David Walend's Blog

Did I Miss Generic Array Creation?

        
        HeapMember<Key,Value,Comp$gt;[] fibNodes = (HeapMember<Key,Value,Comp>[])Array.newInstance(HeapMember.class,size());

The first thing I tried didn't compile:

        //doesn't compile
        HeapMember<Key,Value,Comp>[] fibNodes = new HeapMember<Key,Value,Comp>[size()];

jdigraph/v2/source/collection/net/walend/collection/FibHeap.java:57: generic array creation
        HeapMember<Key,Value,Comp>[] fibNodes = new HeapMember<Key,Value,Comp>[size()];        

I tried dynamically creating the array using java.lang.reflect.Array:

        HeapMember<Key,Value,Comp>[] fibNodes = (HeapMember<Key,Value,Comp>[])Array.newInstance(HeapMember.class,size());

/Users/dwalend/projects/opensource/jdigraph/v2/source/collection/net/walend/collection/FibHeap.java:58: warning: [unchecked] unchecked cast
found   : java.lang.Object
required: net.walend.collection.HeapMember<Key,Value,Comp>[]
         HeapMember<Key,Value,Comp>[] fibNodes = (HeapMember<Key,Value,Comp>[])Array.newInstance(HeapMember.class,size());

I've tried doing other things, especially to the HeapMember.class argument, but haven't found a solution that compiles with no warnings.

Is there some bit of API I missed? Is there a good reason not to create arrays of generics? Or should I report a RFE to Sun asking them to add generic parameters to Array.newInstance()?

Thanks,

Dave

Comments

• section 7.3 of the tutorial seems to cover this?
  
  http://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf
  
  in summary, it sounds like because the true parameterized type in unknown at run-time you don't have enough information to guarantee run-time type safety, so you have to be disallow more than you'd like
  
  (possibly)
  
  btw, isn't forums.java.sun.com the place for this sort of question?

  Posted by: asjf on January 07, 2005 at 05:55 AM

• you can see Articles by Bruce Eckel
  9-19-04 Puzzling Through Erasure
  hope some help.

  Posted by: pprun on January 07, 2005 at 06:37 AM

• asjf,

  The end of section 7.3 of the tutorial says section 8 will explain a work-around to avoid the warning. To use section 8, I'd need a way to say HeapMember<Key,Value,Comp>.class either a newArray(size) method on Class or a type-aware newInstance() method on Array. That's why I think there's a missing piece.

  pprun,

  Thanks for the pointer. I'd rather not use @SuppressWarnings("unchecked") . Just one method, but it'd leave me feeling a little dirty.

  Thanks again,

  Dave

  Posted by: dwalend on January 07, 2005 at 07:06 AM

• public class ArrayFactory
  {
      private ArrayFactory() {}
     public static   T[] toArray( T... array ) 
      {
          return array;
      }
  }
  

  Posted by: wingetr on January 07, 2005 at 06:54 PM

• I've run into the same problem. I think the bug is with the compiler's -Xlint option (which was added late in the release), rather than the language itself. I'm not a Java language expert, however.

  Posted by: tball on January 08, 2005 at 10:04 AM

• wingetr,

  That's a nice hack, but doesn't quite fit my problem. However, that plus pprun's post did inspire my own hack:

      @SuppressWarnings("unchecked")
      private HeapMember[] createHeapMemberArray(int size)
      {
          return (HeapMember[])Array.newInstance(HeapMember.class,size);
      }
  
      private void consolidate()
      {
          HeapMember[] fibNodes = createHeapMemberArray(size());
  ...
  

  I decided that a one-line method with one unchecked cast is OK for now.

  tball,

  Looks like Sun ran out of time to fix the -Xlint option to actually look at @SuppressWarnings("unchecked"). So my hack will help reduce warnings in classes that have the same unchecked cast, but won't get rid of all warnings. See bug #4986256, and add a vote if you've got one to spare.

  And I'll go ahead and put in the RFE for a typesafe Array.newInstance(). Will post the bug number once it works its way through the parade.

  Thanks,

  Dave

  Posted by: dwalend on January 08, 2005 at 10:33 AM

• T[].class.cast(java.lang.reflect.Array.newInstance(T[].class, size));

  Posted by: kinfei on September 05, 2006 at 12:32 AM

• Thanks for the suggestion, kinfei. It still gives the warning. You can't specify the generics on T[] in the T[].class. Thanks for the suggestion in this old blog, though. Dave

  Posted by: dwalend on September 06, 2006 at 06:07 AM

• As I read in http://sofpower.com/java/qa.html, this is a common problem. Quoting sofpower.com: "Q: Why does E[] arr = new E[cap]; give a compile-time error? A: Basically, the operator new is a run-time operation and Java 1.5 removes all generics typing at compile-time. This is the well-known generic array creation an operation not supported by Java 1.5. You need to use obj = (E[]) (new Object[cap]); which will compile with just a warning." Regards, Duilio Protti.

  Posted by: dprotti on May 07, 2007 at 06:54 AM

• dprotti,

  Thanks for posting in this ancient blog. I'm at JavaOne where we're going to have an "animated discussion" about getting rid of erasure for JDK7.

  For now, the cast is the best you can do. I put the casts like that in a short separate private method with a @SuppressWarning( "unchecked" ) annotation. That works well for me.

  Dave

  Posted by: dwalend on May 07, 2007 at 07:46 AM

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