Does the for foreach construct in java create a hard or soft copy?

Question

Say I have the following piece of java code

ArrayList<Double> myList = new Double[100];
for (Double x : myList)
    x = randomDouble();

Does this actually modify myList or just the dummy variable?

I realize I should just try this code segment out, but I think this is the sort of thing I should be able to google or search for on this site, and several queries so far have turned up nothing useful.

Answer 1

It doesn't modify myList. It works by calling myList.iterator(), then (repeatedly) hasNext() and next(), none of which change myList.

Also, Java does not have C++-style references. That means you don't need to worry (even without looking at the API) about x being a reference that could modify myList.

Finally, that's invalid syntax. It should be:

ArrayList<Double> myList = new ArrayList<Double>(); 
/* or new ArrayList<Double>(100), but that's only an optimization 
(initial capacity), not the size. */

Answer 2

It dos not change the value in the list.

You assign a new Double object to the reference x, which leaves the original object unchanged.

Answer 3

Does this actually modify myList or just the dummy variable?

Just the dummy variable.

BTW you code doesn't even compile.

It should be either:

ArrayList<Double> myList = new ArrayList<Double>(100);

or

Double[] myList = new Double[100];

Answer 4

So with the benefit of hindsight, the answer I was looking for would look something like this:

Using the "=" operator will set the object x to a new value without modifying any of the values in the variable myList. To modify myList using the foreach loop construct, one needs to access a modifier method for the object. For example:

for (MyDouble x : myList) {
    x.setX( randomDouble() );
}