can run the program.  Click the mouse on the Interactions tab at the lower left.  You should see something like this in the pane:
  
  
• Type the following exactly as it appears, including the semicolon:

  new Robot();
  

• A new window should appear - this is your program's window. A rectangle should appear because when the Java program begins to execute, your begin method is invoked, which includes the instruction to construct a rectangle. If this happens, smile. If not, press the "File" menu item and select "Quit" to leave your program.  Examine the instructions you typed carefully to see if you can find any mistakes. If so, correct them and select "Run" again. Otherwise, ask for help.

When you run your program, it starts up a new program that is separate from Dr. Java. While this program is running, you can return to Dr. Java by just clicking on any portion of a Dr. Java window that is visible on your screen. However, to avoid confusion, it is a good idea to "Quit" the old copy of your program that Dr. Java created before running it again.

Once your rectangle program is working, you should add more instructions to draw a complete warning sign.

Back in Dr. Java, immediately beneath the line you added to draw the rectangle, add all the other components. Each time you add a line, compile again (function key F5) to see if the program is still correct.

You might find it convenient to look at the objectdraw cheat sheet to remind you about the names of the objects and method to use.

When you are done, your program should draw the face of a robot, and then do nothing until you tell it to quit.

Now, to explore event handling methods a bit more, revise your program so that it reacts to the mouse in more interesting ways. In the revised version, the face will change as the program runs. In particular, when the user moves the mouse into the program window or presses the mouse, your revised code will alter the appearance of the face.

Note that the order of instructions within a method is important, but the order among different methods in your program is not. Only the names of the methods are important (though we urge you to put them in a logical order so that it will be easier for all of us to read!).

Making the face change in response to the mouse is a bit trickier. There is a setColor operation that you can use to change the color of the components. There is also an obvious place to tell Java to make this change. The onMouseEnter method is executed whenever the mouse moves into your program window. Placing an appropriate setColor in that method would do the trick.

The problem is that you can't simply say setColor in the onMouseEnter method. If that was all you said, Java has no way of knowing which of the several objects' color to change. It could change the rectangle, the oval, the text, or all of them, etc. Your code has to be more specific and identify the object that should change.

To be able to specify the object whose color we wish to set, we will have to give it a name. You can use names such as leftEye, rightEye, message, or any name that seems appropriate. However, keep in mind that any name you come up with must be meaningful, but only to yourself but also to other readers. Names such as abc and 123 are considered bad names and should be avoided.

Important: Like for any natural language such as English, style are very important for a programming language. There are certain style conventions and naming conventions that must be following. Failing to follow these rules will result in losing points.

Associating a name with an object requires two steps. First, we have to include a line that declares or "introduces" the name. This line informs Java that we plan to use a particular name within the program. At this point, we don't need to tell Java what object it should be associated with, but we do need to specify what sort of object it will eventually be associated with. We plan to associate the name message with an object created as new Text, so we have to tell Java that the name will be associated with a Text object. The form of a Java declaration is simply the name being declared preceded by the type of object with which it will be associated. So, the form for our declaration is:

Text message;

You should add a line containing this declaration to your program immediately before the heading of the onMouseClick method.

Now you have to tell Java which object to associate with the name message. The following line in your begin method does just that:

message = new Text("TRANSMITTING", ..., canvas);

This is an example of a construct called an assignment statement. It tells Java that in addition to creating the new object, it should associate a name with it. Shortly, you will convert some of your other constructors into assignments. Consider the following question: Why is this line belong to the begin method, instead of the onMousePress method?

Now that the text has a name, we can use the name to change its color. Within the body of the begin method add the line:

message.setColor(Color.red);

Then, run your program (correcting any errors as needed).

Next we need to hind message when the program starts, and show it when the mouse is pressed. This can be accomplished by adding the following line to the begin method:

message.hide();

and the following line to the onMousePress method:

message.show();

You should be able to figure out what to add to make it disappear again when the mouse is released. Give it a try.

To get more practice using names and other event handling methods, we would like you to modify your program a bit more. Complete your program so that it behaves exactly like the sample program given in the beginning of the page.

Value	Feature
6 pts.	Drawing the face correctly when the program starts (it does not have to be exactly the same as the sample program, but it should look similar and be symmetric)
6 pts.	React to mouse events in the same way as the sample program
4 pts.	Meaningful names used in declarations
4 pts.	Good and consistent formatting