Using collection size in for loop comparison

Question

Is there a compiler optimization for the size() methods of Collections in Java?

Consider the following code:

for(int i=0;i

        

Answer 1

Okay, here is an excerpt from the JDK sources (src.zip in the JDK folder):

public int size() {
    return size;
}

This is from ArrayList, but I think other collections have similar implementations. Now if we imagine that the compiler inlines the size() call (which would make perfect sense), your loop turns into this:

for(int i=0;i

(Well, let's forget that the size is private.) How does compiler checks if the collection was modified? The answer that it doesn't and doesn't need to do so because the size is already available in the field, so all it has to do is to access the size field on each iteration, but accessing an int variable is a very fast operation. Note that it probably calculates its address once, so it doesn't even have to dereference list on each iteration.

What happens when the collection is modified, say, by the add() method?

public boolean add(E e) {
    ensureCapacity(size + 1);  // Increments modCount!!
    elementData[size++] = e;
    return true;
}

As you can see, it just increases the size field. So the compiler doesn't actually need to do anything to ensure it has access to the latest size. The only exception would be that if you modify the collection from another thread you need to synchronize, otherwise the loop thread may see its local cached value of size which may or may not be updated.