CS 161 Lab #4

Goals:

Directions:

1. Download the CircleDrawerLab project and extract its contents, then start BlueJ and open the project.
2. Take a moment to familiarize yourself with the Java code in the CircleDrawer class. A CircleDrawer instance contains references to a pair of circles (first, second), and can make the circles visible or invisible. You can prove this by doing the following:
   1. Create two circles on the object workbench. Make them each visible, and move them in different directions (e.g. moveUp, moveDown etc.) so you can tell them apart. Different colors would help, too.
   2. Create a new CircleDrawer instance. BlueJ will ask you for two circle references to pass to the constructor. Type the names of your circles into the boxes so the new CircleDrawer will "control" the two circles you've created.
   3. Invoke eraseCircles on your CircleDrawer object and notice that they both disappear. Run drawCircles and both will reappear.
   4. Note that you can make a circle visible (or invisible) either by asking the CircleDrawer to do it, or by invoking the appropriate method directly on a circle — both refer to the same circle!
3. Back in CircleDrawer, modify the drawCircles method so that it only draws circles with radii greater than 20 and less than 50. (You can use the getDiameter method in the Circle class to find out how large a circle is — do not change the Circle class!) In other words, instead of always calling makeVisible on both circles, check to see if the radius is in the right range before drawing. Remember that you can use the Boolean and operator (&&) to build more complex tests. Also, when testing your modified method, don't forget that you can still invoke changeSize on the circles you've created on the object workbench.
4. Modify the addCircle method so that it returns a reference to the circle that's being eliminated. This is a little tricky since the return statement needs to be at the end of the method, but if you're not careful, you'll have lost track of the circle by then. Verify that your modified works! For example, you could make the returned circle visible again, or just use the object inspector to verify that it has the same attributes as the old "first" circle did. (When you call the method from BlueJ, you can click the "Get" button in the method result box to put the returned object on the desktop.)

• Write a method that draws only the larger of the two circles.
• Write a new method called replaceSmallest that takes a Circle as a parameter, just like addCircle does. The new method will replace one of the current circles with the one passed to the method but, unlike addCircle, we'll get rid of the smaller of first and second. We need to respect seniority, though: If first is the smaller, we'll promote second to take its place and add the new circle in second place. If second is the smaller, we'll just replace it with the new circle. In the case of a tie, replace second.

  Hint: This is pretty easy if you remember that you can call the original addCircle method from within replaceSmallest.

• Extend the CircleDrawer class to manage three circles.