AP CSA 3.8 Debugging: The Local That Ate the Field

Unit 3: Class Creation · Lesson 3.8 · Debugging

The Local That Ate the Field

Scope and Access. 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 3.8 page All Unit 3 lessons

Why debugging is its own skill

The method updates a total, prints a sensible number while it runs, and then the object reports zero. The total it was updating was born inside the method and died there. Scope decides how long a variable lives, and a variable that dies at the closing brace cannot remember anything.

What is wrong with it

addAll declares int total inside the method, so the field total is never touched and stays 0. Remove the local declaration so the field accumulates.
getAverage divides by a count that is declared and left at zero inside the method, which throws an ArithmeticException. Use the field that actually tracks how many values were added.
The field declarations are correct. The fix is to stop hiding them, not to add more.

The program reads

A count, then that many integers.

The program should print

Three lines: the total, the count, and the integer average.

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

4
10 20 30 40

Example 1 output

100
4
25

Example 2 input

1
7

Example 2 output

7
1
7

Your answer

Main.java

public class Tally {
  private int total;
  private int count;

  public Tally() {
    total = 0;
    count = 0;
  }

  public void add(int value) {
    int total = 0;
    total = total + value;
    count++;
  }

  public int getTotal() {
    return total;
  }

  public int getCount() {
    return count;
  }

  public int getAverage() {
    int count = 0;
    return total / count;
  }
}

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

When a local variable has the same name as a field, the local wins everywhere inside that method. The field is still there, still zero, and completely unreachable by that name. This is called shadowing and it never produces a warning by default.

Hint 2

Notice that count works correctly in add and breaks in getAverage. The difference is one line: add does not redeclare it, getAverage does. Compare those two methods directly.

Hint 3

Deleting two declarations fixes both methods. If you find yourself adding a parameter or a new field, you have gone past the bug.

Where to go next

Rework the Lesson 3.8 page Try the from-scratch version of this lesson

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