Would a Java foreach loop get a list each iteration

Question

Say I have a dataObject, who has a getChildren() function which returns a collection, but for whatever reason is a slow process that I want to avoid. I then want to iterate over said collection. For the sake of this getChildren() will return an empty collection when it has no children, not null.

Can I get away with doing:

for(dataObject child : parent.getChildren()){ ... }

or is that going to call getChildren() each loop, meaning that to avoid that costly call each time, I need to instead do:

Collection<dataObject> children = parent.getChildren();
for(dataObject child : children){ ... }

This could also cause problems if I the parent returns a different collection, for instance if a child was added/removed, and I was wanting to look at a snapshot of the children at that moment (let's gloss over other issues with that).

I suspect that the first version is indeed fine, as the for loop would get an iterator for the collection when it starts and just refer to that iterator, but it's something I've never confirmed.

Of course not, this would be too slow for everybody. But you could have checked that yourself : if you can't debug, just add a println inside `getChildren`. — Denys Séguret, Jul 18 '14 at 07:59
Put a breakpoint in getChildren and see if it gets called every time — Eamonn McEvoy, Jul 18 '14 at 08:02
@AnkitLamba not sure if it is, this is specifically asking about how the Collection is queried in the first place. — thecoshman, Jul 18 '14 at 08:05

score 3 · Accepted Answer · answered Jul 18 '14 at 08:00

From 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
}

so it only evaluates the Iterable once.

Would a Java foreach loop get a list each iteration

1 Answers1