关于按位与在这个程序的的实际意义，看我注释

Question

public class VoteMsgBinCoder implements VoteMsgCoder{
public static final int MIN_WIRE_LENGTH = 4;
public static final int MAX_WIRE_LENGTH = 16;
public static final int MAGIC = 0x5400;
public static final int MAGIC_MASK = 0xfc00;
public static final int MAGIC_SHIFT = 8;
public static final int RESPONSE_FLAG = 0x0200;
public static final int INQUIRE_FLAG = 0x0100;
public VoteMsg fromWire(byte[] input) throws IOException {
ByteArrayInputStream bs = new ByteArrayInputStream(input);
DataInputStream in = new DataInputStream(bs);
int magic = in.readShort();
/*为什么是和MAGIC_MASK进行位与运算呢，为什么是和MAGIC进行比较呢，而不是其他数，小弟对位运算不是很明白*/
if ((magic & MAGIC_MASK) != MAGIC) {
throw new IOException("Bad Magic #: " +
((magic & MAGIC_MASK) >> MAGIC_SHIFT));
}

      boolean resp = ((magic & RESPONSE_FLAG) != 0);
      boolean inq = ((magic & INQUIRE_FLAG) != 0);
      int candidateID = in.readShort();
      if (candidateID < 0 || candidateID > 1000) {
      throw new IOException("Bad candidate ID: " + candidateID);
      }
      long count = 0;
      if (resp) {
      count = in.readLong();
      if (count < 0) {
      throw new IOException("Bad vote count: " + count);
      }
      }
      // Ignore any extra bytes
      return new VoteMsg(resp, inq, candidateID, count);
      }  

}

Answer 1

(magic & MAGIC_MASK) != MAGIC)

magic是一组二进制位开关
MAGIC_MASK作为参照
这里的意思就是，判断MAGIC_MASK标记为1的字段，在magic中是否全为1

举例
m M m&M m&M==M
00 01 00 0
01 01 01 1
10 01 00 0
11 01 01 1