AP CSA 1.8 Debugging: Commented Out by Accident

Unit 1: Using Objects and Methods · Lesson 1.8 · Debugging

Commented Out by Accident

Documentation with Comments. 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 1.8 page All Unit 1 lessons

Why debugging is its own skill

A comment is invisible to the compiler, which is the entire point and also the danger. A line that got commented out during some earlier debugging session is still sitting there, perfectly readable to you and completely absent from the program.

What is wrong with it

The line that adds the bonus to the total is commented out, so the bonus is read and never used. Uncomment it.
A block comment that was opened to explain the tax calculation was never closed properly, so it swallows the tax line as well. Close the comment before the tax calculation so that line runs again.
Do not delete the comments themselves. Documentation is worth keeping; it just has to stop eating the code.

The program reads

A base amount, then a bonus.

The program should print

Three lines: the base plus bonus, the tax at 10 percent of that, and the final 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

1000 200

Example 1 output

1200
120
1320

Example 2 input

500 0

Example 2 output

500
50
550

Your answer

Main.java

import java.util.Scanner;

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int base = input.nextInt();
    int bonus = input.nextInt();

    int subtotal = base;
    // subtotal = subtotal + bonus;
    System.out.println(subtotal);

    /* The tax is ten percent, computed on the subtotal
       after the bonus has been included.
    int tax = subtotal * 10 / 100;
    */
    int tax = 0;
    System.out.println(tax);

    System.out.println(subtotal + tax);
  }
}

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

Two slashes disable everything to the end of that line. The bonus line reads like working code and contributes nothing, which is why the subtotal always equals the base exactly.

Hint 2

A block comment runs from its opening to the FIRST closing marker, crossing as many lines as it needs to. Here it opened before the explanation and stayed open through the tax calculation, so that line is prose as far as Java is concerned.

Hint 3

Once the real tax line is live you no longer need the placeholder that sets tax to 0, because you cannot declare tax twice in the same scope.

Where to go next

Rework the Lesson 1.8 page Try the from-scratch version of this lesson

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