AP CSA 3.1 Debugging: Two Copies of One Rule

Unit 3: Class Creation · Lesson 3.1 · Debugging

Two Copies of One Rule

Abstraction and Program Design. This is not a blank editor: it is someone else's attempt. Find what is wrong, fix it, and submit to be graded against hidden test cases.

Back to the Lesson 3.1 page All Unit 3 lessons

Why debugging is its own skill

The tax rate is written out twice. Someone updated one copy and not the other, so two methods that should always agree now disagree, and which answer you get depends on which method you happened to call. This is the entire argument for abstraction, in eleven lines.

What is wrong with it

The tax rate 8 percent appears in two different methods, and one of them still uses the old rate of 5 percent. Rather than fixing the number in both places, give the class ONE method that computes the tax and have both callers use it. Fixing the duplicate by editing the second copy is not the fix this exercise wants.
getTotal is supposed to be the subtotal plus the tax. It currently recomputes the subtotal by hand instead of calling getSubtotal, so it misses the quantity entirely. Have it call the method that already knows how.
Do not change getSubtotal. It is the one method that is already right.

The program reads

A unit price in cents, then a quantity.

The program should print

Three lines: the subtotal, the tax, and the total.

Worked examples

These show what the FIXED program should print. There are more cases you cannot see, and they use different values, so patching around just these numbers will fail.

Example 1 input

250 4

Example 1 output

1000
80
1080

Example 2 input

100 1

Example 2 output

100
8
108

Your answer

Main.java

public class Receipt {
  private int unitPrice;
  private int quantity;

  public Receipt(int unitPrice, int quantity) {
    this.unitPrice = unitPrice;
    this.quantity = quantity;
  }

  public int getSubtotal() {
    return unitPrice * quantity;
  }

  public int getTax() {
    return unitPrice * quantity * 8 / 100;
  }

  public int getTotal() {
    return unitPrice + unitPrice * 5 / 100;
  }
}

Input for the Run button

Run sends whatever is in the code box below, bugs and all, so you can see the crash or the wrong answer for yourself before you fix anything.

Stuck?

Hint 1

Count how many times the number 8 should appear in a correct version of this class. The answer is once. Every additional copy of a rule is a place where a future edit can be forgotten, which is exactly what already happened here.

Hint 2

getTotal has two problems at once: the wrong rate and a subtotal that ignores quantity. Calling getSubtotal() and getTax() fixes both at the same time, which is the point.

Hint 3

A method calling another method of the same object needs no dot and no name in front of it. Just getSubtotal().

Where to go next

Rework the Lesson 3.1 page Try the from-scratch version of this lesson

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