EBoard 12: Exceptional programming

Warning This class is being recorded. At least I think it is.

Approximate overview

• Administrivia
• Questions
• Two design problems
• Lab

Administrivia

Introductory notes

• Today’s lab is somewhat rewritten. Fingers crossed that everything works okay.
  • Make sure that you are using the repo named something like lab-exceptions-vscode.
• Note that you can usually get VSCode to format your code to meet my expectations using the command “Format Document”.
• Your graders ask that you not clog your repositories with cruft.
  • Just the README and the .java files for the specific project.
• When you are opening a Java project in VSCode, use the folder above the src folder.

Upcoming Token activities

Academic

• CS Extras, Thursday 21 Sept 2023, 4:15 pm, Science 3821. Study Abroad in CS.
• GrinTech talk w Karla Erickson and Elizabeth Cave, Thursday, 21 Sept 2023, HSSC N1112.
• Mentor session Sunday.

Cultural

Peer

Wellness

Misc

Upcoming work

• Thursday: MP3
  • Bonus token to the first person who spends four hours on MP3 and TeamsChats me to tell me how far they got.
  • Two people have gotten to four hours. They say that there are some important mental hurdles to overcome.
  • We may want to try to get things to click.
• Friday: More readings, today’s lab

Questions

You can ask questions about anything related to the class! Self gov says that you should ask those questions.

Java

Administrative stuff

Do we get to choose which way I meet each LA?

If I have a paper-based LA, then you should at least try the first time on paper. If that fails, you can provide me with evidence.

Those may start appearing this weekend.

MP2

MP3

What should I get if I center boxed text?

It depends on the particulars. Let’s say you build new Centered(new BoxedBlock(new TextLine("Hello")), 21) and put it above new TextLine("012345678901234567890123456789").`

          +-----+
          |Hello|
          +-----+
   012345678901234567890123456789

On the other hand, if you built a block with an expression like new BoxedBlock(new Centered(new TextLine("Hello"), 21)), that should look more like

   +---------------------+
   |        Hello        |
   +---------------------+
   012345678901234567890123456789

Order of operations matters!

What should Centered and RightJustified do if the block is too big (or the size is too small)?

This will also truncate.

What should new HorizontallyFlipped(new TextLine("Hello")) look like?

    olleH

What should new HorizontallyFlipped(new BoxedBlock(new TextLine("Hello"))) look like?

    +-----+
    |olleH|
    +-----+

What should new VerticallyFlipped(new VerticallyCompose(new TextLine("Hello"), new TextLine("Goodbye"))) look like?

    Goodbye
    Hello

When centering with an odd number of surrounding spaces, do you care if we’re slightly left or slightly right?

No. But that may make my testing harder.

Can you give us some hints as to how we might write Truncate?

Truncate will have two fields, * TextBlock block * int w

Focus on how you write row, height, width in terms of the row, height, and width of your one field.

If I truncate a block, what’s the height of the new block? It’s the same.

If I truncate a block to width w, what’s the width of the truncated block? It’s w.

What is row i of the truncated block? The first w characters or row i of the underlying block. (Usually)

How would you phrase “The first w characters or row i of the underlying block” in Java?

Write a for loop to grab the characters from row i of the this.block.row(i).

return this.block.row(i).substring(0, w);

Special case: The width of the ith row may be less than w. So we need to pad it with spaces (at the right) in that case.

Putting it all together

    /**
     * Blocks of text truncated to a specified width.
     */
    public class Truncated implements TextBlock {
      // +--------+------------------------------------------------------
      // | Fields |
      // +--------+

      /** 
       * The block we are truncating.
       */
      TextBlock block;

      /**
       * The width of the truncated block.
       */
      int width;

      // +--------------+------------------------------------------------
      // | Constructors |
      // +--------------+
      


      // +---------+-----------------------------------------------------
      // | Methods |
      // +---------+

      public int width() {
        return this.width;
      } // width()

      public int height() {
        return this.block.height;
      } // height()

      public String row(int i) {
        return "";  // STUB
      } // row(int)
    } // class Truncated

Testing

When testing, do I write one test method per method I’m testing or should I write multiple methods?

It depends on how much you like green checkmarks.

It’s harder to track down bugs if you put everything in one test method, so I generally recommend separate test methods, although I don’t always follow my own recommendation.

When testing, why would you have multiple test classes?

Most developers prefer one test class per normal class.

Exceptions

Two design problems

In today’s lab, we’re working with quadratic polynomials (of the form \(ax^2 + bx + c\)).

TPS

On paper, by yourself: What does class Quadratic, which we use to represent quadratic polynomials look like?

• Fields - Don’t forget to include types
• Constructor(s) - Don’t forget to include parameter types
• Methods - Don’t forget to include parameter types and return types. If the name isn’t obvious, you may want to explain them, too.

What I saw

• Two approaches to fields:
  • three fields, a, b, and c, or coeff1, coeff2, and coeff3.
  • We tend to think of quadratics with a, b, and c, so those are reasonable field names.
  • One field which is an array of three values int[] coeffs = new int[3];
  • Using an array may be a little harder because we’ll have to spend mental energy remembering whether coeffs[0] is the coefficient of the x^2 term or the constant (or, given the design of light switches in Noyce, the x term).
  • Note that we normally make our fields package or protected.
• What type should these fields be? int, double, BigInteger, BigDouble, …
  • double seems okay.
• Methods
  • evaluate(double x) - find the value of the polynomial
  • Polynomial derivative() - find the derivative
  • double smallerRoot() - these are sometimes complex; we’ll need to think of how to deal with that solution.
  • double biggerRoot()
  • double[] realRoots() - could return the empty array if there are no real roots, a size-one array if there’s one root, and a size-two array if there are two roots. Might also work with a list, which could be easier to handle.
  • void setA() - should we be able to mutate our structures?
  • void setB() - perhaps we should just create a new one
  • void setC()
  • double getA() - Useful. May not be strictly necessary.
  • double getB()
  • double getC()
  • String toString()
  • String toString(double x)
  • Quadratic add(Quadtratic other)

Lab

https://rebelsky.cs.grinnell.edu/Courses/CSC207/2023Fa/labs/exceptions.html

Compute square roots with Math.sqrt.

Surprise! There’s nothing to turn in today.