Question

Supposing I have the following

public class Foo {
   private Map<Integer,SomeObject> myMap;
   public Foo() {
      this.myMap = new HashMap<Integer,SomeObject>();
   }

   private class Runner implements Runnable {
       public void run() {
            SomeObject someObj = new SomeObject();
            Foo.this.myMap.put(10,someObj);
            //'soObj' will always be null upon retrieval later...
       }   
   }
}

If I create a thread with a Runnable of type ‘Runner’ and start the thread, it will run.
In the run method, I simply create an instance of ‘SomeObject’ and place it in the map of the outer class.
However, when I attempt to get a value from ‘myMap’ later on, the ‘SomeObject’ instance will always be null. I can’t understand why as I have placed a reference into the map ‘myMap’ which still lives on in the heap after the thread finishes. Is there a way around this?!

Thanks very much

Answer 1

It could be a problem with the way your objects are instantiated. It should be:

    Foo foo = new Foo();
    Thread thread = new Thread(foo.new Runner());
    thread.start();