optional operation

[bob]

I'm looking at the docs for FloatBuffer, and in many places it says
"(optional operation)". What's the deal with this? Can we use the
operation or not?

Thanks.

 

[bob]

Thanks, but I am still confused. How can a buffer not be backed by an
accessible array?

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);
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.

 

[markspace]

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(int)>

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.

 

[markspace]

I would have guessed due to this method here:

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


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

Should be "beginning" not begging. Typing fast, didn't proofread.

Meant to add too that you seemed to have discovered another reason that
the array would not be accessible --- the Buffer wraps another buffer,
which is fine.

 

[bob]

Here's what I tried:

AssetManager am = this.getResources().getAssets();

AssetFileDescriptor afd = am.openFd(filename);
FileInputStream fis = afd.createInputStream();
FileChannel fchan = fis.getChannel();
ByteBuffer inBuff = ByteBuffer.allocateDirect( 24121*4 );
int bytesread = fchan.read( inBuff );

FloatBuffer floatBuffer = inBuff.asFloatBuffer();

float[] floats = new float[ 24121 ];
floatBuffer.get( floats);
return floats;

However, I always get a BufferUnderflowException at
floatBuffer.get( floats);

10-05 23:37:07.400: ERROR/AndroidRuntime(3570): Caused by:
java.nio.BufferUnderflowException

For some reason, floatBuffer thinks it has a capacity of 0 after it's
created.

Any ideas?

 

[Lew]

On Wednesday, October 5, 2011 11:46:11 PM UTC-7, bob wrote:

PLEASE do not top-post!

PLEASE stop embedding TAB characters in your listings!

Dude. Really.

Here's what I tried:

AssetManager am = this.getResources().getAssets();

AssetFileDescriptor afd = am.openFd(filename);
FileInputStream fis = afd.createInputStream();
FileChannel fchan = fis.getChannel();
ByteBuffer inBuff = ByteBuffer.allocateDirect( 24121*4 );
int bytesread = fchan.read( inBuff );

FloatBuffer floatBuffer = inBuff.asFloatBuffer();

float[] floats = new float[ 24121 ];
floatBuffer.get( floats);
return floats;

However, I always get a BufferUnderflowException at
floatBuffer.get( floats);

10-05 23:37:07.400: ERROR/AndroidRuntime(3570): Caused by:
java.nio.BufferUnderflowException

For some reason, floatBuffer thinks it has a capacity of 0 after it's
created.

Any ideas?

buffer 'flip()'?