How to iterate over an array returned by a function in Java

Question

What's the best way to iterate over an array returned by a function?

Example:

for (String s : collectMyStrings()){
  System.out.println(s);
}

Is collectMyStrings() called in each iteration, or does Java only call it once, and use the returned array for all iterations of the for loop?

Yes, but I like to have it documented, if others try google, like i did — kadrian
– kadrian, Commented Mar 21, 2012 at 20:54
if you were really concerned you could always save it as a variable and then loop over the variable. But I think it will only call it once. — Colleen
– Colleen, Commented Mar 21, 2012 at 20:55

MByD · Accepted Answer · 2012-03-21 20:54:33Z

7

It will be called only once, and an iterator for the returned array will be created (implicitly).

answered Mar 21, 2012 at 20:54

MByD

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2 Comments

Natix Over a year ago

For arrays, the compiler transforms the for-each loop into an ordinary for loop with an index variable. Only for-each loops on Iterable objects are transformed to the Iterator-based loop.

Andy Over a year ago

Just to add a bit more context to this. I believe that you can't/shouldn't modify what is being iterated over. So it would make sense for the compiler to cache off the returned iterable.

Natix · Accepted Answer · 2012-03-21 23:23:43Z

2

For arrays, the for-each loop is transformed by the Java compiler into an ordinary for loop with an index variable. That means that your code snippet is roughly equivalent to the this:

String[] strings = collectMyStrings();
int length = strings.length;

for (int i = 0; i < length; i++) {
    String s = strings[i];
    System.out.println(s);
}

So, as you can see, the method is called only once. Even the length property of the array is read only once.

answered Mar 21, 2012 at 23:23

Natix

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Comments

Rogach · Accepted Answer · 2012-11-16 18:28:07Z

0

The method collectMyStrings() is only called once to get the array/collection to iterate over.

edited Nov 16, 2012 at 18:28

Rogach

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answered Mar 21, 2012 at 20:56

John Haager

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