java 数字与字节的转换(IP与字节的转换)

网络传输中，Java 采用字节形式，所以涉及到字节与其他数据类型的转换。
本文以IP 为例进行讨论。

先看代码
本例 是，接收一个IP，转换为二进制数据传输，再转换为IP，
IP-->Binary-->IP

package com.mkyong.core;
 
public class BitwiseExample {
 
  public static void main(String[] args) {
 
	BitwiseExample obj = new BitwiseExample();
	long ipAddressInLong = obj.ipToLong("192.168.1.2");
	System.out.println(ipAddressInLong);
 
	String binary = Long.toBinaryString(ipAddressInLong);
	printPrettyBinary(binary);
 
	String ipAddressInString = obj.longToIp(ipAddressInLong);
	System.out.println(ipAddressInString);
 
  }
 
  public long ipToLong(String ipAddress) {
 
	String[] addrArray = ipAddress.split("\\.");
 
	long num = 0;
	for (int i = 0; i < addrArray.length; i++) {
 
		int power = 3 - i;
 
		// 1. (192 % 256) * 256 pow 3
		// 2. (168 % 256) * 256 pow 2
		// 3. (2 % 256) * 256 pow 1
		// 4. (1 % 256) * 256 pow 0
		num += ((Integer.parseInt(addrArray[i]) % 256 * Math.pow(256, power)));
 
	}
 
	return num;
  }
 
  public String longToIp(long i) {
 
	return ((i >> 24) & 0xFF) + "." + ((i >> 16) & 0xFF) + "." + ((i >> 8) & 0xFF) + "." + (i & 0xFF);
 
  }
 
  //print pretty binary code, padding left zero 
  private static void printPrettyBinary(String binary) {
 
	String s1 = String.format("%32s", binary).replace(' ', '0');
	System.out.format("%8s %8s %8s %8s %n", 
                s1.substring(0, 8), s1.substring(8, 16), 
                s1.substring(16, 24), s1.substring(24, 32));
  }
 
}

输出

 3232235778
11000000 10101000 00000001 00000010 
192.168.1.2

Testing IP address = 192.168.1.2
Decimal Number     = 3232235778

转换成10进制

192 x (256)^3 + 168 x (256)^2 + 1 x (256)^1 + 2 (256)^0 = ?
3221225472 + 11010048 + 256 + 2 = 3232235778

 long ipAddress = 3232235778L;
  String binary = Long.toBinaryString(ipAddress);
  System.out.println(binary);
  System.out.println((ipAddress>>24) & 0xFF);
  System.out.println((ipAddress>>16) & 0xFF);
  System.out.println((ipAddress>>8) & 0xFF);
  System.out.println((ipAddress) & 0xFF);


11000000101010000000000100000010
192
168
1
2

为什么  (ipAddress>>24) & 0xFF will return 192

 Decimal   = 3232235778
Binary    = 11000000 10101000 00000001 00000010
IpAddress = 192      168      1        2
 
(ipAddress>>24)
            -------------------------->24
Binary    = 00000000 00000000 00000000 11000000
 
(ipAddress>>24) & 0xFF
Binary    = 00000000 00000000 00000000 11000000
& 0xFF    = 00000000 00000000 00000000 11111111
Result    = 00000000 00000000 00000000 11000000 = 192 (2^7 + 2^6)

(ipAddress>>16) & 0xFF = 168

  Decimal = 3232235778
Binary  = 11000000 10101000 00000001 00000010
 
(ipAddress>>16)
          ----------------->16
Binary  = 00000000 00000000 11000000 10101000 
 
(ipAddress>>16) & 0xFF
Binary  = 00000000 00000000 11000000 10101000 = 49320 (2^14 + 2^15 + 2^7 + 2^5 + 2^3)
& 0xFF  = 00000000 00000000 00000000 11111111
Result  = 00000000 00000000 00000000 10101000 = 168

(ipAddress>>8) & 0xFF = 1

  Decimal = 3232235778
Binary  = 11000000 10101000 00000001 00000010
 
(ipAddress>>8)
          -------->8
Binary  = 00000000 11000000 10101000 00000001
 
(ipAddress>>8) & 0xFF
Binary  = 00000000 11000000 10101000 00000001 = 12625921
& 0xFF  = 00000000 00000000 00000000 11111111
Result  = 00000000 00000000 00000000 00000001 = 1

(ipAddress) & 0xFF = 2

 Decimal = 3232235778
Binary  = 11000000 10101000 00000001 00000010
 
(ipAddress)
Binary  = 11000000 10101000 00000001 00000010
 
(ipAddress) & 0xFF
Binary  = 11000000 10101000 00000001 00000010 = 3232235778
& 0xFF  = 00000000 00000000 00000000 11111111
Result  = 00000000 00000000 00000000 00000010 = 2