AP CSA 3.9 Exercise 1: Returning Yourself

Unit 3: Class Creation · Lesson 3.9 · Exercise 1

Returning Yourself

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

Back to the Lesson 3.9 page All Unit 3 lessons

Why this one is worth doing

this is a reference to the object the method was called on, which means a method can hand the object back. Do that and calls chain, which is a real pattern and not a trick.

What to write

Write a class named Robot with a private name and a private x position.
The constructor takes a name and a starting x, and its parameters must have the SAME names as the instance variables, so this is required to tell them apart.
move(dx) adds dx to x and returns this robot, so calls can be chained.
rename(name) changes the name and returns this robot.
describe returns the name, then an @ sign, then x, with no spaces.

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 Robot
Robot(String name, int x)
Robot move(int dx)
Robot rename(String name)
String getName()
int getX()
String describe()

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

Ada 0 3 4 Bo

Example 1 output

Ada@0
Ada@7
7
Bo@7
Bo
true

Example 2 input

Zip 10 -10 0 Zip

Example 2 output

Zip@10
Zip@0
0
Zip@0
Zip
true

Your answer

Main.java

class Robot {
  private String name;
  private int x;

  public Robot(String name, int x) {
    // Both parameters are shadowing. Use this.
  }

  public Robot move(int dx) {
    return null;
  }

  public Robot rename(String name) {
    return null;
  }

  public String getName() {
    return "";
  }

  public int getX() {
    return 0;
  }

  public String describe() {
    return "";
  }
}

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) {
    Robot r = new Robot("Ada", 0);
    System.out.println(r.move(3).move(4).describe());
    System.out.println(r.rename("Bo").describe());
  }
}

Stuck?

Hint 1

this.name = name; assigns the parameter to the field. Writing name = name; assigns the parameter to itself and leaves the field empty.

Hint 2

return this; is what makes r.move(3).move(4) work: the first move hands back the same robot for the second one to be called on.

Hint 3

The last check compares references with ==, so returning a new Robot instead of this prints false.

Where to go next

Rework the Lesson 3.9 page Pick another Unit 3 lesson

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