java的seek()函数

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;


public class Text {
public static void main(String[] args) throws IOException {
long pos=3l; 
RandomAccessFile rad=new RandomAccessFile("E://demo.txt", "r");
rad.seek(pos);
rad.readInt();
rad.readShort();
}

}

关于sekk() 的源代码：

    /**
     * Sets the file-pointer offset, measured from the beginning of this
     * file, at which the next read or write occurs.  The offset may be
     * set beyond the end of the file. Setting the offset beyond the end
     * of the file does not change the file length.  The file length will
     * change only by writing after the offset has been set beyond the end
     * of the file.
     *
     * @param      pos   the offset position, measured in bytes from the
     *                   beginning of the file, at which to set the file
     *                   pointer.
     * @exception  IOException  if pos is less than
     *                          0 or if an I/O error occurs.
     */

大概的意思是：跳到pos的位置，以字节为单位

readint 的用法，返回四个字节长度的数据即返回一个int的值，指针自动跳过四个字节，源代码如下：

  /**
     * Reads a signed 32-bit integer from this file. This method reads 4
     * bytes from the file, starting at the current file pointer.
     * If the bytes read, in order, are b1,
     * b2, b3, and b4, where
     * 0 <= b1, b2, b3, b4 <= 255,
     * then the result is equal to:
     *

      *     (b1 << 24) | (b2 << 16) + (b3 << 8) + b4
      * 

     *

     * This method blocks until the four bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return     the next four bytes of this file, interpreted as an
     *             int.
     * @exception  EOFException  if this file reaches the end before reading
     *               four bytes.
     * @exception  IOException   if an I/O error occurs.
     */
    public final int readInt() throws IOException {
        int ch1 = this.read();
        int ch2 = this.read();
        int ch3 = this.read();
        int ch4 = this.read();
        if ((ch1 | ch2 | ch3 | ch4) < 0)
            throw new EOFException();
        return ((ch1 << 24) + (ch2 << 16) + (ch3 << 8) + (ch4 << 0));
    }

eadShort();跟上面一样，返回两个字节的数，即返回一个short的值。源码如下：

   /**
     * Reads a signed 16-bit number from this file. The method reads two
     * bytes from this file, starting at the current file pointer.
     * If the two bytes read, in order, are
     * b1 and b2, where each of the two values is
     * between 0 and 255, inclusive, then the
     * result is equal to:
     *

      *     (short)((b1 << 8) | b2)
      * 

     *

     * This method blocks until the two bytes are read, the end of the
     * stream is detected, or an exception is thrown.
     *
     * @return     the next two bytes of this file, interpreted as a signed
     *             16-bit number.
     * @exception  EOFException  if this file reaches the end before reading
     *               two bytes.
     * @exception  IOException   if an I/O error occurs.
     */
    public final short readShort() throws IOException {
        int ch1 = this.read();
        int ch2 = this.read();
        if ((ch1 | ch2) < 0)
            throw new EOFException();
        return (short)((ch1 << 8) + (ch2 << 0));
    }