In official JDK docs, the java.io.InputStream#read(byte[], int, int) said:
Let k be the number of bytes actually read; these bytes will be stored in elements b[off] through b[off+k-1], leaving elements b[off+k] through b[off+len-1] unaffected.
But after the sentence, the next paragraph said:
In every case, elements b[0] through b[off-1] and elements b[off+len] through b[b.length-1] are unaffected.
The first sentence requires a more strong limitation of the read operation.
I'm writing an implementation that extends from the InputStream. The documentation said, "unaffected". I interpret it as it can't be modified during the read operation. But even for the b[off+k]~b[off+len-1] also is not able to be modified? I want to use the whole buffer [off, off+len) to processing data temporary.
Concerning InputStream.read(byte[] b, int off, int len), the following scenarios are possible:
There are len or more bytes available in the stream. In this case, the bytes b[off] to b[off + len - 1] will be written. All other bytes in the buffer b will not be modified.
There are less than len bytes in the stream. Let k be the total number of bytes that can be read. In this case, the bytes b[off] to b[off + k - 1] will be written. All other bytes in the buffer b will not be modified.
In particular, your InputStream implementation is not allowed to first clear the contents of b from off to off + len - 1 before attempting to read the stream.
There are other possible scenarios: those involving exceptions. These scenarios are out of scope of this question.