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.

• Lab_CircleDrawer.zip

Instructions

1. Download the CircleDrawer project from the link above and extract its contents, then start BlueJ and open the project. See how a CircleDrawer contains two instance variables of type Circles (named first and second). This demonstrates that classes like Circle can serve as a variable’s data type!

2. Notice that the CircleDrawer class has two constructors. The default constructor just sets both instance variables to null. The secondary constructor takes to two Circle objects as arguments, and it sets both instance variables appropriately.

   • Using the menus in BlueJ, instantiate two Circles. 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 using its secondary constructor . Input c1 and c2 to be your arguments. The new CircleDrawer will now “control” the two circles.

   • Inspect the CircleDrawer object, and you see that first and second store references (see those arrows?). Double-clicking on one of the arrows, and it opens the inspector to the Circle you just created!

     

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

     

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

   • That’s an example of Object Aliasing that we talked so much about in class! first is aliased to c1 and second is aliased to c2.

3. Now go back in the CircleDrawer source code and take a look at drawCircles() and eraseCircles(). Calling them will make both circles visible or invisible, but that’s assuming that first and second actually refer to actual Circle objects, and not a null reference.

   • Create a CircleDrawer object using its default constructor (that’s the one with no input arguments).
   • Inspect it, and see how first and second both point to null.
   • Try calling drawCircles() or eraseCircles(). The program should crash with a NullPointerException. That’s bad news – it means our code isn’t robust.
4. Add necessary changes to drawCircles() so that you only draw the circle(s) when it’s not 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 by creating a default CircleDrawer object, and calling drawCircles() – if the program doesn’t crash with a NullPointerException, you’ve done your job!
   • Now make the same changes to eraseCircles()

5. Add a new method called strictDrawCircles() method so that it only draws circles with radii strictly greater than 20 and strictly less than 50.

   • Recall that the radius is half of a circle’s diameter. But how do you get a Circle’s diameter? You can’t access that field from this class!
     • Try it: What if you tried to print first.diameter from this strictDrawCircles() method?
   • Ah, this is why we write “getters.” Add a method in the Circle class that simply returns its diameter.
   • After you’ve done that, you need to go back to the strictDrawCircles method and call your new “get diameter” method on first and second using “dot notation!” Use some if statements to ensure that each of your circles diameters are within the specified range, and make them visible!
   • Test this out. Ahem – if your Circle’s diameter is say, 41, it should be drawn. If you just can’t get yours to draw, write some code to print out the calculated radius. Are you falling victim to integer divide?
6. 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 you should draw both of them.
   • Beware of null pointer exceptions!

7. 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 9 and another has a diameter of 3, then this method returns 0.333… You should always divide the smaller diameter by the larger.

   • Now, if either circle is null then you can simply return 1 in this method.
   • Are you always getting a 0 when you’re expecting to get a fraction? It could be due to integer divide, which means you are going to need to type cast.
8. Add a method in CircleDrawer called addCircle() that inputs a Circle object, and it doesn’t return. This method must first erase both Circles (Yep, just call eraseCircles() instead re-writing all that code). Now:
   • If either of the two circles are null, then simply assign the new Circle to that field.
   • If neither of the two are pointing to null, then promote the second Circle to the first, then set the newly-input Circle to be the second.
   • After you’re done with all that, make both Circles visible.
   • To test this method, 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).
   • Here, we see another example of object aliasing! The new Circle that you input is now also referenced by your input parameter!

9. Add a method called swapCircles() that inputs and returns nothing. Calling this method will cause first to now point to what second held, and vice versa. They exchange whatever they’re pointing at (even if they’re pointing to null).

10. Add a method called replaceSmallest() that takes a Circle as input.
    • If either of the two circles are null, simply add the new circle into the empty slot.
    • If neither of the two are pointing to null, this method must replace the smaller circle with the new, given 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.