AP CSA 1.13 Exercise 1: Two Names, One Object

Unit 1: Using Objects and Methods · Lesson 1.13 · Exercise 1

Two Names, One Object

Object Creation and Storage (Instantiation). Write real Java, run it, and submit it to be graded against hidden test cases.

Back to the Lesson 1.13 page All Unit 1 lessons

Why this one is worth doing

A reference variable holds the address of an object, not the object. Copy the variable and you have two names for one thing, which is the single most useful fact in this unit and the one that surprises everybody.

What to write

The Counter class is written for you. Do not change it.
Read an integer start and an integer bump.
Create one Counter with the starting value, then create a SECOND variable that refers to the SAME object.
Call add(bump) through the first variable, then print the value read through the second variable.
Now create a genuinely new Counter from the same starting value, call add(bump) on the first variable again, and print the new object value.

Your program reads

Two integers on separate lines: start, then bump.

Your program prints

Two lines: the value seen through the alias, and the value in the separately created object.

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

10
5

Example 1 output

15
10

Example 2 input

0
1

Example 2 output

1
0

Your answer

Main.java

import java.util.Scanner;

class Counter {
  private int value;

  public Counter(int value) { this.value = value; }

  public void add(int n) { value += n; }

  public int getValue() { return value; }
}

public class Main {
  public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    int start = input.nextInt();
    int bump = input.nextInt();

    // One object with two names, then a second, separate object.
  }
}

Input for the Run button

Run sends whatever is in the input box below. Submitting runs your program against every test case, including hidden ones with different input.

Stuck?

Hint 1

Counter alias = first; does not build anything. It copies a reference, so both names point at one object.

Hint 2

Only the word new creates an object. Count the news in your answer: there should be exactly two.

Hint 3

The second printed line should be the starting value untouched, because add was never called on that object.

Where to go next

Rework the Lesson 1.13 page Pick another Unit 1 lesson

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