Fundamentals of Computer Science II (CSC-152 99F)

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Notes on Assignment 2

Rather than copying the whole set of instructions, I've just included the answers for each section. Note that I'm using the revised versions of these labs, which are slightly different from the ones that you received at the beginning of the semester.

I'm probably a little bit faster at this stuff than you folks, but not much (since most of this is copying and running). It took me about an hour of repetitious work. I apologize for the repetition, but I do think that it serves some purposes. I hope you found some interesting things in the lab.

O2

O2.1

Step 1. 

(2.0,3.0)
  distance from origin: 3.605551275463989
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 4.036087214122113
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 6.126989472816156
Shifting left by 10.2
(-7.499999999999999,5.5)
  distance from origin: 9.300537618869136

Step 2. 

(0.0,0.0)
  distance from origin: 0.0
Shifting right by 0.7
(0.7,0.0)
  distance from origin: 0.7
Shifting up by 2.5
(0.7,2.5)
  distance from origin: 2.596150997149434
Shifting left by 10.2
(-9.5,2.5)
  distance from origin: 9.82344135219425

By default, the point starts at (0,0).

Step 3.

Added my name to the author list.

Changed the version and date. Updated the import statement. (No, I didn't expect you to list these.)

Changed the class name and ending comment.

Changed the line that reads 

Point pt = new Point();

to read 

NewPoint pt = new Point();

Step 4. 

(0.0,0.0)
  distance from origin: 0.0
Shifting right by 0.7
(0.7,0.0)
  distance from origin: 0.7
Shifting up by 2.5
(0.7,2.5)
  distance from origin: 2.596150997149434
Shifting left by 10.2
(-9.5,2.5)
  distance from origin: 9.82344135219425

The output appears to be the same.

Step 5. 

(1.0,1.0)
  distance from origin: 1.4142135623730951
Shifting right by 0.7
(1.7,1.0)
  distance from origin: 1.972308292331602
Shifting up by 2.5
(1.7,3.5)
  distance from origin: 3.891015291668744
Shifting left by 10.2
(-8.5,3.5)
  distance from origin: 9.192388155425117

Because the zero-parameter constructor of NewPoint uses the zero-parameter constructor of Point to create the ``base'' object, updates to the constructor for Point affect NewPoint.

Step 6.

Replaced the line that read 

    base = new Point();

with 

    base = new Point(0,0);

That seemed to be enough.

Step 7.

Even without inheritance, it is possible to base one object upon another; it's just somewhat time-consuming to do so.

O2.2

Step 1. 

(2.0,3.0)
  distance from origin: 3.605551275463989
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 4.036087214122113
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 6.126989472816156
Shifting left by 10.2
(-7.499999999999999,5.5)
  distance from origin: 9.300537618869136

Step 2.

Done. No error messages.

Step 3.

Updated the import statement.

Replace the line that reads 

Point pt = new Point(2,3);

with 

ExtendedPoint pt = new ExtendedPoint(2,3);

Step 4. 

PointFun.java:17: Wrong number of arguments in constructor.
    ExtendedPoint pt = new ExtendedPoint(2,3);
                       ^
1 error

We're trying to use a two-parameter constructor, and we haven't defined one.

Step 5. 

(0.0,0.0)
  distance from origin: 0.0
Shifting right by 0.7
(0.7,0.0)
  distance from origin: 0.7
Shifting up by 2.5
(0.7,2.5)
  distance from origin: 2.596150997149434
Shifting left by 10.2
(-9.5,2.5)

If we use the default constructor for a subclass, then we end up with the default constructor for the superclass.

Step 6. 

PointFun.java:17: No constructor matching ExtendedPoint() found in class ExtendedPoint.
    ExtendedPoint pt = new ExtendedPoint();
1 error

We just created a two-parameter constructor. Java doesn't let us use the ``default'' zero-parameter constructor if there is another constructor. Since we haven't created a zero-parameter constructor, the compiler is unhappy with us.

Step 7.

It compiled successfully. 

(1.0,1.0)
  distance from origin: 1.4142135623730951
Shifting right by 0.7
(1.7,1.0)
  distance from origin: 1.972308292331602
Shifting up by 2.5
(1.7,3.5)
  distance from origin: 3.891015291668744
Shifting left by 10.2
(-8.5,3.5)
  distance from origin: 9.192388155425117

Although the default position for Points is (0,0), the default position for ExtendedPoints is (1,1). This means that you can give different defaults in subclasses.

Step 8. 

(4.0,1.0)
  distance from origin: 4.123105625617661
Shifting right by 0.7
(4.7,1.0)
  distance from origin: 4.805205510693586
Shifting up by 2.5
(4.7,3.5)
  distance from origin: 5.860034129593445
Shifting left by 10.2
(-5.499999999999999,3.5)
  distance from origin: 6.519202405202647

We can also use the two-parameter constructor. However, we also note that Java has some trouble with simple mathematics (using 5.49999... instead of 5.5).

Step 9.

It still compiles and executes. The output is still the same. This means that super(x,y) has a similar effect to setValue(x,y). As we know from the discussion, this is because we're calling the corresponding constructor of the superclass.

O2.3

Step 1. 

(2.0,3.0)
  distance from origin: 3.605551275463989
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 4.036087214122113
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 6.126989472816156
Shifting left by 10.2
(-7.499999999999999,5.5)
  distance from origin: 9.300537618869136

Step 2.

The results are the same as in the previous step. That's what I expected (more or less), since I didn't use the toString method.

Step 3. 

x=2.0, y=3.0
  distance from origin: 3.605551275463989
Shifting right by 0.7
x=2.7, y=3.0
  distance from origin: 4.036087214122113
Shifting up by 2.5
x=2.7, y=5.5
  distance from origin: 6.126989472816156
Shifting left by 10.2
x=-7.499999999999999, y=5.5
  distance from origin: 9.300537618869136

This seems to work fine (although the output is slightly different since we've chosen to write ``x=...; y =...'' rather than ``(...,...)''.

Step 4. 

X:2.0, Y:3.0
  distance from origin: 3.605551275463989
Shifting right by 0.7
X:2.7, Y:3.0
  distance from origin: 4.036087214122113
Shifting up by 2.5
X:2.7, Y:5.5
  distance from origin: 6.126989472816156
Shifting left by 10.2
X:-7.499999999999999, Y:5.5
  distance from origin: 9.300537618869136

It seems a little bit odd to be using this.xcoord, given that I haven't defined a field named xcoord. However, I understand that I inherit this field from Point.

Step 5.

It seems a little bit more efficient to use the fields directly, since you don't have an intervening method call. However, if a subclass uses the fields directly and the superclass later changes the names of the fields (e.g., xcoord to xCoord, then the subclass will stop working.

O2.4

Step 1. 

(2.0,3.0)
  distance from origin: 3.605551275463989
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 4.036087214122113
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 6.126989472816156
Shifting left by 10.2
(-7.499999999999999,5.5)
  distance from origin: 9.300537618869136

Looks the same as it has in the past few experiments. This is getting somewhat repetitious :-).

Step 2.

The distanceFromOrigin method has changed (it looks a little bit simpler). It was the square root of the sum of the squares of the x and y coordinates in Point. Now it's the sum of the x and y coordinates.

Step 3. 

(2.0,3.0)
  distance from origin: 5.0
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 5.7
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 8.2
Shifting left by 10.2
(-7.499999999999999,5.5)
  distance from origin: -1.9999999999999991

Those distances are certainly different.

O3

O3.2

Step 1. 

(2.0,3.0)
  distance from origin: 3.605551275463989
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 4.036087214122113
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 6.126989472816156
Shifting left by 10.2
(-7.499999999999999,5.5)
  distance from origin: 9.300537618869136

Still looks the same.

Step 2.

Updated the import statement. (Also added one for PointPrinter which I'd forgotten to add in the previous step. It's good that Java is casual about such things.)

Replaced the line that read 

    Point pt = new Point(2,3);

with one the reads 

    NewPoint pt = new NewPoint(2,3);

Step 3.

Java complains that it can't convert NewPoints to Points in all the lines that read 

    printer.print(out,pt);

It's interesting that in the previous experiment (O2), we were able to change all the instances without receiving any error messages but we were not able to do so here. What's the difference? This time, we're calling a method using pt as a parameter.

Step 4.

As recommended, I added 

  /**
   * Print a point using a particular output object.
   */
  public void print(SimpleOutput out, NewPoint pt) {
    out.println("(" + pt.getX() + "," + pt.getY() + ")");
    out.println("  distance from origin: " +
            pt.distanceFromOrigin());
  } // print(SimpleOutput, NewPoint)

Oh the wonders of overloading! Now I have two methods with the same name that do basically the same thing.

Step 5.

Successful. We needed a print method that took a NewPoint as a parameter. There wasn't one in step 3, but we added one in step 4.

Step 6. 

(2.0,3.0)
  distance from origin: 3.605551275463989
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 4.036087214122113
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 6.126989472816156
Shifting left by 10.2
(-7.499999999999999,5.5)

Looks okay.

Step 7.

It was somewhat painful to have to make a copy of the print method, given that NewPoint provides exactly the same methods as Point. Can't Java figure that out?

O3.3

Step 1. 

(2.0,3.0)
  distance from origin: 3.605551275463989
Shifting right by 0.7
(2.7,3.0)
  distance from origin: 4.036087214122113
Shifting up by 2.5
(2.7,5.5)
  distance from origin: 6.126989472816156
Shifting left by 10.2
(-7.499999999999999,5.5)
  distance from origin: 9.300537618869136

This is getting repetitious.

Step 2.

Changed the import statement.

Changed the line that read 

    Point pt = new Point(2,3);

to read 

    ExtendedPoint pt = new ExtendedPoint(2,3);

Step 3.

It compiled, even though it didn't do so in the previous experiment, and it seems that we did something very similar (replacing Point with something that provides the same methods as Point).

Step 4.

It seems to be a lot easier to use a subclass than something that just mimics the original class.

History

Sunday, 19 September 1999

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