java关于输入输出流的习题

没有看懂这一段代码，请问一下为什么运行出来的是4：abcd。是怎么看出来的呀

int m=fileInputStream.read(tom);

这里的read返回值 Int 是读入缓冲区的总字节数，并不是byte tom 缓冲区的大小 100.

 /**
     * Reads up to <code>b.length</code> bytes of data from this input
     * stream into an array of bytes. This method blocks until some input
     * is available.
     *
     * @param      b   the buffer into which the data is read.
     * @return     the total number of bytes read into the buffer, or
     *             <code>-1</code> if there is no more data because the end of
     *             the file has been reached.
     * @exception  IOException  if an I/O error occurs.
     */
    public int read(byte b[]) throws IOException {
        return readBytes(b, 0, b.length);
    }