CS 161 - Intro to Computer Science

Lab: Circle Drawer

This assignment is all about conditionals and object references! We’ll work with the CircleDrawer class that is given to you. Along the way you’ll write code that uses and manipulates object references, and requires more complex boolean expressions than you’ve written previously.

Student Outcomes

• Practice object interaction (puppeteering)
• Practice using object references
• Practice returning values across objects
• Working with null reference testing

Required Files

The following file(s) have been provided for this homework.

• Lab5_CircleDrawer.zip

Instructions

• Download the CircleDrawer project from the link above and extract its contents, then start BlueJ and open the project. Take a moment to familiarize yourself with the code in the CircleDrawer class. A CircleDrawer contains two instance variables as references to a pair of Circles (named first and second).

• Notice that the CircleDrawer class has two constructors. The default constructor is not given references to the two Circles to manage, it just sets both fields to null. The second constructor is given references to two Circles, and it sets both instance variables appropriately.

  • Using the menus in BlueJ, instantiate two Circle objects. You can name them c1 and c2.
  • Change c1 to green, change size to 50.
  • Change c2 to red, change size to 75. Then move it down by 50 pixels.

  • Now create a new CircleDrawer, called drawer with its second constructor (the one that requires references to two Circles). Input c1 and c2 as your inputs. The new CircleDrawer will now “control” the two circles.

  • Inspect the CircleDrawer object on the workbench, and you see that first and second store references (arrows). Follow the arrows by double-clicking on them. It opens the inspector to the Circles you created!

    

  • Call the drawCircles() method on the drawer, and notice that both Circles appear. (If you call eraseCircles() both disappear!)

    

  • So, this gives us some idea about what a CircleDrawer object does. It’s basically a “puppet master” that bosses around one or two Circles.

• Now go back in the CircleDrawer code and take a look at drawCircles() and eraseCircles(). Their code looks pretty straightforward. Calling them will make both circles visible or invisible, but… that’s assuming that our first and second fields actually refer to actual Circle objects, and not null.

  • Create a CircleDrawer object using its default constructor (that’s the one with no inputs).
  • Inspect it, and see that first and second both point to null (no arrows to circle objects as before).
  • Let’s see what happens if you tried to call drawCircles() or eraseCircles().
  • The program should crash with a "NullPointerException." That’s bad news – it means our code isn’t all that robust.

• Back in the CircleDrawer code, add necessary changes to drawCircles() and eraseCircles() so that no action is taken if either field refers to null. To do this, use an if-statement to check if each field is == or != to the keyword null. Make sure you test these out after you’re done.

• Now modify the drawCircles() method so that it only draws circles with radii strictly greater than 20 and strictly less than 50.

  • But how do you get a Circle’s radius? One way is to go into Circle and making the diameter public instead of private. However this is a practice considered highly dangerous because it exposes diameter to unwanted changes. You should instead add a method in the Circle class that returns its diameter.
  • Again, test this out. Ahem – if your Circle’s diameter is say, 41, it should be drawn.

• Go back in the CircleDrawer class. Add a method called drawLarger() that draws only the larger of the two circles. The smaller one should be hidden. If there’s a tie, then draw both of them.

• Add a method in CircleDrawer called sizeRatio() that returns the ratio of the two circle’s diameters as a double. If one circle has a diameter of 10 and another has a diameter of 3, then this method returns 0.333… You should always divide the smaller diameter by the larger.

  • If either circle is null, simply return 0.

• Add a method in CircleDrawer called addCircle() that inputs a Circle object, and it doesn’t return anything. This method will first erase both Circles. If either of the two instance variables (first, second) are pointing to null, then simply assign the new Circle to that field. If neither reference is pointing to null, then promote the second Circle to the first, then sets the newly-input Circle to be the second. After you’re done, make both Circles visible.

  • To test, I would create 3 circles in BlueJ. Then create a CircleDrawer using the default constructor so that both first and second are null. Then call addCircle() 3 consecutive times (on the 3 circles you created).

• Add a method called swapCircles() that inputs and returns nothing. Calling this method will cause first to now point to whatever second held, and vice versa.

• Add a method called replaceSmallest() that takes a Circle as input. In CircleDrawer, if either of the two instance variables are null, simply add the new circle into the empty slot. If both instance variables are non-null, this method must replace the smaller circle with the new one. We need to respect seniority, though:

  • If first is smaller, we’ll first promote second to take its place, and assign the new circle in second’s place.
  • If second is smaller, we’ll just replace it with the new circle. In the case of a tie, replace second.
  • This method seems to have quite a bit of similarities to addCircle(). You don’t need to, but kudos if you can figure out how to utilize addCircle() and swapCircles() to simplify writing this method.

Grading

This assignment will be graded out of 2 points, provided that:
- You were in attendance and on-time.
- Completed all required methods.

Submitting Your Assignment

Follow these instructions to submit your work. You may submit as often as you’d like before the deadline. I will grade the most recent copy.

• Navigate to our course page on Canvas and click on the assignment to which you are submitting. Click on “Submit Assignment.”

• Upload all files ending in .java from your project folder.

• Click “Submit Assignment” again to upload it.

Credits

Written by Brad Richards with modifications.

Lab Attendance Policies

Attendance is required for lab. Unexcused absence = no credit even if you turned in the lab. Unexcused tardiness = half credit.