Is it valid to modify array elements while iterating with 'enhanced for' in Java?

Question

Cosider this code:

    for (int value : values) {
        values[value] = -values[value] - 1;
    }

Is it guaranteed to pick updated values when the iteration process reaches appropriate elements?

Experimentally I figured ut that it works as expected (iteration yields the updated values).

Also from Java spec I can conclude that it should work:

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
}

I just wonder, if there is an 'official' confirmation that it's valid and guaranteed to pick the updated values?

Update: Excluded according to comments:

I am in doubt because analogous code would be invalid for e.g. ArrayList.

"equivalent" == "behaves exactly the same", so that part* of the spec is the 'official' confirmation that it's valid and guaranteed to pick the updated values. Although the part of the spec you quoted is applicable to Iterable's, not arrays - that's further down. — Bernhard Barker
– Bernhard Barker, Commented Feb 4, 2018 at 16:31
What do you mean "would be invalid for e.g. ArrayList"? List<Integer> values = new ArrayList<>(Arrays.asList(6, 5, 3, 2, 2, 4, 0, 1)); for (int value : values) { values.set(value, values.get(value) + 1); } works perfectly fine. You obviously can't use [] on a List, but that's unrelated to the enhanced for loop. — Bernhard Barker
– Bernhard Barker, Commented Feb 4, 2018 at 19:07
@Dukeling, I mean The iterators returned by this class's iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Apparently, ArrayList#set is not a structural modification , and so my analogy was admittedly wrong: one cannot structurally modify a Java array. — Alexey
– Alexey, Commented Feb 5, 2018 at 11:07

Sergey Kalinichenko · Accepted Answer · 2018-02-04 15:33:40Z

1

Short answer: yes, it is guaranteed to pick up the updated value.

Long answer: an enhanced for on an array is equivalent to this code (see this documentation, scroll down to "Otherwise, the Expression necessarily has an array type" part):

T[] #a = Expression;
for (int #i = 0; #i < #a.length; #i++) {
    VariableModifiersopt TargetType Identifier = #a[#i];
    ...
}

Therefore, it behaves the same way as if the array was iterated using an index.

Note: One thing that would be illegal is replacing the whole array that you are iterating with a new array object, i.e. assigning values = new int[...] has a potential of going past the end of the newly allocated array.

answered Feb 4, 2018 at 15:33

Sergey Kalinichenko

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Comments

Suresh Atta · Accepted Answer · 2018-02-04 15:32:45Z

Reading source code can confirm this.

Yes it is.

Just look at the source code of next() method. It keep on getting the data from ArratList

Object[] elementData = ArrayList.this.elementData;

Full method

@SuppressWarnings("unchecked")
753         public E More ...next() {
754             checkForComodification();
755             int i = cursor;
756             if (i >= size)
757                 throw new NoSuchElementException();
758             Object[] elementData = ArrayList.this.elementData;
759             if (i >= elementData.length)
760                 throw new ConcurrentModificationException();
761             cursor = i + 1;
762             return (E) elementData[lastRet = i];
763         }

That tells that, if you are modifying an element in side ArrayList while iterating, you see those changes right away.

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