Question

This is my Java code:

List<Object> objects = new ArrayList();

// Assign values to objects
...

for (int i = 0; i < objects.size(); i++) {
    Object object = objects.get(i);
    ...
}

I have two questions:

1. Is objects.size() calculated only once before stating the loop, or is it calculated each loop?
2. If objects.size() is calculated each loop, then if other thread change it at the same time without multi-threads protection, the code may be crashed.

Am I correct?

Answer 1

Answers:

1. objects.size() is called every loop (whether it is calculated depends on the ArrayList implementation, which you shouldn’t care about)
2. Yes, another thread may change the list and this will affect your loop

Real answer:

You shouldn’t have to care, and here’s how you don’t have to:

1. Use a CopyOnWriteArrayList, which is thread-safe. If you iterate over it using an Iterator (as the foreach syntax uses internally), you’ll iterate over the list as it was when the iteration started
2. Use the foreach syntax, which means you don’t have to use an index etc – it’s done for you:

for (Object object : objects) {
    // do something with each object 
}