Question

• Let’s say I’ve created a final static instance of the class
  ComparatorChain.
• Through Collections.sort(List<T> list, Comparator<? super T> c) I’m using this instance.
• Somewhere in Collections.sort(List<T> list, Comparator<? super T> c) Comparator.compare(T o1, T o2) is called on the ComparatorChain instance.

Now my beginners question:

When multiple threads use this static instance can they all call the Comparator.compare(T o1, T o2) method at the same time?

I’d suppose that as long as there is no synchronized modifier involved, they could. Is this right?

Why do I want to know this?

Through such a static instance I could avoid the useless creation of ComparatorChain objects.

Answer 1

Yes, if there’s no synchronization involved, there’s nothing to stop multiple threads calling the same method multiple times concurrently – whether that’s a static method or an instance method on an object which is accessible by multiple threads, however it’s accessible by those threads.

It’s worth noting that although a variable can be static, there’s no such concept as a static object. While on this occasion I knew what you meant, the difference between a variable and an object is often vital.

So long as your ComparatorChain.compare method is thread-safe, it sounds like you should be fine. Most comparisons can easily be thread-safe, as they rarely mutate state.