Variable scope and lifetime: Correct Solution


Given the following code:

public class Ussdron {
    private int ad = 0;
    private static int bou = 0;

    public void intid(int or) {
        A
        int er = 0;
        bou += or;
        ad += or;
        er += or;
        System.out.println("bou=" + bou + "  ad=" + ad + "  er=" + er);
    }

    public static void main(String[] args) {
        Ussdron u0 = new Ussdron();
        B
        Ussdron u1 = new Ussdron();
        C
        u0.intid(1);
        u1 = new Ussdron();
        u0 = u1;
        u1.intid(10);
        u0.intid(100);
        u1.intid(1000);
    }
}
  1. What does the main method print?
  2. Which of the variables [er, bou, ad, u0, u1] are in scope at A ?
  3. Which are in scope at B ?
  4. Which are in scope at C ?

Solution

  1. Output:

    er=1  bou=1  ad=1
    er=11  bou=10  ad=10
    er=111  bou=110  ad=100
    er=1111  bou=1110  ad=1000
  2. In scope at A : er, bou, ad

  3. In scope at B : er, u0, u1

  4. In scope at C : er, u0, u1


Explanation (which you do not need to write out in your submitted solution):

  1. er is a static variable, bou is an instance variable, and ad is a local variable.

  2. At A , u0 and u1 out of scope because they are local to the main method.

  3. At B , bou is out of scope because it is an instance variable, but main is a static method. ad is out of scope because it is local to intid.

  4. At C , bou is out of scope because it is an instance variable, but main is a static method. ad is out of scope because it is local to intid.


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