Doubt with shadowing

[Gitesh Ramchandani]

source: Dan Chisholm

class A extends Thread {
  String[] sa;
  public A(String[] sa) {this.sa = sa;}
  public void run() {
    synchronized (sa) {System.out.print(sa[0] + sa[1] + sa[2]);}
}}
class B {
  private static String[] sa = new String[]{"X","Y","Z"};
  public static void main (String[] args) {
    synchronized (sa) {
      Thread t1 = new A(sa); t1.start();
      sa[0] = "A"; sa[1] = "B"; sa[2] = "C";
}}}

What is the result of attempting to compile and run the program?

a. Prints: XYZ
b. Prints: AYZ
c. Prints: ABZ
d. Prints: ABC
e. Compile-time error
f. Run-time error
g. None of the above

Ans: d
Doubt: inside run, why not String sa of class A is used, shadowing the String sa of class B?

************************************

class A extends Thread {
  String[] sa;
  public A(String[] sa) {this.sa = sa;}
  public void run() {
    synchronized (sa) {
      while (!sa[0].equals("Done")) {
        try {sa.wait();} catch (InterruptedException ie) {}
    }}
    System.out.print(sa[1] + sa[2] + sa[3]);
}}
class B {
  private static String[] sa = new String[]{"Not Done","X","Y","Z"};
  public static void main (String[] args) {
    Thread t1 = new A(sa); t1.start();
    synchronized (sa) {
      sa[0] = "Done";
      sa[1] = "A"; sa[2] = "B"; sa[3] = "C";
      sa.notify();
}}}

What is the result of attempting to compile and run the program?

a. Prints: XYZ
b. Prints: AYZ
c. Prints: ABZ
d. Prints: ABC
e. Compile-time error
f. Run-time error
g. None of the above

Ans: d
Doubt: inside run, why not String sa of class A is used, shadowing the String sa of class B?


Thanks,
Gitesh
[ February 08, 2008: Message edited by: Gitesh Ramchandani ]

[Josh Buckner]

After the constructor of A is passed sa, the sa of both objects A and B point to the same object. When the code in class B modifies the sa in B, it's modifying the same object on the heap referred to by the reference sa in the object instance of A.

[Dean Jones]

Dear Gitesh, what is the purpose of the wait and notify here?