Understanding the Scope of Multi-Threaded Variables

Short Answer:ArrayList is not inherently thread safe.If you need a thread-safe list, use Collections.synchronizedList.

List<X>list=Collections.synchronizedList (new ArrayList<>());

I heard that if you use new ArrayList() as the variable in the method, it becomes a local variable, so it becomes thread safe.

It's a mistake.There is no correlation between the scope of the variable and thread safe.Thread safe or not is an external specification for the class or method.

ArrayList is an asynchronous class, but can it still be a thread-safe variable within the method?

No, but if one instance is referenced only by the local variable in the method, it cannot be manipulated by other threads, so the result is no accident.