CPSC 170 Lab 3
Sorting and Events

Thinking about Events

However! ActionListeners only do one thing - respond to an "actionPerformed". Moreover, since we are always going to specify what to do in case there was an "actionPerformed", the actionListener is actually abstract! We have an abstract class, where all methods are abstract - this is a good candidate for an interface! Therefore, when we define a class to respond to events, we don't extend ActionListener, we ___________________________________ ActionListener.

Timers

Download the file drawBars.tgz . Uncompress it using the command

tar xzf drawBars.tgz

You should have noticed that the bars changed color, one by one -- if you didn't know better, you would expect that there was a for loop or a while loop controlling the color change. Examine the code to find out what is really going on. Before you move on, you should make sure that you understand:

• How much time lapses between color changes?
• How does the timer know which bar to alter?
• How do we know that all the bars have changed color?
• What does the timer do after all the bars have changed color?

Sorting

We are going to sort the bars (according to height), using a selection sort (refer to the code on p 501 or your class notes). Instead of doing the entire sort at once, we are going to use the timer to delay processing and see incremental steps. To achieve this you should:

• Modify bars.java to make it implement Comparable. Remember from class that a Comparable interface uses one method - compareTo.
• Create a method: private int minIndex(int start) -- takes an index start and returns the index of the smallest bar in the list starting at index start.
• Create a method: private void swap(int i, int j) -- takes two indices and swaps the values in the list at those indices.
• Modify the refreshListener to implement a selection sort.-- At the conclusion of each timer event, you should see that one more bar is correctly positioned. To make things easier to follow, you should also change the color of the corectly positioned bars to blue. Note - your final code won't be too much different than it does now, right?

Print drawBars.java and bars.java to turn in.

Mouse Events

1. File MousePlay.java contains a program that displays a message saying when the mouse button is pressed and released. It uses MousePlayPanel.java to define the JPanel that is actually displayed. Save these files to your directory, run MousePlay to see how it works, and open MousePlayPanel in emacs. Note that MousePlayPanel has an inner class MousePlayListener that implements MouseListener by giving bodies for the five MouseListener methods. Even though nothing is done when the mouse is clicked or when it enters or exits, those methods still appear with empty bodies -- this is required for the MouseListener interface to be implemented.

   Modify MousePlayPanel so that it also prints appropriate messages when the mouse is clicked, when it enters the panel, and when it exits. Play with the resulting program until you understand how the mouse events are generated.

2. Files Dots.java and DotsPanel.java contain revised versions of the code in listings 7.18 and 7.19 in the text. Dots draws a dot every time the uses presses the mouse button; only the current dot is shown in the revised version. Save these files to your directory and run Dots to see how it works. Then modify DotsPanel as follows:
   1. Make the dots that appear alternately red and green. To do this, you will need to make the following changes:
      • Add an instance variable to hold the dot color.
      • Add an instance variable to count the number of dots that have been drawn.
      • When the mouse is pressed, in addition to setting the draw point you need to update the counter and change the dot color to green if the counter is odd, to red if it is even. Then call repaint.
   2. Modify your code so that the dot is drawn where the mouse is released, not where it is pressed.

   Print DotsPanel.java to turn in.

3. File Circles.java sets up a GUI that creates and draws a circle as defined in Circle.java of random size and color at each mouse press Each circle replaces the one before it. The code to handle the mouse press and do the drawing is in CirclePanel.java. Save these files to your directory, compile them and run them and experiment with the GUI. Then modify these files as described below.
   1. This program creates a new circle each time -- you can tell because each circle is a different color and size. Write a method void move(Point p) for your Circle class that takes a Point and moves the circle so its center is at that point. Now modify your CirclesListener class (defined inside CirclePanel) so that instead of creating a new circle every time the user presses the mouse, if a circle already exists it moves the existing circle to the point where the user pressed (the "presspoint"). If no circle exists, a new one should be created at the presspoint. So now a circle of the same color and size should move around the screen.

   2. Write a method boolean isInside(Point p) for your Circle class that takes a Point and tells whether it is inside the circle. A point is inside the circle if its distance from the center is less than the radius. Recall that the distance between two points (x1,y1) and (x2,y2) is sqrt((x2-x1)²+(y2-y1)².

   3. Now modify the mousePressed method of CirclesListener so that the GUI behaves as follows:
      • If there is no circle (i.e., it is null) and the user presses anywhere, a new (random) circle should be drawn at the presspoint.
      • If there is a circle on the screen and the user presses inside that circle, the circle should go away. (Hint: To make the circle go away, set it to null and repaint.)
      • If there is a circle on the screen and the user presses somewhere else, the circle should move to that point (no change from before).
      So the logic for mousePressed should look like this:

        
        if there is currently no circle
           create a new circle at the presspoint
        else if the click is inside the circle
           make the circle go away
        else
           move the circle to the presspoint
      
        repaint
      

   4. Add bodies for the mouseEntered and mouseExited methods so that when the mouse enters the applet the background turns white, and when it exits the background turns blue. Remember that you can set the background color with the setBackground method.

   5. Now modify the code in CirclePanel.java so that a circle is drawn when the user presses a mouse, but the user can drag it around as long as the mouse button is depressed. If the mouse button is released and then pressed again, it deletes or moves the circle as before. You will need to make the following changes:
      1. In the CirclePanel constructor, make the CirclesListener object listen for both mouse events and mouse motion events. Don't create two separate listeners -- use the same object for both.

      2. Make the CirclesListener class implement the MouseMotionListener interface in addition to the MouseListener interface. This requires two steps:
         • Note in the header that CirclesListener implements MouseMotionListener.
         • Add bodies for the two MouseMotionListener methods, mouseDragged and mouseMoved. In mouseDragged, simply move the circle to the point returned by the getPoint method of the MouseEvent and repaint. (Remember that you already wrote a move() method for the Circle class.) Provide an empty body for mouseMoved.

      Print CirclesListener.java and Circle.java to turn in.

HAND IN:

• Printouts of your programs - make sure that your name is included in the header comments (not just written on)
• Tar the files in your lab3 directory with the command

   
  tar czf lab3.tgz .