🚀 go-pugleaf

"Alan Shiers" <ashiers@hfx.eastlink.ca> wrote in message
news:3D21EC78.228A5CD3@hfx.eastlink.ca...

> >
> >     byte[] buffer = new byte[128];
> >     int n;
> >     while ((n = in.read(buffer)) >= 0)
> >       out.write(buffer, n);
>
>
> This while loop is confusing to me.  You dimension an array at 128
> elements and pass it to the read(byte[]) method of DataInputStream.
Which populates the array with /up to/ 128 bytes of data on each read.

> Where does the size of the file figure into all this?
In the boolean condition (n = in.read(buffer)) >= 0, which does two things:
  n = in.read(buffer) assigns the int n to the number of bytes that have
just been read
(..) >= 0 tests that this is greater than 0; if EOF is detected then the
stream will return -1, for example, so this condition gurantees that the
loop will continue until all the data in the file has been read. At no point
do we actually need to know the size of the file; it simply isn't relevant.


> The thing that is throwing me is that the file maybe much bigger than
> 128bytes.
Hence the loop.

>Is the read() method going to automatically increase the size
> of the array?
No, this is impossible with Java arrays.

What happens is that for each iteration of the loop up to 128 bytes of data
are read from the input stream. The actual number of bytes read is stored in
a variable, n, so that we can ensure that the correct number are written to
the output stream. The loop has an exit condition based on the fact that the
general contract of input stream is for EOF to be signalled with a negative
number (and the assumption that a read of 0 bytes would also indicate the
absence of any more useful data).

This technique offers an improvement over trying to read an entire file in
at one time, as the resource overhead remains pretty much constant, i.e. the
size of your buffer, and as such far more able to deal with files of any
size. It also provides an improvment to reading a byte at a time by
requiring far less method calls (roughly 128th of the number in this case).

hth
Owen