1) What is the point of class-level lock of the Iron class? This class does not contain any methods. Then what does it lock in fact?? 2) Why is the (Person.class) lock not correct solution? The output appeared in the correct order (as if there were only 1 piece of iron). 3) When I wrote this: synchronized(Clothes.class), the output appeared in the correct order. When I wrote this: synchronized (Solution.class), the output still appeared in the correct order. So that means perhaps is it irrelevant what I write into the parentheses?? ... But the same question just in other words: what is the meaning of "synchronized" with lock of one class, and what is the meaning of "synchronized" with lock of another class? What is the difference?