Variable scope and lifetime: Correct Solution

Given the following code:

public class Onoc {
    private int es = 0;
    private static int spen = 0;

    public void adcost(int ir) {
        int ong = 0;
        A
        es += ir;
        ong += ir;
        spen += ir;
        System.out.println("es=" + es + "  ong=" + ong + "  spen=" + spen);
    }

    public static void main(String[] args) {
        Onoc o0 = new Onoc();
        B
        Onoc o1 = new Onoc();
        o0.adcost(1);
        o1.adcost(10);
        o1 = o0;
        o0 = o1;
        o0.adcost(100);
        o1.adcost(1000);
        C
    }
}
  1. What does the main method print?
  2. Which of the variables [spen, es, ong, o0, o1] are in scope at A ?
  3. Which are in scope at B ?
  4. Which are in scope at C ?

Solution

  1. Output:

    spen=1  es=1  ong=1
spen=10  es=10  ong=11
spen=101  es=100  ong=111
spen=1101  es=1000  ong=1111

  2. In scope at A : ong, spen, es

  3. In scope at B : ong, o0, o1

  4. In scope at C : ong

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

  1. ong is a static variable, spen is an instance variable, and es is a local variable.

  2. At A , o0 and o1 out of scope because they are local to the main method.

  3. At B , spen is out of scope because it is an instance variable, but main is a static method. es is out of scope because it is local to adcost.

  4. At C , o0 and o1 are out of scope because they are not declared yet. spen is out of scope because it is an instance variable, but main is a static method. es is out of scope because it is local to adcost.

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