AP CSA 3.6 Exercise 1: Boxes That Know About Boxes

Unit 3: Class Creation · Lesson 3.6 · Exercise 1

Boxes That Know About Boxes

Methods: Passing and Returning Object References. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 3.6 page All Unit 3 lessons

Why this one is worth doing

A method that takes an object can change it, and a method that returns one hands back a reference rather than a copy. absorb proves the first and bigger proves the second, and the harness checks both by looking at the original objects afterwards.

What to write

Write a class named Box with a private size.
absorb(other) adds the other box size to this box size, and sets the OTHER box size to 0.
bigger(other) returns whichever of the two boxes has the larger size, returning this box when they are equal.
Return the actual object, not a new Box built from its size. The grader checks that by changing the returned box and looking at the original.

Your program reads

The grader supplies the input. Your class does not read anything.

Your program prints

Your class prints nothing. The grader prints what your methods return.

The class the grader will call

Your answer must declare exactly these, spelled exactly like this. The grader creates the object and calls the methods itself, so a method under a different name is a method it cannot find.

class Box
Box(int size)
int getSize()
void absorb(Box other)
Box bigger(Box other)

Do not write a class named Main and do not write a main method. The grader supplies its own.

Worked examples

These are the cases you can see. There are more you cannot, and they use different values, so an answer that prints these numbers as constants will fail.

Example 1 input

3 9 4

Example 1 output

Example 2 input

5 5 5

Example 2 output

Your answer

Main.java

class Box {
  private int size;

  public Box(int size) {
    this.size = size;
  }

  public int getSize() {
    return size;
  }

  public void absorb(Box other) {
    // Grow by other size, then empty the other box.
  }

  public Box bigger(Box other) {
    return null;
  }
}

Input for the Run button

Run uses the sample main shown below, which is not the one that grades you. Submitting runs your class against the real cases, including hidden ones.

The sample main the Run button uses

public class Main {
  public static void main(String[] args) {
    Box a = new Box(3);
    Box b = new Box(9);
    System.out.println(a.bigger(b).getSize());
    a.absorb(b);
    System.out.println(a.getSize() + " " + b.getSize());
  }
}

Stuck?

Hint 1

Inside Box you can reach other.size directly. A class can see the private members of every object of its own type.

Hint 2

return this; hands back the object the method was called on. It is a reference, so the caller and you are looking at one box.

Hint 3

Returning new Box(size) would pass the size checks and fail the last two lines, because changing the copy leaves the original alone.

Where to go next

Rework the Lesson 3.6 page Pick another Unit 3 lesson

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