AP CSA 3.3 Debugging: The Field That Stayed Zero

Unit 3: Class Creation · Lesson 3.3 · Debugging

The Field That Stayed Zero

Anatomy of a Class. 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 3.3 page All Unit 3 lessons

Why debugging is its own skill

Every getter returns zero, the constructor clearly assigns the values, and the compiler is perfectly happy. The constructor is assigning them to brand new local variables that vanish the instant it finishes, leaving the real fields untouched at their defaults.

What is wrong with it

The constructor declares int width and int height inside its own body. Those are new local variables, not the fields, so the fields keep their default value of 0. Remove the declarations so the assignments reach the fields.
getArea returns the literal 0 rather than computing anything. Make it return the actual area.
The field declarations at the top of the class are correct. Do not add or rename them.

The program reads

A width and a height.

The program should print

Three lines: the width, the height, and the area.

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 4

Example 1 output

3
4
12

Example 2 input

1 1

Example 2 output

1
1
1

Your answer

Main.java

public class Box {
  private int width;
  private int height;

  public Box(int w, int h) {
    int width = w;
    int height = h;
  }

  public int getWidth() {
    return width;
  }

  public int getHeight() {
    return height;
  }

  public int getArea() {
    return 0;
  }
}

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

A type in front of a name is a DECLARATION: it creates something new. Without the type it is an ASSIGNMENT to something that already exists. int width = w; inside the constructor creates a second, temporary width that disappears at the closing brace.

Hint 2

Numeric fields default to 0 and object fields default to null. Getting 0 back from every getter is the signature of fields that were never assigned, and it is worth recognizing instantly.

Hint 3

Deleting two words fixes the constructor. If you renamed the fields or added new ones, back that out first.

Where to go next

Rework the Lesson 3.3 page Try the from-scratch version of this lesson

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