Java IO difference in reading streams

Question

Could you help me figure out the streams. Why in the tutorials I find that when reading from a file, we use len != -1 (for example).And when reading from a stream and then writing to a stream, we use len> 0.What is the difference when reading?

PS The codes below are taken from examples

byte[] buffer = new byte[1024];
int len;
while ((len = in.read(buffer)) != -1) {
    out.write(buffer, 0, len);
}

 byte[] buf = new byte[8192];
    int length;
    while ((length = source.read(buf)) > 0) {
        target.write(buf, 0, length);
    }
}

UPD

link1

link2

link3

UPD 2

You can also look at IOUtils.copy and Files.copy They are different too

UPD 3

I read that the read method does not return 0, or the available number of bytes, or -1. Thanks everyone

the read method returns -1 at the end of the file, but in the second case, if it returns 0 to us, then we finish reading — Влад
– Влад, Commented Aug 16, 2020 at 14:08
baeldung.com/java-inputstream-to-outputstream baeldung.com/convert-input-stream-to-string attacomsian.com/blog/…. — Влад
– Влад, Commented Aug 16, 2020 at 14:11
Thanks for the source links. It’s even better to edit the question and include them there. — Anonymous
– Anonymous, Commented Aug 16, 2020 at 14:17

Stephen C · Accepted Answer · 2020-08-16 14:33:03Z

3

There is no difference. The javadoc for InputStream.read(byte[]) says the following:

If the length of [the buffer] is zero, then no bytes are read and 0 is returned; otherwise, there is an attempt to read at least one byte. If no byte is available because the stream is at the end of the file, the value -1 is returned; otherwise, at least one byte is read and stored into b.

and

Returns the total number of bytes read into the buffer, or -1 if there is no more data because the end of the stream has been reached.

A careful reading of the above tells us that read will only ever return zero if the buffer size is zero.

The buffer size is not zero in your two examples. Therefore len != -1 and length > 0 will have the same effect.

answered Aug 16, 2020 at 14:33

Stephen C

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