AP CSA 1.13 Debugging: Rebuilt Every Time Around

Unit 1: Using Objects and Methods · Lesson 1.13 · Debugging

Rebuilt Every Time Around

Object Creation. 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 1.13 page All Unit 1 lessons

Why debugging is its own skill

The object is constructed inside the loop, so each pass throws away the one built on the previous pass and starts over from scratch. The accumulated total is only ever one item deep, and the loop looks completely reasonable.

What is wrong with it

The Basket is created inside the loop, so every pass replaces it with a brand new empty one and only the last item survives. Move the construction above the loop so one basket collects everything.
The constructor is called without new on the second line, which means the variable is reassigned to a value from a method call rather than a fresh object. Restore the new keyword.
The Basket class is correct.

The program reads

A count, then that many item prices.

The program should print

Two lines: the number of items in the basket and the total of their prices.

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

3
100 200 300

Example 1 output

3
600

Example 2 input

1
50

Example 2 output

1
50

Your answer

Main.java

import java.util.Scanner;

class Basket {
  private int count;
  private int total;

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

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

  public int getCount() {
    return count;
  }

  public int getTotal() {
    return total;
  }
}

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

    Basket basket = new Basket();
    for (int i = 0; i < n; i++) {
      basket = new Basket();
      basket.add(input.nextInt());
    }

    System.out.println(basket.getCount());
    System.out.println(basket.getTotal());
  }
}

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

Ask how many baskets should exist while the program runs. The answer is one, so new Basket() should appear exactly once, and never inside a loop that runs many times.

Hint 2

The starter always reports a count of 1, because the basket printed at the end was created on the final pass and has had exactly one item added to it.

Hint 3

An object created on an earlier pass is not saved anywhere once the variable is pointed at a new one. It is simply gone, along with everything that was added to it.

Where to go next

Rework the Lesson 1.13 page Try the from-scratch version of this lesson

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