Why doesn't the array() method of MappedByteBuffer work?

According to Javadoc
"The content of a mapped byte buffer can change at any time, for example if the content of the corresponding region of the mapped file is changed by this program or another. Whether or not such changes occur, and when they occur, is operating-system dependent and therefore unspecified.

All or part of a mapped byte buffer may become inaccessible at any time, for example if the mapped file is truncated. An attempt to access an inaccessible region of a mapped byte buffer will not change the buffer's content and will cause an unspecified exception to be thrown either at the time of the access or at some later time. It is therefore strongly recommended that appropriate precautions be taken to avoid the manipulation of a mapped file by this program, or by a concurrently running program, except to read or write the file's content."

To me it seems to many conditions and undesirable misbehavior. Do you need particularly this class?

If you just need to read file contents in fastest way, give a try:

FileChannel fChannel = new FileInputStream(f).getChannel();
    byte[] barray = new byte[(int) f.length()];
    ByteBuffer bb = ByteBuffer.wrap(barray);
    bb.order(ByteOrder.LITTLE_ENDIAN);
    fChannel.read(bb);

It works at speed almost equal to disk system test speed.

For double you can use DoubleBuffer (with double[] array if f.length()/4 size) or just call getDouble(int) method of ByteBuffer.