There are 5 checkpoints , including the clean-up checkpoint, in this lab. Work together with your partner using one of your accounts. CHANGE WHO CONTROLS THE COMPUTER AFTER EACH CHECKPOINT! If you need help with any exercise, raise your hand.

Copy the Lab10 directory from the course directory. If you have forgotten how, refer to the handouts for labs 1 or 2. Everything you need for the lab exercises today is contained in this directory (except your outstanding intellect).

Change directories into Lab10 and start running BlueJ.

Using Objects in Interactive Method Calls

To work with this project, remember the coordinate system used by Java. Java places (0,0) in the upper left corner of the window. The x values get larger as you move to the right. The y values get larger as you move down. The units are in terms of "pixels", or dots on the screen. With our lab's screen resolution, 100 pixels is about 1.25 inches.

1. Compile the project by clicking on the Compile button.
2. Right click on the GraphicsProgram class and choose new GraphicsProgram(). BlueJ will prompt you for an object name, giving you the default name graphics1. Replace this with the name picture and then click on the "OK" button. A window should appear on the screen and an object icon should appear in BlueJ's object bench. We will add objects to the window to create a drawing.
3. Right click on the GRect class and choose new GRect(double x, double y, double width, double height). When asked for a name, type wall as the new GRect object's name. Supply the parameters 100, 200, 200, 200, in that order. wall now appears on BlueJ's object bench. Note that wall has been instantiated, but is not yet displayed.
4. Right click on picture and choose the add() method. In the window that appears, enter wall as the object to be added. The graphics window should now contain a square at the location (100,200) with a side length of 200 pixels. that will make it appear. Note that the square is currently a simple line drawing.
5. We want to make the wall red. This requires two steps:
   1. Right click on wall on the object bench and choose the method the setFillColor(). Enter as its parameter java.awt.Color.RED (remember to use the specified capitalization).
   2. Right click on wall again and choose the method setFilled(). Enter as its argument true.
   
   
   STOP!!! MAKE SURE YOU FOLLOW THE NEXT STEP VERY CAREFULLY!!!

6. Now you are on your own. The goal is to create a simple image of a house with a sun in the sky. For the next checkpoint, we want to see a written list of the exact steps you used to create the image. Take out a clean sheet of paper and make three columns. Here is a model, with its first line filled in:

   Object	Method	Arguments
   wall	GRect Constructor	100, 200, 200, 200

   Write down exactly what you do at each step. Include:
   
   1. Which object did you manipulate?
   2. Which method did you call?
   3. What argument(s) (if any) did you pass to the method?
   You must have the list to be checked off at the next checkpoint.
   NOTE: You can "erase" things you place in the drawing window. If you don't want to use an object that you placed in the picture:
   1. Right-click on the picture object on the Object Bench and choose the remove() method.
   2. Type the name of your object as the method argument.
   3. You can now delete the object from the Object Bench.
   It's okay if you have a misplaced line or two in the drawing you show us at the checkpoint.

   A sample of what the scene might look like is shown below, but do not feel bound by your instructor's efforts. Create your own scene.

   

1 Show us your scene and written list of method calls.

Program Method Calls

Now you'll recreate the same image by writing your steps in a Java program.

1. Close the PictureDemo project and open the PictureProgram project.
2. Compile the Picture class.
3. Right-click on the Picture class and run its main() method. The program runs and places a red wall in the figure. Unfortunately, that is all it does. There is no door, window, roof or sun - yet! Your mission, should you decide to accept it, is to finish the drawing.
4. Open the Picture class in the editor window.
5. Look at the code in the main() method. It begins with two lines that create the window. The wall is then added to the picture. It is declared in the line that says:
   GRect wall;
   It is then instantiated on the line that says:
   wall = new GRect(100, 200, 200, 200);
   Finally, several method calls are made. Notice that within this program, we are able to specify color with simply the capitalized color name. Check online documentation for the java.awt.Color class to discover all allowed colors.

6. Modify this code. Use the same method calls you used when making the interactive picture to create the same picture from within the main program.
   Hints:
   1. Follow the code that is already there. Method calls always look like:
      object.method(arguments);
   2. Try compiling and running your code frequently. It is better to catch your errors early than to be overwhelmed after typing many lines.
   3. To run this program, DO NOT construct a picture object. Run the main() method in the Picture class.
   4. Don't hesitate to refer back to the ACM graphics documentation in Firefox.

2 Show us the image you created.

Adding Methods to the Point Class

Our goal for the rest of this lab is to create a program that produces the same results as this main() method in a different way: by using an instantiable class that represents points. We will (hopefully) see the benefits of using classes and methods when we see a final program that is shorter and more elegant than that one.

This project contains three other classes (2 unfinished):

1. The Point class contains data and methods to represent 2D points. This class is described in the prelab reading.
2. The Pointedly class contains a program that uses the Point class to produce the same output as the Pointless program.
3. The Triangle class contains a program to find the perimeter of a triangle.

We will begin by working with the Point class. Open it in the editor. The Point class is designed to contain data and methods frequently needed by programs using points (like the Pointless program).

Constructor

Accessor

Mutator

The readFromKeyBoard() method

The distanceTo(double px, double py) method

Test each of the five methods you have just written by creating objects of type Point and calling the methods. When you are satisfied that the methods are running correctly, call us over.

3 Be prepared to answer the following questions:

1. Will there be any difference between a Point created by the default constructor, and one that is created by passing 2 zeroes as arguments to your new constructor?
2. Why can't other classes access the values of xCoord and yCoord directly?
3. How many different tests did you run for the distanceTo(double pX, double pY) method?
   • Did you test for two points separated vertically?
   • horizontally?
   • with negative, positive, and 0 values for x and y?
4. Experts Question: Our distanceTo method takes two doubles as parameters. Should it be possible to have a distanceTo method that takes a Point as a parameter?

Writing Client Code

Open the Pointedly class. It contains a complete program that should run identically to the Pointless program. Look over the code, then try compiling and running it. Does it run correctly? If not, make changes in the Point class, not in Pointedly!! Is this program shorter and more elegant than the Pointless program? Hint: The correct answer is yes. Be prepared to tell your instructor or lab assistant why.

Close the Pointedly class.

Open the Triangle class. It contains a shell for a main program that declares three points and reads the first one from data supplied by the user. Complete the main program so that it reads in all three points from the user, calculates the perimeter of the triangle they represent and neatly outputs the three points and the perimeter to standard output. Hint: You can see how we have code that calls the readFromKeyboard() method. Use similar code and call the distanceTo() method. Do NOT retype or copy the distance formula into this class.

Compile and run the program.

4 Be prepared to answer the following questions:

1. Why do we need to place objectName. in front of the calls to run getX() and getY()?
2. Will your program give a correct solution if the triangle is degenerate? (A degenerate triangle might have three colinear vertices, or two vertices that have the same x and y values.)

Extras for Experts

There are at least four other methods that could be added to the Point class. The two simplest ones are setX(double xArg) and setY(double yArg). These can be used to change the x and y values of Points. Implement the setters.

Points can also be represented in polar coordinates, (r, theta). r is the distance to the Origin. theta is the angle from the X axis to the ray from the Origin to the Point measured in a counter clockwise direction. theta = arcCos (x/r) assuming that y is positive. Implement getR() and getTheta() for the Point class. Do not add r and theta to the class as data members. Translate from x and y to r and theta in the methods.

Here is an even more interesting project. Most likely, your Triangle class contains a main() method that in turn contains three Points and code to calculate the perimeter. Move the declarations for the three Points above the main method so that they are now instance variables in the class. Add a constructor to instantiate the three Point objects, and a method to calculate the perimeter. Rewrite the main program so that it declares a Triangle object and then calculates the perimeter.

After the Lab