Interacting with Filesystems - Beginning Java 7 - page 712

Most of Table 8-6 ' s methods are fairly self-explanatory. However, the getFD()
method requires further enlightenment.
Note RandomAccessFile 's read -prefixed methods and skipBytes() ori-
ginate in the java.io.DataInput interface, which this class implements. Fur-
thermore, RandomAccessFile 's write -prefixed methods originate in the
java.io.DataOutput interface, which this class also implements.
Whenafileisopened,theunderlyingplatformcreatesaplatform-dependentstructure
to represent the file. A handle to this structure is stored in an instance of the
java.io.FileDescriptor class, which getFD() returns.