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Homework Assignment 6
Late homework assignments will not be accepted, unless you have a valid written excuse (medical, etc.). You must do
this assignment alone. No team work or "talking with your friends" will be accepted. No copying from the Internet.
Cheating means zero.
Create a new Eclipse workspace named "Assignment6_1234567890" on the desktop of your computer (replace
1234567890 with your student ID number). For each question below, create a new project in that workspace. Call
each project by its question number: "Question1", "Question2", etc. Answer all the questions below. At the end of
the assignment, create a ZIP archive of the whole workspace folder. The resulting ZIP file must be called
"Assignment6_1234567890.zip" (replace 1234567890 with your student ID number). Upload the ZIP file on
iSpace.
Here are a few extra instructions:
 Do not forget to write tests for all the code of all the classes.
 Give meaningful names to your variables so we can easily know what each variable is used for in your program.
 Put comments in your code (in English!) to explain WHAT your code is doing and also to explain HOW your
program is doing it.
 Make sure all your code is properly indented (formatted). Your code should be beautiful to read.
Failure to follow these instructions will result in you losing points.
Question 1
Write an IShape interface with the following UML specification:
+------------------------------------+
| <> |
| IShape |
+------------------------------------+
| + getX(): int |
| + getY(): int |
| + setX(int x): void |
| + setY(int y): void |
| + isVisible(int w, int h): boolean |
| + isIn(int x, int y): boolean |
| + draw(Graphics g): void |
+------------------------------------+
and a Shape class that implements IShape and has the following UML specification:
+------------------------------------+
| Shape |
+------------------------------------+
| - x: int |
| - y: int |
| - color: Color |
+------------------------------------+
| + Shape(int x, int y) |
| + getX(): int |
| + getY(): int |
| + setX(int x): void |
| + setY(int y): void |
| + isVisible(int w, int h): boolean |
| + isIn(int x, int y): boolean |
| + draw(Graphics g): void |
| + testShape(): void |
+------------------------------------+
The x and y instance variables indicate the position of the center of the shape, and the color instance variable
indicates the color of the shape. The color of the shape is computed randomly in the constructor of the shape class and
never changes after that, like this:
color = new Color((float)Math.random(),
(float)Math.random(),
(float)Math.random());
The isVisible method is abstract, and indicates whether the shape is currently visible or not inside a window of
width w and of height h. The isIn method is abstract, and indicates whether the point at coordinates (x, y) is currently
inside the shape or not. The draw method simply changes the color of the graphics object g to be the correct color for
the shape.
Also add to your program a Start class to test your Shape class.
Question 2
Add a class Bubble that extends Shape. The Bubble class has an instance variable called radius of type double
that represents the radius of the bubble. The constructor of the Bubble class takes an x and a y as arguments, which
represent the position of the new bubble. The radius of a new bubble is always 10 and never changes after that.
The isVisible method indicates whether the bubble is currently visible inside a window of width w and height h
(position (0, 0) is in the upper-left corner of the window). The bubble is considered visible if at least one pixel of the
bubble is visible. Therefore a bubble might be visible even when its center is outside the window, as long as the edge of
the bubble is still visible inside the window.
The code of the isVisible method is a little bit complex, mostly because of the case where the center of the circle is
just outside one of the corners of the window. So here is the code of the isVisible method, which you can directly
copy-paste into your assignment:
// Find the point (wx, wy) inside the window which is closest to the
// center (x, y) of the circle. In other words, find the wx in the
// interval [0, w - 1] which is closest to x, and find the wy in the
// interval [0, h - 1] which is closest to y.
// If the distance between (wx, wy) and (x, y) is less than the radius
// of the circle (using Pythagoras's theorem) then at least part of
// the circle is visible in the window.
// Note: if the center of the circle is inside the window, then (wx, wy)
// is the same as (x, y), and the distance is 0.
public boolean isVisible(int w, int h) {
double x = getX();
double y = getY();
double wx = (x < 0 ? 0 : (x > w - 1 ? w - 1 : x));
double wy = (y < 0 ? 0 : (y > h - 1 ? h - 1 : y));
double dx = wx - x;
double dy = wy - y;
return dx * dx + dy * dy <= radius * radius;
}
The isIn method indicates whether the point at coordinates (x, y) (which are the arguments of the method) is
currently inside the bubble or not. The edge of the bubble counts as being inside of the bubble. HINT: use Pythagoras's
theorem to compute the distance from the center of the bubble to the point (x, y).
The draw method uses the graphics object g to draw the bubble. HINT: remember that the color of the graphics object
g is changed in the draw method of the superclass of Bubble.
Also add a testBubble method to test all your methods (including inherited methods, but excluding the isVisible
method, which I provide, and excluding the draw method since it requires as argument a graphics object g that you do
not currently have).
Question 3
Write a Model class with the following UML specification:
+----------------------------------------------+
| Model |
+----------------------------------------------+
| - score: int |
| - bubbles: ArrayList |
+----------------------------------------------+
| + Model() |
| + getScore(): int |
| + addBubble(int w, int h): void |
| + moveAll(int dx, int dy): void |
| + clearInvisibles(int w, int h): void |
| + deleteBubblesAtPoint(int x, int y): void |
| + drawAll(Graphics g): void |
| + testModel(): void |
+----------------------------------------------+
When a new model object is created, the score must be zero and the arraylist must be empty.
The getScore method returns as result the current score for the game.
The addBubble method adds a new bubble to the arraylist of bubbles. The position of the center of the new bubble is
random but must be inside a window of width w and height h (the arguments of the addBubble method), like this:
new Bubble((int)(w * Math.random()), (int)(h * Math.random()))
The moveAll method moves the positions of all the bubbles in the arraylist of bubbles by the amount dx in the x
direction and by the amount dy in the y direction.
The clearInvisibles method takes as argument the width w and the height h of the window, and deletes from the
arraylist of bubbles any bubble which is not visible in the window anymore. For each bubble which is deleted, the
score decreases by 1.
WARNING: when you use the remove method of Java’s ArrayList class to remove an element of an arraylist at
index i, the arraylist immediately shifts down by one position all the elements with higher indexes to make the arraylist
one element shorter. So, for example, when removing the element at index i, the element at index i+1 immediately
moves to the position at index i, the element at index i+2 immediately moves to the position at index i+1, etc. This
means that on the next iteration of the loop, when i has become i+1, the element that you will be testing at index i+1
is in fact the element that used to be at index i+2. Which means that the element that used to be at index i+1 (and
which is now at index i) will never be tested! Therefore, when removing elements from an arraylist, if your loop starts
at index 0 and goes up the indexes in the arraylist, then your loop will fail to test some elements! CONCLUSION: when
removing elements from an arraylist, your loop must start from the END of the arraylist and go DOWN to index 0.
The deleteBubblesAtPoint method takes as argument the coordinates (x, y) of a point, and deletes from the
arraylist of bubbles any bubble which contains this point (multiple bubbles might contain the point, because bubbles can
overlap in the window). For each bubble which is deleted, the score increases by 1.
The drawAll method draws all the bubbles in the arraylist of bubbles.
Make sure you test as many methods of the Model class as possible.
Question 4
Create a ViewBubbles class that extends JPanel and has the following UML specification:
+---------------------------------------------+
| ViewBubbles |
+---------------------------------------------+
| - model: Model |
+---------------------------------------------+
| + ViewBubbles(Model model) |
| - moveUp(int w, int h): void |
| - clickBubbles(int x, int y): void |
| # paintComponent(Graphics g): void |
+---------------------------------------------+
The constructor of the ViewBubbles class must create a timer (from the javax.swing library, not from the
java.util library!). This timer ticks every 500 milliseconds. Every time the timer ticks, the timer's action listener must
call the moveUp method with the width and height of the panel as arguments (remember that ViewBubbles extends
JPanel, so a view is a panel). Remember to start the timer!
The constructor of the ViewBubbles class also adds a mouse listener to the panel (use the MouseAdapter class to
implement the mouse listener). Every time the user clicks into the panel, the mouseClicked method of the mouse
listener must call the clickBubbles method with the x and y coordinates of the mouse event as arguments.
The constructor of the ViewBubbles class must also set the background color of the ViewBubbles to white (this
will help you see the bubbles).
The moveUp method is private and takes a width w and height h as arguments and does the following:
 it calls the moveAll method of the model to move all the bubbles up by 1 pixel (remember that the y direction
is pointing down);
 it calls the clearInvisibles method of the model with the width w and height h as arguments to delete all
the bubbles that have become invisible after moving;
 it calls the addBubble method of the model to add a new bubble;
 it uses the model’s getScore method to print the current score on the standard output, but only if the
current score is different from the score before the call to the clearInvisibles method;
 it calls the repaint method (this in turn will make Java automatically call the paintComponent method of
the ViewBubbles).
The clickBubbles is private takes the position (x, y) of a mouse click as arguments and does the following:
 it calls the deleteBubblesAtPoint method of the model to delete all the bubbles which contain the point
where the mouse was clicked;
 it uses the model’s getScore method to print the current score on the standard output, but only if the
current score is different from the score before the call to the deleteBubblesAtPoint method;
 it calls the repaint method (this in turn will make Java automatically call the paintComponent method of
the ViewBubbles).
The paintComponent method calls the drawAll method of the model, which will draw all the bubbles.
Create a MyFrame class that extends JFrame. Set the size, title, default close operation of the frame and make it
visible. Do not add a layout manager to the frame.
In the constructor of MyFrame, create a Model object. Then create and add a ViewBubbles object to the frame.
Then in the main method of the Start class, after the tests, create an anonymous class that implements the
Runnable interface with a run method that creates a MyFrame object, and use the
javax.swing.SwingUtilities.invokeLater method to run that code on the event dispatch thread.
Run your program and check that:
 a new bubble is correctly created every half second;
 all the bubbles move up by 1 pixel every half second;
 bubbles disappear immediately when you click on them (not the next time all the bubbles move up);
 clicking on two bubbles that overlap makes both bubbles disappear;
 your score increases when you click on bubbles;
 your score decreases when bubbles disappear at the top of the window;
 bubbles appear everywhere in the window, even when you resize the window to be bigger.
Note: when you click the mouse button, Java considers it a mouse click only if the mouse has not moved between the
moment when you press the button and the moment when you release the mouse button. So make sure that you do not
move the mouse at the same time you are clicking the mouse button, otherwise Java will just ignore the button click!
Your program must then look like this:
Note: once your program is working, you can decrease the timer from 500 milliseconds to 200 milliseconds, or even
down to 100 milliseconds, and then see how long you can keep your score positive!

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