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Groups > comp.lang.java.programmer > #8584

Re: optional operation

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On 10/5/2011 2:20 PM, bob wrote:
> Thanks, but I am still confused.  How can a buffer not be backed by an
> accessible array?

I would have guessed due to this method here:

<http://download.oracle.com/javase/6/docs/api/java/nio/ByteBuffer.html#allocateDirect%28int%29>

You'll need to read the comments at the begging of that page to 
understand what it is doing.


>
> I tried this code, and hasArray returns true.  However, hasArray2
> returns false.
>
> 			byte[] b = new byte[numfloats*4];
> 			dis.read(b, 0, numfloats*4);
>
> 			ByteBuffer bb = ByteBuffer.wrap(b);

It's been a while since I've worked with ByteBuffers, but I think the 
wrap() method is mostly for writing, not reading.

> 			boolean hasArray = bb.hasArray();
> 			FloatBuffer fb2 = bb.asFloatBuffer();
> 			boolean hasArray2 = fb2.hasArray();
>
> I'm trying to get a float[] out of this, but I can't if hasArray2
> returns false.


The trick to ByteBuffers is they work with Channels, not streams.

<http://download.oracle.com/javase/6/docs/api/java/nio/channels/Channel.html>

<http://download.oracle.com/javase/6/docs/api/java/nio/channels/FileChannel.html>

Not tested, but I think this is the idea:

   FileInputStream fis = new FileInputStream( "your/filename/here" );
   FileChannel fchan = fis.getChannel();
   ByteBuffer inBuff = ByteBuffer.allocateDirect( 64 * 1024 );
   fchan.read( inBuff );
   FloatBuffer floatBuffer = inBuff.asFloatBuffer()

   float[] floats = new float[ numfloats ];
   floatBuffer.read( floats );

To reuse the underlying ByteBuffer, I think you have to reset() it.  The 
FloatBuffer does not have to be reset, it'll reset when you reset the 
ByteBuffer backing it, and you shouldn't have to call asFloatBuffer() 
again, you can just re-use the existing FloatBuffer.  This'll help you 
with speed.

You're doing a good job reading the documentation so far, you should 
verify what I've done above because I haven't tested it.  However I 
think it's at least close and gives you the basic pattern for reading 
with a ByteBuffer.

If you have more than 16 * 1024 floats, you'll need to use a loop to 
read them, or increase the side of the buffer in allocateDirect.

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optional operation bob <bob@coolgroups.com> - 2011-10-05 09:49 -0700
  Re: optional operation Patricia Shanahan <pats@acm.org> - 2011-10-05 10:30 -0700
    Re: optional operation bob <bob@coolgroups.com> - 2011-10-05 14:20 -0700
      Re: optional operation markspace <-@.> - 2011-10-05 15:51 -0700
        Re: optional operation markspace <-@.> - 2011-10-05 16:05 -0700
          Re: optional operation bob <bob@coolgroups.com> - 2011-10-05 23:46 -0700
            Re: optional operation Lew <lewbloch@gmail.com> - 2011-10-06 06:47 -0700

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