AP CSA 3.4 Exercise 1: Three Ways to Build It

Unit 3: Class Creation · Lesson 3.4 · Exercise 1

Three Ways to Build It

Constructors. Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 3.4 page All Unit 3 lessons

Why this one is worth doing

Overloaded constructors are chosen by what you pass, not by what you name. Three constructors here take three different argument lists, and the compiler picks between them before your program ever runs.

What to write

Write a class named Rectangle with private width and height.
The two argument constructor sets width and height.
The one argument constructor makes a square with that side length.
The no argument constructor makes a 1 by 1 rectangle.
getWidth, getHeight, getArea and getPerimeter return what their names say. The perimeter is twice the width plus twice the height.

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 Rectangle
Rectangle(int width, int height)
Rectangle(int side)
Rectangle()
int getWidth()
int getHeight()
int getArea()
int getPerimeter()

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 4 5

Example 1 output

Example 2 input

1 1 1

Example 2 output

Your answer

Main.java

class Rectangle {
  private int width;
  private int height;

  public Rectangle(int width, int height) {

  }

  public Rectangle(int side) {

  }

  public Rectangle() {

  }

  public int getWidth() {
    return 0;
  }

  public int getHeight() {
    return 0;
  }

  public int getArea() {
    return 0;
  }

  public int getPerimeter() {
    return 0;
  }
}

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) {
    System.out.println(new Rectangle(3, 4).getArea());
    System.out.println(new Rectangle(5).getArea());
    System.out.println(new Rectangle().getPerimeter());
  }
}

Stuck?

Hint 1

A constructor has no return type, not even void, and its name matches the class exactly.

Hint 2

this(side, side) calls another constructor of the same class, so the square version does not have to repeat the assignments.

Hint 3

Writing any constructor removes the free no argument one Java would have given you, which is why the empty one has to be declared.

Where to go next

Rework the Lesson 3.4 page Pick another Unit 3 lesson

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