Java IO中的“ while（-1！=（len = in.read（b）））”和“ while（（len = in.read（b））> 0）”之间有什么区别？

Question

I have this (working) code:

BufferedInputStream in = new BufferedInputStream(conn.getInputStream());
byte[] b = new byte[1024];
int len;
while (-1 != (len = in.read(b))) {
    fos.write(b, 0, len);
}
fos.flush();

But if I change the while (-1 != (len = in.read(b))) to while ((len = in.read(b)) > 0) , the stream cannot finish. Why is this?

Answer 1

On first sight, these two conditions may seem very different. Let's rearrange them so that (len = in.read(b)) is always on the left:

(len = in.read(b)) != -1

(len = in.read(b)) > 0

The expression (len = in.read(b)) evaluates to just in.read(b) . Therefore, the only difference between the two conditions is that the first checks if read does not return -1, while the second checks if read returns a value greater than 0.

Let's look at what read can return :

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.

This means that read does not return anything less than -1, which in turn means that the two conditions in question will only evaluate to different values if read returns 0. But look, read only returns 0 when no byte is read, and the only time that no byte is read is when you pass in an array of length 0:

If the length of b 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.

Your array has a constant length of 1024 , so in this particular case, the two conditions will produce identical results.

Answer 2

According to the documentations for the method InputSteam.read(byte[] b) :

If the length of b 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.

So, basically, when you use while ((len = in.read(b)) > 0) you are exiting the loop if read() return 0 (meaning that the length of the last read byte is 0), even though the end of stream has not been reached and, thus, there is still data to be read (the stream is not finished).

Answer 3

You are entirely changing condition rule. The first statement check value exactly for not matching -1, so your new condition should met the requirements that value is in range of (-infinity,-1) and (-1, infinity)

(-1 != (len = in.read(b))) // read until reach end of stream
(len = in.read(b)) > 0) // read until data are present but this is wrong

Change it to:

(len = in.read(b)) >= 0)

I am not sure why to change this condition anyway. As docs say:

returns the number of bytes read, or -1 if the end of the stream has been reached.

Situation described here