Stuck with wait() and notify()

Question

Hi,
I have a problem understanding synchronous access.
I have an object that should provide a list of data. The list is used later in my code, but it would increase performance to load it in a separate thread at the beginning. However it is possible that the list is not ready when the first other object tries to access the list.
I thought that this code should solve the problem
public class SingleObject implements Runnable {
     ArrayList list;
     public SingleObject(){
          list = new ArrayList();
     public synchronized void run() {
          try {
               makeALongProc(100);
          } catch (InterruptedException e) {
               e.printStackTrace();
          notify();
     private synchronized void makeALongProc(int size) throws InterruptedException{
               System.out.println("...doing something ...");
               Thread.sleep(5000);
               for (int i = 0 ; i < size ; i++ ){
                    list.add(""+i);
               System.out.println("finished!");
               notifyAll();
     public synchronized String getValueAt(int pos) throws InterruptedException{
          wait();
          return list.get(pos).toString();
     public synchronized List getList() throws InterruptedException{
          wait();
          return list;
} The object is invoked by the following code.
public class ThreadProject {
      * @param args
     public static void main(String[] args) {
          System.out.println("...starting thread...");
          SingleObject so = new SingleObject();
          Thread th = new Thread(so);
          th.start();
          try {
               System.out.println("waiting ...");
               List list = so.getList();
               System.out.println("got list"+list.size());
               String sso = so.getValueAt(5);
               System.out.println(sso);
          } catch (InterruptedException e) {
               e.printStackTrace();
}Unfortunateley this is not doing what I have expected. Instead of calling the methodsso.getList() and so.getValueAt() it enters so.getList() and waits and waits....
Can anybody give my a clue or an explanation what I am doing wrong?
Regards
Sas
Message was edited by:
Poncho1975

Answer 1

I have tried a little bit further.
I changed the following part:
public class SingleObject {
     ArrayList list;
     public SingleObject(){
          list = new ArrayList();
     public synchronized void makeALongProc(int size) throws InterruptedException{
          System.out.println("...doing something ...");
          Thread.sleep(5000);
          for (int i = 0 ; i < size ; i++ ){
               list.add(""+i);
          System.out.println("finished!");
          notify();
     public synchronized String getValueAt(int pos) throws InterruptedException{
                if (list==null || list.size()<=pos){
                      wait();
          return list.get(pos).toString();
     public synchronized List getList() throws InterruptedException{
          if (list==null){
                      wait();
          return list;
}and ...
public class ThreadProject {
      * @param args
     public static void main(String[] args) {
          System.out.println("...starting thread...");
          final SingleObject so = new SingleObject();
          Thread th = new Thread(new Runnable(){
               public synchronized void run() {
                    try {
                         so.makeALongProc(100);
                    } catch (InterruptedException e) {
                         e.printStackTrace();
          th.start();
          try {
               System.out.println("waiting ...");
               List list = so.getList();
               System.out.println("got list"+list.size());
               String sso = so.getValueAt(5);
               System.out.println(sso);
          } catch (InterruptedException e) {
               e.printStackTrace();
}Now I am satisfied. Every call on a method from this object would lead to the situation, that either makeALongProc is called first or all other methods must wait until makeALongProc has been called once to fill the array.
However I suppose that accessing elements of a list with a synchronized object is not a good decision. I would rather take only the whole List and do all other access outside of the object.
Regards
Poncho