Does Java foreach make copies?

Question

I have a question about this foreach:

for(Element e : nullProof(setOfElements)){ // do stuff } 

Suppose I defined a nullProof function to return a non-zero Set (because I heard that you should not pass null to the extended for loop. Is this true?):

  private Set<Element> nullProof(Set<Element> s){ return s == null ? Collections.<Element> emptySet() : s; } 

My question is ... is it safe to call the nullProof function inside foreach? In particular, the following header is equivalent:

  for(Element e : setOfElements){ //assuming setOfElements != null 

I was wondering if anyone could point me to some kind of Java standard that says this is a defined behavior.

Also, can someone explain what actually happens “behind the scenes” when this foreach is called?

Let's say setOfElements has size 6. For each iteration through setOfElements does the JVM setOfElements work 6 times or does it create a copy of this set and refer to the copy? I am trying to understand the behavior. Thanks for the help!

Answer 1

Yes, it is absolutely safe.

 for(Element e : nullProof(setOfElements)) { 

equivalently

 Set<Element> s = nullProof(setOfElements); for(Element e : s) { 

As stated in JLS-14 , backstage for each loop is equivalent to:

 for(Iterator<Element> it = s.iterator() ; it.hasNext() ; ) { Element e = it.next(); // ... } 

Answer 2

This is safe because the actual collection must be resolved before any iteration begins, but it does mask your intention a bit. In my opinion, this is clearer:

 if (setOfElements != null) { for (Element e : setOfElements) { ... } } 

Or you can overestimate why you allow setOfElements be null at all. Does it make sense to have this empty list first of all instead of null ?

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Let's say setOfElements has size 6. For each iteration through setOfElements does the JVM work up setOfElements 6 times or creates a copy of this set and refers to the copy? I am trying to understand the behavior.

No copying. The enhanced for -loop uses an Iterator (as returned by the Iterator any collection) to complete its iteration. Iterator is an object that can give you the elements of a collection one by one through the next method.