AP CSA 3.7 Exercise 1: One Counter for Everybody

Unit 3: Class Creation · Lesson 3.7 · Exercise 1

One Counter for Everybody

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

Back to the Lesson 3.7 page All Unit 3 lessons

Why this one is worth doing

A static variable belongs to the class, so every object shares the one copy. That is exactly what a ticket counter needs and exactly what a ticket holder name must never be.

What to write

Write a class named Ticket. Each ticket has its own holder name and its own number.
Keep a STATIC count of how many tickets have been created. It starts at zero.
The constructor increases that count and gives this ticket the new count as its number, so the first ticket ever made is number 1.
getIssued is a static method returning the count, and it must be callable as Ticket.getIssued() without any object.
reset is static and sets the count back to zero. It does not change any ticket that already exists.

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 Ticket
Ticket(String holder)
String getHolder()
int getNumber()
static int getIssued()
static void reset()

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
Ada
Bo
Cy
Dee

Example 1 output

Example 2 input

1
Sol
Ray

Example 2 output

Your answer

Main.java

class Ticket {
  // One of these belongs to the class. Two of them belong to each object.

  public Ticket(String holder) {

  }

  public String getHolder() {
    return "";
  }

  public int getNumber() {
    return 0;
  }

  public static int getIssued() {
    return 0;
  }

  public static void reset() {

  }
}

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) {
    Ticket a = new Ticket("Ada");
    Ticket b = new Ticket("Bo");
    System.out.println(a.getNumber() + " " + b.getNumber());
    System.out.println(Ticket.getIssued());
  }
}

Stuck?

Hint 1

The holder name and the ticket number are per object, so they are NOT static. Only the running count is.

Hint 2

A static method cannot read an instance variable, because there is no object for it to read one from. If getIssued will not compile, the count is not static.

Hint 3

reset only changes the shared count. A ticket already holding number 3 keeps it, which is the point of the last printed line.

Where to go next

Rework the Lesson 3.7 page Pick another Unit 3 lesson

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