CPSC 170 Lab 5: Inheritance and Mouse Events

As usual, create a lab5 subdirectory for today's lab, open this document in Netscape, and start emacs.


File Dog.java contains a declaration for a Dog class. Save this file to your directory and study it -- notice what instance variables and methods are provided. Files Labrador.java and Yorkshire.java contain declarations for classes that extend Dog. Save and study these files as well.

File DogTest.java contains a simple driver program that creates a dog and makes it speak. Study DogTest.java, save it to your directory, and compile and run it to see what it does. Now modify these files as follows:

  1. Add statements in DogTest.java after you create and print the dog to create and print a Yorkshire and a Labrador. Note that the Labrador constructor takes two parameters: the name and color of the labrador, both strings. Don't change any files besides DogTest.java. Now recompile DogTest.java; you should get an error saying something like
    ./Labrador.java:18: cannot resolve symbol
    symbol  : constructor Dog  ()
    location: class Dog
    But if you look at line 18 of Labrador.java, it's just a {. In fact, the constructor the compiler can't find (Dog()) isn't called anywhere in this file.
    1. What's going on? (Hint: What did we say about constructors in subclasses?)

    2. Fix the problem (which really is in Labrador) so that DogTest.java creates and makes the Dog, Labrador, and Yorkshire all speak.

  2. Add code to DogTest.java to print the average breed weight for both your Labrador and your Yorkshire. Use the avgBreedWeight() method for both. What error do you get? Why?


    Fix the problem by adding the needed code to the Yorkshire class.

  3. Add an abstract int avgBreedWeight() method to the Dog class. Remember that this means that the word abstract appears in the method header after public, and that the method does not have a body (just a semicolon after the parameter list). It makes sense for this to be abstract, since Dog has no idea what breed it is. Now any subclass of Dog must have an avgBreedWeight method; since both Yorkshire and Laborador do, you should be all set.

    Save these changes and recompile DogTest.java. You should get an error in Dog.java (unless you made more changes than described above). Figure out what's wrong and fix this error, then recompile DogTest.java. You should get another error, this time in DogTest.java. Read the error message carefully; it tells you exactly what the problem is. Fix this by changing DogTest (which will mean taking some things out).

Print DogTest.java, Labrador.java, and Yorkshire.java.

Overriding the Equals Method

File Player.java contains a class that holds information about an athlete: name, team, and uniform number. File ComparePlayers.java contains a skeletal program that uses the Player class to read in information about two baseball players and determine whether or not they are the same player.
  1. Fill in the missing code in ComparePlayers so that it reads in two players and prints "Same player" if they are the same, "Different players" if they are different. Use the equals method, which Player inherits from the Object class, to determine whether two players are the same. Are the results what you expect?

  2. The problem above is that as defined in the Object class, equals does an address comparison. It says that two objects are the same if they live at the same memory location, that is, if the variables that hold references to them are aliases. The two Player objects in this program are not aliases, so even if they contain exactly the same information they will be "not equal." To make equals compare the actual information in the object, you can override it with a definition specific to the class. It might make sense to say that two players are "equal" (the same player) if they are on the same team and have the same uniform number.

Mouse Events

You have written Java programs that respond to GUI events -- a user pressing a button, checking a box, etc. Programs can also respond to mouse events -- moving and clicking the mouse -- that are not tied to any particular GUI component. Java divides these into two categories: mouse event and mouse motion events. A MouseListener (a class that implements the MouseListener interface) must provide bodies for methods corresponding to each of the mouse events. A MouseMotionListener (implementing the MouseMotionListener interface) must provide bodies for methods corresponding to each of the mouse motion events. These events are described in the tables below and on pages 425-426 of the text.

Mouse Event Description
mouse pressed The mouse busson is presed down
mouse released The mouse button is released
mouse clicked The mouse button is pressed down and released without moving the mouse in between
mouse entered The mouse pointer is move onto (over) a component
mouse exited The mouse pointer is moved off of a component

Mouse Motion Event Description
mouse moved The mouse is moved
mouse dragged The mouse is moved while the mouse button is pressed down

  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.

    Print MousePlayPanel.java to turn in.

  2. Files Dots.java and DotsPanel.java contain revised versions of the code in listings 7.23 and 7.24 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 clicked, in addition to setting the draw point you need to update the counter and chnage the dot color to green if the counter is odd, to red if it is even. Then call repaint.
    2. Make the program draw a large dot if the user clicks, and a small dot if the user presses and releases the mouse elsewhere. Continue to alternate between red and green. Remember the difference between a mouse click and a press/release -- a click is released in the same location where it was pressed. Note that a mousePressed event is generated in either case. The current dots have a radius of 6 pixels; modify this to 20 pixels for clicked dots. You'll need to:
      • Change RADIUS from a constant to a variable (also make it lower case).
      • Add code for the mouseClicked event so that a large dot is drawn. Remember that mousePressed will always be called before mouseClicked, so you'll see a small dot briefly until you release the mouse.
    3. Modify your code so that the small dot is drawn where the mouse is released, not where it is pressed. We said that mousePressed is called on both press/releases and clicks; experiment with your program to determine whether or not this is true for mouseReleased.

Print DotsPanel.java to turn in.

What to Turn In

Turn in hardcopy of DogTest.java, Labrador.java, Yorkshire.java, Player.java, MousePlayPanel.java, and DotsPanel.java. Tar your lab5 directory and e-mail it to me with cpsc170 lab5 in the Subject line.