Lab 10: Inheritance (I)

Transcription

1 CS Java Programming Spring 2005 Dr. Zhizhang Shen Background Lab 10: Inheritance (I) In this lab, we will try to understand the concept of inheritance, and its relation to polymorphism, better; and will also see some impact of inheritance on GUI design and implementation. 1 Dog? Which one? The file Dog.java contains a declaration for a Dog class, while the files Labrador.java and Yorkshire.java contain declarations for classes that extend Dog. Finally, file DogTest.java contains a simple driver program that creates a dog and makes it speak. 1. Save all the files in your directory, compile and run the program to see what it does. 2. Check out those four files and then draw an inheritance hierarchy for the involved classes, namely Dog, Labrador and Yorkshire. Now do the following: 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 are 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 (a) What s going on? (Hint: What did we say about constructor inheritance by the child classes?) (b) Fix the problem (which really is in Labrador) so that DogTest.java creates an object of Dog, Labrador, and Yorkshire. Then make all of them 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. 1

2 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 should look like the following. For more about the concept of abstract methods, have a look at pp.294 of the textbook. public abstract int avgbreedweight(); It makes sense for this method 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 Labrador 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 taking some things out of the DogTest method. 4. Discuss the usage of various speak() methods within the method of DogTest and their relation with polymorphism. For example, how do you make a labrador, a yorkshire, or any dog, speak? Similarly discuss the relationship among three methods all called avgbreedweight(). 2 Another type of employees The files Firm.java, Staff.java, StaffMember.java, Volunteer.java, Employee.java, Executive.java, and Hourly.java illustrate inheritance and polymorphism. In this exercise you will add one more employee type to the class hierarchy, namely the one of Commission, who is an hourly employee but also earns a commission on sales. Naturally, it will be derived from the Hourly class. 1. Save all the files in your directory, compile and run the program to see what it does. 2. Check out all the involved files and then draw an inheritance hierarchy for the involved classes, namely Firm.java, Staff.java, StaffMember.java, Volunteer.java, Employee.java, and Executive.java. Now, write a class named Commission with the following features: 1. It extends the Hourly class. 2. It has two instance variables (in addition to those inherited): one is for the total sales the employee has made (type double) and the second is for the commission rate for the employee (the commission rate will be type double and will represent the percent (in decimal form) commission the employee earns on sales (so.2 would mean the employee earns 20% commission on sales)). 2

3 3. The constructor takes six parameters: the first five are the same as for Hourly (name, address, phone number, social security number, hourly pay rate) and the last one is the commission rate for the employee. The constructor should call the constructor of the parent class with the first five parameters then use the 6th to set the commission rate. 4. One additional method is needed: public void addsales (double totalsales) that adds the parameter to the instance variable representing total sales. 5. You also need to add in two methods for this class: The pay method calls the pay method of the parent class to compute the pay for hours worked, then add in the commission on sales. (See the pay method in the Executive class, as an example.) The total sales should be set back to 0 (You don t need to set the hoursworked back to 0. Question:Why not?). The tostring method calls the tostring method of the parent class then add in the total sales. 6. Discuss the usage of the above pay and tostring methods in terms of polymorphism. For example, when printing an hourly and an executive employee, which tostring method is used, and why? Same questions for the pay() method. To test your class, update Staff.java as follows: 1. Increase the size of the array to Add two commissioned employees to the stafflist. You simply make up the required names, addresses, phone numbers and social security numbers. Have one of the employees earn $6.25 per hour and 20% commission and the other one earn $9.75 per hour and 15% commission. 3. For the first additional employee you added, put the hours worked at 35 and the total sales $400; for the second, put the hours at 40 and the sales at $950. Compile and run the program. Make sure it works properly. 3 Circles again We once drew 10 randomly positioned circles in Lab 6. We also did another slightly changed lab in Lab 9. We now come back again to these circles. 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 click. Each circle replaces the one before it. The code to handle the mouse clicks and do the drawing is in CirclePanel.java. Save these files to your directory, compile them and run them and experiment with the GUI. 3

4 Now we will write a new program that creates a new circle each time, which you can tell because each circle is of a different color and size. More specifically, 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 clicks, it moves the existing circle to the clickpoint if a circle already exists. On the other hand, if no circle exists, a new one should be created at the clickpoint. So now a circle of the same color and size should move around the screen. You may go through the following steps to get this assignment done. 1. Write a method boolean isinside(point p) for your Circle class that takes a Point and tells whether this Point is inside the circle, that is, if its distance from the center is less than the radius. (Recall that the distance between two points (x1,y1) and (x2,y2) is (x2 x1) 2 +(y2 y1) Modify the mousepressed method of CirclesListener so that the GUI behaves as follows: (a) If there is no circle (i.e., it is null) and the user clicks anywhere, a new (random) circle should be drawn at the click point. (b) If there is a circle on the screen and the user clicks inside that circle, the circle should go away. (Hint: To make the circle go away, set it to null and repaint.) (c) If there is a circle on the screen and the user clicks 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 click point else if the click is inside the circle make the circle go away else move the circle to the click point repaint 3. 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. 4. (Extra point) 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 4

5 is depressed. If the mouse button is released and then pressed again, a new circle is created, which can then be dragged around. You will need to make the following changes: (a) 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. (You may have already done this in a previous exercise.) (b) In the CirclePanel constructor, create a CirclesListener object and make it listen for both mouse events and mouse motion events. (c) Make the CirclesListener class implement the MouseMotionListener interface in addition to the MouseListener interface. This requires two steps: i. Note in the header that CirclesListener implements MouseMotionListener. ii. 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. Provide an empty body for mousemoved. 5