Re: I/O Confusion

Eric Sosman <esosman@ieee-dot-org.invalid>
Tue, 25 Oct 2011 21:20:57 -0400
On 10/25/2011 7:12 PM, Novice wrote:

I'm try to wrap my head around the basic I/O classes, especially how to
choose the one I need for a given job.

I understand that Streams are for bytes and Readers and Writers are for
characters and I think I understand the distinction between bytes and

Let's say that I want to write characters, as opposed to bytes. How do I
decide which subclass of Writer I want to use? I'm not clear on when
BufferedWriter, say, is preferable to PrintWriter.

     You choose the one with the behavior you want. BufferedWriter
is a fairly low-level object: It offers a few output methods that
accept things already rendered in character form, and writes them to
the destination. Its main promise is that it will make an attempt to
accumulate a batch of output characters before sending them onward,
which may improve efficiency by making N/B trips through the I/O stack
instead of N.

     PrintWriter offers a richer set of methods, many of which take
non-character inputs and handle the character rendering themselves
instead of requiring you to do it. There's even locale control, and
things like the printf() methods. Also, the PrintWriter methods don't
throw IOExceptions: Instead of guarding every output with try/catch,
you can blithely ignore all the possible troubles until you finally
call checkError() just before you're done. (Whether this is a Good
Thing or a Bad Thing depends on the program -- but it's your choice.)

I also get very confused when I see Streams, Readers and Writers wrapped
within one another. For example, this snippet confuses me:

Writer out = new BufferedWriter(new OutputStreamWriter(new FileOutputStream
("outfilename"), "UTF8"));

Why wrap FileOutputStream within OutputStreamWriter within BufferedWriter?
Why not just BufferedWriter, say, to do everything? What is actually
happening at execution time when you execute this code?

     I believe this is what the Patterns aficionados call "Decoration."
You start with a FileOutputStream, which is capable of only one thing:
Writing bytes to a file.

     You "decorate" that by wrapping it inside an OutputStreamWriter,
whose job is to translate characters to sequences of bytes. Note that
the translation is independent of the ultimate destination, which could
be a FileOutputStream or a SocketOutputStream or an IScreamYouStream;
*any* kind of OutputStream is fair game. Separating the translation
from the delivery means that all the translations of OutputStreamWriter
are available for *all* the implementations of OutputStream: You'll
never be tripped up by "We can write TUE7-encoded characters to the
console or to disk, but not to sockets or ZIP files."

     Finally, you "decorate" still further by putting a BufferedWriter
around the OutputStreamWriter. As above, the BufferedWriter's job is
to improve efficiency by gathering a batch of characters and delivering
them all at once instead of delivering them one by one. (You need a
gallon of milk, a jar of kosher pickles, and the New Rochelle Times:
do you make one trip or three to the convenience store, and why?)

Also, when do I use this basic I/O and when is it better to use the NIO

     As a non-NIO user I'm not a good person to answer this. My
impression from reading the docs is that NIO is mostly of interest
when you need extremely high performance and/or a lot of parallelism.
For ordinary I/O tasks -- only a few tens of sources and destinations,
only a few gigabytes of total transfer, no troublesome requirements
about latencies -- suffices and is simpler. If you're using
thousands of sockets and moving terabytes of data that must go to the
output wire in no more than X microseconds, look at java.nio more
closely than I've found need for.

I've looked at a few different tutorials but the penny hasn't dropped for
me yet. Any advice on how to answer these questions for myself?

     Roedy Green had a cute little helper applet he liked to call an
"amanuensis" at (I say
"had" because it seems to be down at the moment: all I get is
"java.lang.ClassNotFoundException: com.mindprod.fileio.FileIO".) The
usage pattern was: "Tell me what sort of data you want to read or write,
tell me what the data comes from or goes to, and I'll write a skeleton
of the code you'll need." Pretty sweet, when it works.

Eric Sosman

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