AP CSA 3.4 Debugging: The Constructor That Is Not One

Unit 3: Class Creation · Lesson 3.4 · Debugging

The Constructor That Is Not One

Constructors. 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 3.4 page All Unit 3 lessons

Why debugging is its own skill

A constructor has no return type. Write void in front of it and it silently becomes an ordinary method that merely shares the name of the class. Java then supplies the default constructor, your fields stay at zero, and nothing anywhere complains.

What is wrong with it

The two-argument constructor is declared with a return type of void, which makes it a method rather than a constructor. Java therefore builds the object with a default constructor and the fields stay 0. Remove the return type.
The one-argument constructor is supposed to build a square by delegating to the two-argument constructor. It duplicates the assignments by hand instead. Replace the body with a call that reuses the other constructor.
The three getters are correct.

The program reads

A width, a height, then a side length for the square.

The program should print

Five lines: the rectangle width, its height, its area, then the square width and its 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 5

Example 1 output

Example 2 input

1 1 1

Example 2 output

Your answer

Main.java

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

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

  public Rect(int side) {
    width = side;
    height = 0;
  }

  public int getWidth() {
    return width;
  }

  public int getHeight() {
    return height;
  }

  public int getArea() {
    return width * height;
  }
}

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

Read the two declarations side by side. One says public void Rect and the other says public Rect. Only the second is a constructor. The first is a method you could call as r.Rect(3, 4) after the object already exists, which is never what anyone wants.

Hint 2

A square has equal sides, so its height should equal its side, not 0. The starter leaves height at 0, which is why the square area is 0.

Hint 3

this(side, side) as the FIRST statement of a constructor calls the other constructor. Constructor chaining keeps the field assignments in exactly one place.

Where to go next

Rework the Lesson 3.4 page Try the from-scratch version of this lesson

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