Re: Generics, damned if you do, damned if you don't

From:

Eric Sosman <Eric.Sosman@sun.com>

Newsgroups:

comp.lang.java.programmer

Date:

Mon, 02 Jul 2007 17:34:37 -0400

Message-ID:

<1183412078.499734@news1nwk>

Roedy Green wrote On 07/02/07 17:01,:

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.

You may find

http://www.angelikalanger.com/GenericsFAQ/FAQSections/ParameterizedTypes.html#Can%20I%20create%20an%20array%20whose%20component%20type%20is%20a%20concrete%20instantiation%20of%20a%20parameterized%20type?

and

http://www.angelikalanger.com/GenericsFAQ/FAQSections/ProgrammingIdioms.html#How%20do%20I%20generically%20create%20objects%20and%20arrays?

helpful.

--
Eric.Sosman@sun.com