Passing Wrapper Classes as arguments

Question

I have a main class with an Integer object within it as an instance variable. I create a new task that has the Integer object reference passed to it as an argument, the task then increases this objects value by 1. However the objects value never increases apart from within the constructor for the task class, it's as if it's treating the object as a local variable. Why?
mport java.util.concurrent.*;
public class Thousand {
     Integer sum = 0;
     public Thousand() {
          ExecutorService executor = Executors.newCachedThreadPool();
          for(int i = 0; i < 1; i++) {
               executor.execute(new ThousandThread(sum));
          executor.shutdown();
          while(!executor.isTerminated()) {
          System.out.println(sum);
     public static void main(String[] args) {
          new Thousand();
     class ThousandThread implements Runnable {
          public  ThousandThread(Integer sum) {
                    sum = 5;
                    System.out.println(sum);
          public void run() {
               System.out.println("in Thread : ");
}

Answer 1

AlyoshaKaz wrote:
here's the exact queston
(Synchronizing threads) Write a program that launches one thousand threads. Each thread adds 1 to a variable sum that initially is zero. You need to pass sum by reference to each thread.There is no pass by reference in Java
In order to pass it by reference, define an Integer wrapper object to hold sum. This is not passing by reference. It is passing a reference by value.
If the instructor means that you are to define a variable of type java.lang.Integer, this will not help, as Integer is immutable.
If, on the other hand, you are meant to define your own Integer class (and it's not a good thing to use a class name that already exists in the core API), then you can do so, and make it mutable. In this case, passing a reference to this Integer class will allow your method to change the contents of an object and have the caller see that change. This ability to change an object's state and have it seen by the caller is why people mistakenly think that Java passes objects by reference. It absolutely does not.
Bottom line: Your instructor is sloppy with terminology at best, and has serious misunderstanding about Java and about CS at worst. At the very least, I'd suggest getting clarification on whether he means for you to use java.lang.Integer, which might make the assignment impossible as worded, or if you are supposed to create your own Integer wrapper class.
Edited by: jverd on Oct 27, 2009 3:38 PM