how to test result when thread is finished - java

How can I test Thread? A have got MyClass and I would like to test it. I know what is in List<Integer> numbers and I would like to test if in onThreadFinished(String someString) is right string value when Thread is finished. So how to create test for method onThreadFinished(String someString) after Thread is finished.

class MyClass {

  public void startMyClass(List numbers) {
      Thread thread = new Thread() {

        @Override
        public void run() {

          while(...) {
            // ...do something...
          }

          onThreadFinished(someString);
        }
      }
      thread.start();
  }


  protected void onThreadFinished(String someString) {
  }

}