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Java Forum / General / July 2007Generics, damned if you do, damned if you don't
Consider this code snippet: // create a new HashMap HashMap<String, Integer> h = new HashMap<String, Integer>(149, .75f); ... // extract key/value pair entries into an array Set<Map.Entry<String, String>> justEntries = h.entrySet(); Map.Entry<String, String>[] keyValuePairs = justEntries.toArray ( new Map.Entry[justEntries.size()] ); This generates an "unchecked conversion" warning. If put in the type like this: // extract key/value pair entries into an array Set<Map.Entry<String, String>> justEntries = h.entrySet(); Map.Entry<String, String>[] keyValuePairs = justEntries.toArray ( new Map.Entry<String,String>[justEntries.size()] ); I get an "generic array conversion error" How are you supposed to code that. Map.Entry is an interface and HashMap.Entry is not public. However the problem appears to be general -- even creating arrays af HashMaps. -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com > Consider this code snippet: > [quoted text clipped - 4 lines] > // extract key/value pair entries into an array > Set<Map.Entry<String, String>> justEntries = h.entrySet(); Don't you mean Map.Entry<String, Integer> ? > Map.Entry<String, String>[] keyValuePairs = justEntries.toArray > ( new Map.Entry[justEntries.size()] ); > > This generates an "unchecked conversion" warning. > How are you supposed to code that. Go with the first one, bite the bullet, and add a @SuppressWarnings ("unchecked") in. Arrays + generics don't mix. In the Sun generics FAQ, there's a part about why there are such problems with generic arrays (simple answer: type erasure). Roedy Green wrote On 07/02/07 17:01,: > Consider this code snippet: > [quoted text clipped - 23 lines] > Map.Entry is an interface and HashMap.Entry is not public. However > the problem appears to be general -- even creating arrays af HashMaps. You may find http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#Ca n%20I%20create%20an%20array%20whose%20component%20type%20is%20a%20concrete%20ins tantiation%20of%20a%20parameterized%20type? and http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#How %20do%20I%20generically%20create%20objects%20and%20arrays? helpful.  SignatureEric.Sosman@sun.com >http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#Ca n%20I%20create%20an%20array%20whose%20component%20type%20is%20a%20concrete%20ins tantiation%20of%20a%20parameterized%20type? I have often wondered what this ultra competent and supremely altruistic woman looked like: http://angelikalanger.com/ -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com > ... > I get an "generic array conversion error" > > How are you supposed to code that. > ... AFAIK, there's no "generics" way to do this, suppressing it is the only way unfrtunatelly, I guess. > ... > even creating arrays af HashMaps. > ... You mean like this? HashMap<String, Integer> h = new HashMap<String, Integer>(); String[] keys = h.keySet().toArray(new String[]{}); Integer[] values = h.values().toArray(new Integer[]{}); which gives no compiler errors/warnings (with Eclpise's compiler, that is). Regards, Bart. >You mean like this? > [quoted text clipped - 4 lines] >which gives no compiler errors/warnings (with Eclpise's compiler, that >is). no. You don't allocate any arrays of generics there. See http://mindprod.com/jgloss/hashmap.html for sample code. -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com > > ... > > HashMap<String, Integer> h = new HashMap<String, Integer>(); [quoted text clipped - 5 lines] > > no. You don't allocate any arrays of generics there. I don't get your point. What IS it that I do then? > See http://mindprod.com/jgloss/hashmap.html for sample code. > ... >From you webpage: // extract the keys into an array Set<String> justKeys = h.keySet(); // Use toArray that takes an empty array, // otherwise we end up with a useless Object[] instead of a String[]. String[] keys = justKeys.toArray( new String[justKeys.size()] ); How is the above different than the code I posted? Regards, Bart. > ... > How is the above different than the code I posted? > ... Besides the fact that you explicitly say how many elements the newly created array should have, whereas I leave that responsibility to Arrays' toString() method, of course. >>From you webpage: > [quoted text clipped - 7 lines] > >How is the above different than the code I posted? It is not different. It just not the problem we were talking about, arrays of generics. Look in the code sample about autoboxing. -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com On Tue, 03 Jul 2007 20:20:03 GMT, Roedy Green <see_website@mindprod.com.invalid> wrote, quoted or indirectly quoted someone who said : >It is not different. It just not the problem we were talking about, >arrays of generics. Look in the code sample about autoboxing. The code you quoted is an array of ordinary Strings, not an array of generics. The problem comes here when you try to allocate an array of generics: // extract key/value pair entries into an array Set<Map.Entry<String, Integer>> justEntries = h.entrySet(); // Infuriatingly, this generates an unchecked conversion warning message. Map.Entry<String, Integer>[] keyValuePairs = justEntries.toArray( new Map.Entry[justEntries.size()] ); // Type erasure won't let us say: // Map.Entry<String, Integer>[] keyValuePairs = // justEntries.toArray ( new // Map.Entry<String,Integer>[justEntries.size()] ); -- Roedy Green Canadian Mind Products The Java Glossary http://mindprod.com > // create a new HashMap > HashMap<String, Integer> h = new HashMap<String, Integer>(149, .75f); [quoted text clipped - 4 lines] > Map.Entry<String, String>[] keyValuePairs = > justEntries.toArray ( new Map.Entry[justEntries.size()] ); > Map.Entry is an interface and HashMap.Entry is not public. However > the problem appears to be general -- even creating arrays af HashMaps. Avoid arrays of reference types. We now have the much nicer generic List. So the code becomes: List<Map.Entry<String,String>> entries = new java.util.ArrayList<Map.Entry<String,String>>( map.entrySet(); ); Tom Hawtin 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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