Question

What is the best way to do that?

I tried the following:

ByteBuffer cacheBuffer=ByteBuffer.allocateDirect(nm(nLimit,0));  //where nm(nLimit,0) is a large number
double[] cache=cacheBuffer.asDoubleBuffer().array();

But I got this exception:

java.lang.UnsupportedOperationException
at java.nio.DoubleBuffer.array(Unknown Source)

Why?

Edit:

It looks like the javadoc “Returns the double array that backs this buffer (optional operation).” actually means array() method is simply returning the double array that is already backing this buffer. I thought it is going to convert the buffer to double[] somehow. So now it makes sense I got an exception.

Answer 1

This will only work if you expect your array to merely be a copy.

final DoubleBuffer buffer = cacheBuffer.asDoubleBuffer();
final double[] copy = new double[buffer.remaining()];
buffer.get(copy);

The reason your attempt fails is because array is only supported for non-direct buffers; direct buffers are not backed by an array.