List object reference changed during Java loop

Question

I couldn't find any topic about this. I want to know if it is safe to change the reference for the list class during a loop like the one bellow:

Tree minimalTree = someTree;
for (Tree st : minimalTree.getSubtrees()) {
  if (condition)
    minimalTree = st;
}

Does the iterator gets reset and starts again for the new reference?

Edit: I forgot to say: this code is suited for situations where I want to narrow down the search for elements in the tree, let's say, the smaller tree that contains certain elements. In this case, it would be faster to keep looking only for the inner structures of "minimalTree" instead of the entire "someTree" structure.

Answer 1

No, the iteration won't be reset. According to the JLS:

The enhanced for statement is equivalent to a basic for statement of the form:

for (I #i = Expression.iterator(); #i.hasNext(); ) {
     {VariableModifier} TargetType Identifier = (TargetType) #i.next();
     Statement
}

The definition makes it obvious that the iterator is only initialised once, before the first iteration of the loop.

The behaviour when iterating across an array with an enhanced for statement is similar in this respect.

However I'd personally consider it poor practice as it makes the code hard to understand.

Answer 2

Actually, there are two questions:

I want to know if it is safe to change the reference for the list class during a loop like the one bellow:

Yes, it is safe. And for safe I mean: changing the reference does not interfere with the already running loop.

Does the iterator gets reset and starts again for the new reference?

No, the iterator is never reset. It would be exactly the opposite of safe.

In my opinion, it is not a good practice to change iterator or collection variables inside a loop. It makes the code harder to understand, and probably the result is not what you expect when you do that (like in your case I understood you expected the loop to start over reevaluating the collection).

In your case, encapsulate in a method and recursively call it on subtree:

Tree findMinimalTree(Tree tree) {
   for (Tree st : tree.getSubtrees()) {
     if (condition)
       return findMinimalTree(st);
   }
   return tree;
}