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Mediumjavastatic

~90s

A static field shared by every object

What does this Java program print?

public class Main {
    static int count = 0;
    int id;

    Main() {
        count++;
        id = count;
    }

    public static void main(String[] args) {
        Main p = new Main();
        Main q = new Main();
        Main r = new Main();
        System.out.println(p.id + " " + r.id + " " + Main.count);
    }
}
Show the answer and the worked solution

Correct answer: C.

1 3 3

Worked solution

  1. There is a single count shared by every object, and each constructor call raises it: after three objects it is 3.
  2. Each object has its own id, fixed at construction time, so p.id = 1, q.id = 2 and r.id = 3.
  3. The program prints p.id, r.id and count, that is 1 3 3.

Answer: 1 3 3.

The idea

A static field exists once per class, an instance field once per object. Reading a snapshot of the static into an instance field is the standard way to number objects as they are created.

Shortcut. Main.count is shared and ends at 3 after three constructions, ruling out "1 1 1" and "1 3 1"; p.id was fixed at 1 when p was made.

Common trap. "3 3 3" treats id as a live view of count; each constructor copies the current count into id, so p keeps its snapshot of 1.

Programming Fundamentals

22 of 40

Level

Medium

Target time

~90s

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A static field shared by every object — Aptitude Question

A static field shared by every object: What does this Java program print? Answer and worked solution on CodeKairo.

Question

What does this Java program print?

public class Main {
    static int count = 0;
    int id;

    Main() {
        count++;
        id = count;
    }

    public static void main(String[] args) {
        Main p = new Main();
        Main q = new Main();
        Main r = new Main();
        System.out.println(p.id + " " + r.id + " " + Main.count);
    }
}

Options

  1. 1 1 1
  2. 1 3 1
  3. 1 3 3
  4. 3 3 3
Show the answer and the worked solution

Correct answer: C. 1 3 3

Worked solution

  1. There is a single count shared by every object, and each constructor call raises it: after three objects it is 3.
  2. Each object has its own id, fixed at construction time, so p.id = 1, q.id = 2 and r.id = 3.
  3. The program prints p.id, r.id and count, that is 1 3 3.

Answer: 1 3 3.

The idea

A static field exists once per class, an instance field once per object. Reading a snapshot of the static into an instance field is the standard way to number objects as they are created.

Shortcut. Main.count is shared and ends at 3 after three constructions, ruling out "1 1 1" and "1 3 1"; p.id was fixed at 1 when p was made.

Common trap. "3 3 3" treats id as a live view of count; each constructor copies the current count into id, so p keeps its snapshot of 1.

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