public class Runner {

public static void main(String argv[]) {
//create thread 1 running Test
//create thread 2 running Test
}

}

public class Test {

public Object obj = null;

public Test(Object obj) {
  this.obj = obj;
  obj.addStuff("Hello!");
}

}

public class Object {

public List<String> l = new ArrayList<String>();

public void addStuff(String str) {
  l.add(str);
}

}