Lab 13     ©2011 Felleisen, Proulx, Chadwick, et. al.

User Interactions



In this lab you will learn a little about programming user interactions using the Model-View-Controler pattern for organizing the responsibilities.

The JPT library allows you to concentrate on the key concepts and avoid the pitfalls/details typically associated with GUI programming.

The Model and the View

The diagram below (on the next page) describes the classes already included in this application:

Here is a brief description of the role these files play in the application.

The model

The program deals with balloons (for now just three of them).

The views

We can view the information about a particular Balloon object in several different ways. The BalloonInput interface provides two methods for getting the data needed to construct an instance of a new Balloon.

To display the information about a Balloon object, we can print a String that represents the Balloon object in the console, or paint it in the given window, or display the values of its fields in a GUI.

To get the data from the user that is needed to instantiate a new Balloon we can read from the console, or from a GUI.

        | DisplayPanel |
               / \   
   | BalloonControl            |
+--| BalloonInputView bView    |
|  | Balloon b                 |---------+
|  | BufferredPanel window     |---+     |
|  | JPTCanvas theCanvas       |-+ |     |
|  | SimpleAction paintAction  | | |     v
|  | SimpleAction newAction    | | |  +------------+
|  | SimpleAction cancelAction | | |  | Balloon    |
|  +---------------------------+ | |  +------------+
|  | void paintAction()        | | |  | int x      |
|  | void paintAction()        | | |  | int y      |
|  | void cancelAction()       | | |  | int radius |
|  +---------------------------+ | |  | Color c    |
|                      +---------+ |  +------------+
|                      v           v
|    +-----------------------+   +---------------+
|    | JPTCanvas             |   | BufferedPanel |
|    +-----------------------+   +---------------+
|    | BufferredPanel window |
|    +-----------------------+
|  +-------------------------------------+
|  | interface: BalloonInput             |
|  +-------------------------------------+
|  | Balloon demandBalloon()             |
|  | Balloon requestBalloon()            |
|  |           throws CancelledException |
|  +-------------------------------------+
|                  / \
|                   |
|          +- - - - - - - - - - - - - - - - -+
|          |                                 |
|  +-----------------------+     +---------------------+
|  | GUIBalloonInput       |     | ConsoleBalloonInput |
|  +-----------------------+     +---------------------+
|  | BalloonInputView bGUI |--+  +---------------------+
|  +-----------------------+  |      +---------+
|                             |      | Display |
|                             |      +---------+
+---------------------+       |           / \ 
                      |       |           ---
                      v       v            |
                   | BalloonInputView           |
                   | TextFieldView   xTFV       |
                   | TextFieldView   yTFV       |
                   | SliderView   rSlider       |
                   | ColorView      cVieW       |
                   | TablePanel createDisplay() |

The control

Build a Configuration with main method in the Interactions class and run the code. Note the behavior in response to the various buttons.

Getting Familiar with the Environment

  1. The model

    Read the code for the class Balloon. Add the method
    eraseBalloon which will paint the balloon in a white color
    (Color.white). Make sure you have the examples and tests for this method.

  2. The console input

    Read the code for the method runConsoleInput in the
    class Interactions. Describe to your partner what the method does. Look at the ConsoleBalloonInput class and see how the methods demandBalloon and requestBalloon are implemented. Run the code and see what happens if you type in a wrong data, or when you do not provide any input.

  3. The actions

    Find the code for the action for the New button in the class BalloonControl. Currently, it only reads the data from the bView, constructs a new Balloon with the fields given by the GUI and makes it the new value of the Balloon b.

    Add to this action a call to the method which paints the balloon, from the class Balloon. Make sure it works.

  4. Text input from a GUI

    Find all places where the xTFV is defined or used. It is constructed in the class BalloonInputView. This class also defines the methods demandBalloon and requestBalloon, each of them produces a new instance of a Balloon from the user inputs.

    In the class BalloonControl user input to the BalloonView initializes the value of a Balloon object that represents our model.

    Extend the model in the class BalloonControl with one more balloon. Define a new actions b2Action that initializes and paints the second balloon.

  5. Connecting slider with a text field

    Look at the class BalloonInputView.

    Look how the two TextFieldViews that represent the x and y inputs are defined.

    Define a new TextFieldView named rTFV, to represent the numerical value of the Balloon radius. Give it the default value 20.

    Test the behavior of the slider. Does it have any effect on the balloon? Does it have any effect on the value displayed in the rTFV field? Change the value of the rTFV field. Does it affect the slider? Does it affect the balloon?

    The two views represent the same value and so should be designed to mimic each other. The slider has to act by changing its position whenever a new value is typed into the text field. The value in the text field has to change when the slider is moved, so it reflects its current position.

    In the class BalloonInputView define two new SimpleActions and the corresponding methods — an rTFVaction and a SliderAction. It does not matter what you choose for the label, because we are not going to use the actions with a button.

    The first one void rTFVaction will be invoked when the value in the field rTFV changes. It should set the value of rSlider to the value displayed in the rTFV. To set the state of the rSlider use the method

        rSlider.setViewState("" + rTFV.getViewState());

    The second method void rSliderAction() will be invoked every time the location of the slider (and the value it represents) changes. It must then set the view state of the rTFV calling the method setViewState in a manner similar to the above. If you run the program now, you may be surprised to see that these changes have no effect. Can you think of the way to test that the methods work correctly?

  6. Listening to changes in the values

    Now you have to tell the rSlider and the rTFV to perform this action when their values change. The following two statements have to be added at the end of the method void createViews():


    The first one tells the rTFV to perform the rTFVaction whenever its value changes. The second one tells the rSlider to perform the sliderAction whenever the position of the slider (and thus the value it represents) changes.

    Test that this works. When entering an ew value in the rTFV you need to hit return before the new value registers and affects the slider.

  7. Reporting changes in the model to the view

    Now that you have seen the method setViewState, add such
    method to the class BalloonInputView. Here the method setViewState should take as input an instance of the Balloon and set the GUI display values to the values of the fields of the given Balloon. To see that is works, we need to modify some of the fields of a Balloon instance and invoke the method. Try it.

  8. Adding mouse actions

    In the last part you will control the balloon with the mouse. You need to define what should happen when the mouse is clicked (or dragged, or released, etc.). You need to specify which GUI component should listen to the mouse and the user mouse actions. You then need to connect the MouseListener with the action it should trigger.

    Build a separate frame

    The first thing you need to do is to change the manner in which the GUI is displayed. Look at the code in the class Interactions for the method runBalloonControl(). Copy the methd and rename it runBalloonControlMouse(). Replace the line which calls the method showOKDialog with the following:

        JPTFrame.createQuickJPTFrame("Balloon Control", bc);

    This places the BalloonControl GUI into a window that runs in its own thread, i.e. independently of the rest of the application. That allows the rest of the application to watch out for the mouse movement and clicks inside of the graphics window.

    Run the program and you will see that while the BalloonControl GUI is open, you can use all the other buttons in the GUI built by the Interactions application.

    Define a mouse action

    Start by adding the following import statement at the top of the file:

        import java.awt.event.*;

    The first mouse action you will build will increase the radius of the balloon by ten, every time you click the mouse. All of this is in the class BalloonControl. Start by defining the method protected void click(MouseEvent mevt) which does the following:

    • Print into the console a message that the mouse was clicked.

    • Erase the balloon

    • Increase the balloon radius by 10

    • Set the view state of the BalloonInputView bView to the current values of the balloon. (Only the radius has changed, but it is easier to let the BalloonView do the whole job by invoking the method setViewState.

    • Finally, paint the changed balloon.         

  9. Defining and installing Mouse action adapter

    Install a MouseActionAdapter for the BufferedPanelas follows:

    • After the definition of the BufferedPanel, add the definition:

          public MouseActionAdapter mouseAdapter;

    • Inside of the constructor for the class BalloonControl first initialize the mouseAdapter as follows:

          mouseAdapter = window.getMouseActionAdapter();

    • Add the action to perform when the mouse is clicked as follows:

          // Respond to mouse clicks
             new MouseAction() {
                 public void mouseActionPerformed(MouseEvent mevt){

      At this point you should test that your program runs as you expected.

  10. Tracking the mouse movement

    Finally, you will make the balloon move when the mouse moves. Do all the steps you have done for the clicked action, but do not get a new mouseAdapter. The following code will add the action:

        // track mouse motions
           new MouseAction() {
               public void mouseActionPerformed(MouseEvent mevt){

    Inside of the track method get the coordinates of the mouse as follows:

        b.x = mevt.getX();
        b.y = mevt.getY();

    and see what your program does. (Probably nothing - you still have to erase the old balloon, before you make the changes, paint the new balloon, and as a courtesy, set the view state for the view.) Now you should have fun.

Sound Library

The tunes library complements the world games packages by allowing the programmer to play a tune (or several tunes) in response to either a tick event or a key event. Our goal is to show students how a completely different kind of information can be represented as data and how musical sequences can be composed into tunes.

Each Tune is defined by the instrument that plays the tune and the chord – a collection of notes. Note is defined by its pitch and (optionally) its duration. You can play up to 15 instruments plus the percussions at a time. The interface SoundConstants contains the names for all MIDI instruments, for the 15 instruments in the current MIDI program and mnemonics for the pitches in the middle of the piano keyboard range. The World comes with two buckets for carrying the tunes: tickTunes and keyTunes.

On each tick, the programmer adds to the tickTunes bucket the tunes that should start playing at this time. The library then starts playing these tunes, using the timer for measuring the duration of the notes.

A separate keyTunes bucket is used to play the desired tunes in response to the key press. The tune plays until the key is released.

At the end of the game the final tunes in the tickTune bucket are played for a short time and then both buckets are emptied (the notes they have played are silenced and the bucket contents is cleared).

Initially, the MIDI player is initialized with 15 instruments. You can change the instrument selection using the programChange method. Here the word program refers to the instrument selection for the MIDI synthesizer, not a program for our computer.

Music explorations

The Tunes Demo applet on the SoundLib web pages allows you to try out 30 different instruments and explore the way music can be illustrated graphically.

Run the program and explore what it can do, how it represents the music, how it interacts with the user.

Run the other samples and read the code - especially the parts that are used to generate the music and the sound effects.


The MIDI synthesizers can read MIDI files that represent the sequence of notes to be played, the instruments to play, etc. However, the format is more complex, you need to worry about timing of all notes, specifying the start and stop times, etc. You can also specify the velocity — this represents the volume at which the note should be played. You may want to read about it, but for this assignment make it simple: initialize a sequence of notes or chords to play (for example saved as an ArrayList of Notes or Chords and design a method that will select next chord to play on each tick.

For example, if your note sequence represent the tune Frere Jacques, you may design a canon by adding the notes from the same sequence, offset by some number of ticks. Or you can play it in two octaves concurrently by changing the pitch of each note by the difference between the octaves (13 pitches). Or you can compose Bach-like fugues by playing the same tunes at two different speeds concurrently, playing if forwards and backwards concurrently, and making other such combinations of musical sequences.

Last modified: Tuesday, April 12th, 2011 12:56:05am