RandomAccessFile
RandomAccessFileis used for files containing records of known size so that you can move from
one record to another using
seek( ),then read or change the records. The records don’t have to be the same
size; you just have to be able to determine how big they are and where they are
placed in the file.
At
first it’s a little bit hard to believe that
RandomAccessFile
is not part of the
InputStream
or
OutputStream
hierarchy. It has no association with those hierarchies other than that it
happens to implement the
DataInputand DataOutput
interfaces (which are also implemented by
DataInputStream
and
DataOutputStream).
It doesn’t even use any of the functionality of the existing
InputStream
or
OutputStream
classes – it’s a completely separate class, written from scratch,
with all of its own (mostly native) methods. The reason for this may be that
RandomAccessFile
has essentially different behavior than the other IO types, since you can move
forward and backward within a file. In any event, it stands alone, as a direct
descendant of
Object.
Essentially,
a
RandomAccessFile
works like a
DataInputStream
pasted together with a
DataOutputStream
and
the methods
getFilePointer( )
to find out where you are in the file,
seek( )
to move to a new point in the file, and
length( )
to determine the maximum size of the file. In addition, the constructors
require a second argument (identical to
fopen( )
in
C) indicating whether you are just randomly reading (
“r”
)
or reading and writing (
“rw”
).
There’s no support for write-only files, which could suggest that
RandomAccessFile
might have worked well if it were inherited from
DataInputStream
.
What’s
even more frustrating is that you could easily imagine wanting to seek within
other types of streams, such as a
ByteArrayInputStream
,
but the seeking methods are available only in
RandomAccessFile
,
which works for files only.
BufferedInputStream
does allow you to
mark( )a position (whose value is held in a single internal variable) and reset( )
to that position, but this is limited and not too useful.
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