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作用

以readInt为例,看看源码

/**     * Reads four input bytes and returns an     * {@code int} value. Let {@code a-d}     * be the first through fourth bytes read. The value returned is:     * 
{@code     * (((a & 0xff) << 24) | ((b & 0xff) << 16) |     *  ((c & 0xff) <<  8) | (d & 0xff))     * }
     * This method is suitable     * for reading bytes written by the {@code writeInt}     * method of interface {@code DataOutput}.     *     * @return     the {@code int} value read.     * @exception  EOFException  if this stream reaches the end before reading     *               all the bytes.     * @exception  IOException   if an I/O error occurs.     */    int readInt() throws IOException;

接口中当然没有方法体,具体干了什么要看起实现类.

public final int readInt() throws IOException {
           int ch1 = in.read();        int ch2 = in.read();        int ch3 = in.read();        int ch4 = in.read();        if ((ch1 | ch2 | ch3 | ch4) < 0)            throw new EOFException();        return ((ch1 << 24) + (ch2 << 16) + (ch3 << 8) + (ch4 << 0));    }

调用了read方法,其作用为

从输入流中读取下一个数据字节。 以0到255的整数形式返回值字节。如果由于已到达流的末尾而没有字节可用，则返回值-1。

此方法将阻塞，直到可用输入数据，检测到流的末尾或引发异常为止。

举个应用

@Override  public void readFields(DataInput in) throws IOException {
       value = in.readLong();  }  @Override  public void write(DataOutput out) throws IOException {
       out.writeLong(value);  }

使用的时候,传入一个DataInput对象,就可以从中读取解析数据. 也可以说叫做反序列化.

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