以下是源码中的解释：

/**
    * Reads the next byte of data from the input stream. The value byte is
    * returned as an <code>int</code> in the range <code>0</code> to
    * <code>255</code>. If no byte is available because the end of the stream
    * has been reached, the value <code>-1</code> is returned. This method
    * blocks until input data is available, the end of the stream is detected,
    * or an exception is thrown.
    *
    * <p> A subclass must provide an implementation of this method.
    *
    * @return     the next byte of data, or <code>-1</code> if the end of the
    *             stream is reached.
    * @exception  IOException  if an I/O error occurs.
    */
   public abstract int read() throws IOException;

其中返回的int值代表了下一字节数据所代表的数值，由于1byte的数值范围是0-255（不考虑符号位），所以int的范围在0-255。