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Java Forum / General / October 2007Iterable arrays
You can use arrays in for:each. I presume that means somehow arrays must have an iterator method that produces an Iterator. What are the methods of arrays? Or is this just fudged as a special case by the compiler?  SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com On Oct 25, 11:19 pm, Roedy Green <see_webs...@mindprod.com.invalid> wrote: > You can use arrays in for:each. I presume that means somehow arrays > must have an iterator method that produces an Iterator. > > What are the methods of arrays? > > Or is this just fudged as a special case by the compiler? I'm not sure of the internal workings of it, but I read recently that it was specifically designed to work with arrays and collections. -- Chris Roedy Green wrote On 10/25/07 09:19,: > You can use arrays in for:each. I presume that means somehow arrays > must have an iterator method that produces an Iterator. > > What are the methods of arrays? > > Or is this just fudged as a special case by the compiler? Special-cased by the compiler. (How do I know? I wrote foreach loops for a List<String> and for a String[], and looked at the generated bytecode.) For the List<String>, the generated code calls the .iterator() method and then uses .hasNext() and .next() on the Iterator<String>. For the String[], the generated code is roughly equivalent to for (int n = array.length, i = 0; i < n; ++i) ... which may be a hair more efficient than for (int i = 0; i < array.length; ++i) SignatureEric.Sosman@sun.com > For the String[], the generated code is roughly >equivalent to > > for (int n = array.length, i = 0; i < n; ++i) that's odd. I've never noticed for:each iterating in reverse order. SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com Roedy Green wrote On 10/25/07 13:48,: >> For the String[], the generated code is roughly >>equivalent to >> >> for (int n = array.length, i = 0; i < n; ++i) > > that's odd. I've never noticed for:each iterating in reverse order. "Reverse order?" SignatureEric.Sosman@sun.com >>> for (int n = array.length, i = 0; i < n; ++i) >> >> that's odd. I've never noticed for:each iterating in reverse order. > > "Reverse order?" sorry. I read the first few chars and presumed you wrote: for (int n = array.length-1; n>=0; n--) SignatureRoedy Green Canadian Mind Products The Java Glossary http://mindprod.com >> For the String[], the generated code is roughly >> equivalent to >> >> for (int n = array.length, i = 0; i < n; ++i) > > that's odd. I've never noticed for:each iterating in reverse order. I think you misread that. it iterates i from 0 to n. I recall hearing that array.length is a no-no on JavaME, so perhaps they just use the same output for both cases? I wonder what happens when you do something like this: Object[] arr = new Object[2]; for (Object o: arr) { arr = new Object[0]; } (I know I could test that, but I will in a bit.) Anyone want to speculate before I test? SignatureDaniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/> >>> For the String[], the generated code is roughly >>> equivalent to [quoted text clipped - 14 lines] > (I know I could test that, but I will in a bit.) > Anyone want to speculate before I test? I'll speculate that the generated bytecode makes a copy of "arr" into a temporay, so that nothing untoward happens. >>>> For the String[], the generated code is roughly >>>> equivalent to [quoted text clipped - 17 lines] > I'll speculate that the generated bytecode makes a copy of "arr" into a > temporary, so that nothing untoward happens. Copy of the reference "arr", of course, not of the array itself. Mike Schilling wrote: >>>> For the String[], the generated code is roughly >>>> equivalent to [quoted text clipped - 16 lines] > I'll speculate that the generated bytecode makes a copy of "arr" into a > temporay, so that nothing untoward happens. The JLS gives the equivalent basic for code. See http://java.sun.com/docs/books/jls/third_edition/html/statements.html#14.14.2 And, yes, it does call for the array reference expression to be copied to a temporary. Patricia > Mike Schilling wrote: >> [quoted text clipped - 6 lines] > And, yes, it does call for the array reference expression to be copied > to a temporary. Makes sense when you think about it, in light of for (String s : expensiveMethodReturningArray()) or even for (String s : methodWithSideEffectsReturningArray()) SignatureEric Sosman esosman@ieee-dot-org.invalid > You can use arrays in for:each. I presume that means somehow arrays > must have an iterator method that produces an Iterator. > > What are the methods of arrays? > > Or is this just fudged as a special case by the compiler? It has nothing to do with fudge. It's the caffeine in Java that makes the arrays iterable. (Okay, no more puns, I promise.) -Wayne Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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