What does the AP CSA 3.9 FRQ practice question ask you to do?

Which Object Am I: (a) A constructor Point(int x, int y) that stores both, using this to tell the parameters from the fields. (b) public int getX() and public int getY() return this point's coordinates. (c) public boolean sameAs(Point other) returns whether both coordinates match. (d) public int distanceTo(Point other) returns the horizontal distance plus the vertical distance, never negative.

Do you write a whole program or just a method for AP CSA 3.9?

Nothing is given. Write a class named Point. Do not write a main method. A hidden driver calls your code, so the value each method RETURNS is what is scored and nothing you print is measured.

How is the AP CSA 3.9 FRQ practice scored?

Out of 4 points, one per rubric row: (a) A constructor Point(int x, int y) that stores both, using this to tell the parameters from the fields. (b) public int getX() and public int getY() return this point's coordinates. (c) public boolean sameAs(Point other) returns whether both coordinates match. (d) public int distanceTo(Point other) returns the horizontal distance plus the vertical distance, never negative. Your answer runs against 6 test cases, 4 of them hidden, and the fraction you pass becomes the score. That is not how a human AP reader marks a rubric, so treat the rows above as the thing being practised.

What is a common mistake on AP CSA 3.9?

distanceTo subtracts without taking the magnitude, so one direction is negative. It is one of 4 error(s) this question is built to catch.

AP CSA 3.9 FRQ Practice: Which Object Am I

Unit 3: Class Creation · Lesson 3.9 · FRQ Practice

Which Object Am I

A free response question in the shape the exam uses: a stated contract, four rubric parts, and no main method handed to you. Worth 4 points.

Back to the Lesson 3.9 page All Unit 3 lessons

Why this question is worth four points

this is the object the method was called on, and it becomes necessary the moment a method takes another object of the same class as a parameter. Inside a comparison of two Points, a bare field name means MINE and the parameter's field means theirs, and getting that backwards produces an answer that is right whenever the two happen to be equal.

What you are given

Nothing is given. Write a class named Point. Do not write a main method.

The question

(a) A constructor Point(int x, int y) that stores both, using this to tell the parameters from the fields.
(b) public int getX() and public int getY() return this point's coordinates.
(c) public boolean sameAs(Point other) returns whether both coordinates match.
(d) public int distanceTo(Point other) returns the horizontal distance plus the vertical distance, never negative.

What the reader is looking for

Write class Point with the constructor and methods described. No main method.
In part (c) and part (d), the bare names x and y mean THIS point. The other point's values come through other.
Part (d) must never be negative. Math.abs on each difference does that without an if.

Worked examples

These show what a correct answer prints. There are more cases you cannot see, and they use different values, so an answer built around just these numbers will fail.

Example 1 input

1 2 4 6

Example 1 output

1
2
false
7
7

Example 2 input

3 3 3 3

Example 2 output

3
3
true
0
0

Your answer

Main.java

class Point {
  // fields

  // (a) Point(int x, int y)

  // (b) getX() -> int, getY() -> int

  // (c) sameAs(Point other) -> boolean

  // (d) distanceTo(Point other) -> int
}

Input for the Run button

How this is scored: your answer runs against every test case, and the fraction it passes becomes your score out of 4. That is not how a human AP reader marks a rubric, so treat the score as a check on whether your code works, and the rubric above as the thing you are actually practising.

Stuck?

Hint 1

The driver measures the distance in BOTH directions. A method that subtracts without taking the magnitude gives a negative answer one of those two times.

Hint 2

A constructor whose parameters are named like its fields needs this on the left of each assignment. Without it, both assignments do nothing at all.

Hint 3

Comparing two points needs both coordinates. Checking only x is correct whenever the y values happen to agree, which is exactly the kind of nearly right the hidden cases are built to catch.

Before you submit

4 mistake(s) that lose points on this question

Each of these is a real error the grader catches. Check your answer against them before you submit, not instead of trying.

distanceTo subtracts without taking the magnitude, so one direction is negative
sameAs compares only the x coordinate
the constructor omits this, so both assignments do nothing
distanceTo measures from the other point to itself, always zero

Where to go next

Rework the Lesson 3.9 page Try the from-scratch exercise for this lesson Try the debugging exercise for this lesson

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