Java How to convert a java.nio.Buffer into a ByteBuffer

Question

I'm facing a little problem I have two libraries one send me the output as java.nio.Buffer and the other receives the input as a java.nio.ByteBuffer how do I make the conversion?

Thanks

the Buffer is from javaCV from this piece of code:

private BytePointer[]   image_ptr;
private Buffer[]        image_buf;

// Determine required buffer size and allocate buffer
int size = avpicture_get_size(fmt, width, height);
image_ptr = new BytePointer[] { new BytePointer(av_malloc(size)).capacity(size) };
image_buf = new Buffer[] { image_ptr[0].asBuffer() };

// Assign appropriate parts of buffer to image planes in picture_rgb
// Note that picture_rgb is an AVFrame, but AVFrame is a superset of AVPicture
avpicture_fill(new AVPicture(picture_rgb), image_ptr[0], fmt, width, height);
picture_rgb.format(fmt);
picture_rgb.width(width);
picture_rgb.height(height);

Answer 1

First of all, since Buffer is an abstract class, and ByteBuffer is one of its subclasses, it's entirely possible that the output you're getting from the first library is in fact a ByteBuffer. If possible, check to see which implementation of Buffer the library is returning, because if it's actually returning a ByteBuffer you can just cast the output to ByteBuffer and be done.

If you don't know which implementation of Buffer the library returns, you'll have to resort to instanceof tests to determine what subclass it is, and copy the data from the returned Buffer to a new ByteBuffer after downcasting it to a subclass. This is because the Buffer interface doesn't actually provide any methods to read the data from the buffer; only the subclasses (ByteBuffer, ShortBuffer, LongBuffer, etc.) do. Fortunately, there are only 7 possible subclasses of Buffer, one for each primitive type.

Once you've determined which subclass of Buffer you have, you can copy the data to a ByteBuffer using the "asXXXBuffer()" method described in this answer, as @Tunaki pointed out.

The code would look something like this:

Buffer outputBuffer = library.getBuffer();
ByteBuffer byteBuffer;
if (outputBuffer instanceof ByteBuffer) {
    byteBuffer = (ByteBuffer) outputBuffer;
} else if (outputBuffer instanceof CharBuffer) {
    byteBuffer = ByteBuffer.allocate(outputBuffer.capacity());
    byteBuffer.asCharBuffer().put((CharBuffer) outputBuffer);
} else if (outputBuffer instanceof ShortBuffer) {
    byteBuffer = ByteBuffer.allocate(outputBuffer.capacity() * 2);
    byteBuffer.asShortBuffer().put((ShortBuffer) outputBuffer);
} else if (outputBuffer instanceof IntBuffer) {
    byteBuffer = ByteBuffer.allocate(outputBuffer.capacity() * 4);
    byteBuffer.asIntBuffer().put((IntBuffer) outputBuffer);
} else if (outputBuffer instanceof LongBuffer) {
    byteBuffer = ByteBuffer.allocate(outputBuffer.capacity() * 8);
    byteBuffer.asLongBuffer().put((LongBuffer) outputBuffer);
} else if (outputBuffer instanceof FloatBuffer) {
    byteBuffer = ByteBuffer.allocate(outputBuffer.capacity() * 4);
    byteBuffer.asFloatBuffer().put((FloatBuffer) outputBuffer);
} else if (outputBuffer instanceof DoubleBuffer) {
    byteBuffer = ByteBuffer.allocate(outputBuffer.capacity() * 8);
    byteBuffer.asDoubleBuffer().put((DoubleBuffer) outputBuffer);
} 

Note that the size of the ByteBuffer you allocate depends on which subclass of Buffer you're copying from, since different primitive types are stored using different numbers of bytes. For example, since an int is 4 bytes, if your library gives you an IntBuffer, you need to allocate a ByteBuffer with 4 times the capacity.