Java中的类型转换，Integer、Long、String

这段时间将项目中一个模块参照C++源代码，实现一个JAVA版。主要功能是将一些字段信息转换为String类型，传输后可以进行解析。

Integer、Long转为String，Java本身提供了这种转换方法。

Example:

int a = 127;
String s = new String(a);
System.out.println(s);
//输出 127

而我的项目需求是Integer、Long字段在String类型中所占大小为固定自己。比如Integer类型的127在内存中应该为0x7f,0x00,0x00,0x00(Little Endian)，在String类型输出可能为乱码，但需要在解析后可以还原为Integer类型的127。

C++中存在无符号数，而JAVA中没有无符号数。所以以byte[]替代unsigned char[]会出现小问题。见下：

       int n = 0xff7f0012;
        byte[] b = new byte[4];
        b[3] = (byte) (n & 0xff);
        b[2] = (byte) (n >> 8 & 0xff);
        b[1] = (byte) (n >> 16 & 0xff);
        b[0] = (byte) (n >> 24 & 0xff);
        String s = new String(b);
        for (byte i : b) {
                System.out.printf("%02x  ", i);
        }
        System.out.println();
        System.out.println(s.length());
        for (byte i : s.getBytes()) {
                System.out.printf("%02x  ", i);
        }
//输出如下
ff  7f  00  12  
3
3f  00  12  

由上可见，byte[]转换String类型时，会发生问题，应为byte表示的范围是-128~127，当byte[]中元素大于127时，会被转换成63。尝试使用不同的编码方案解决。

测试发现UTF-8,GBK,US-ASCII均未得到正确结果。尝试charset为8859_1得到正确结果。

将上面程序中的String s = new String(b);替换为下面程序段：

String s = "";
try {
    s = new String(b, "8859_1");
} catch (UnsupportedEncodingException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}    

即可得到正确结果，下面是程序功能的简单封装。

public class Test{

        private static byte[] intToByte(int n) {
                byte[] b = new byte[4];
                b[3] = (byte) (n & 0xff);
                b[2] = (byte) (n >> 8 & 0xff);
                b[1] = (byte) (n >> 16 & 0xff);
                b[0] = (byte) (n >> 24 & 0xff);
                return b;
        }
        
        private static byte[] longToByte(long n) {
                byte[] b = new byte[8];
                b[7] = (byte) (n & 0xff);
                b[6] = (byte) (n >> 8 & 0xff);
                b[5] = (byte) (n >> 16 & 0xff);
                b[4] = (byte) (n >> 24 & 0xff);
                b[3] = (byte) (n >> 32 & 0xff);
                b[2] = (byte) (n >> 40 & 0xff);
                b[1] = (byte) (n >> 48 & 0xff);
                b[0] = (byte) (n >> 56 & 0xff);
                return b;
        }
        
        private static int byteToInt(byte[] b, int beginPos) {
                return b[beginPos + 3] & 0xff |
                                (b[beginPos + 2] & 0xff) << 8 |
                                (b[beginPos + 1] & 0xff) << 16 |
                                (b[beginPos] & 0xff) << 24;
        }
        
        private static int byteToLong(byte[] b, int beginPos) {
                return b[beginPos + 7] & 0xff |
                                (b[beginPos + 6] & 0xff) << 8 |
                                (b[beginPos + 5] & 0xff) << 16 |
                                (b[beginPos + 4] & 0xff) << 24 |
                                (b[beginPos + 3] & 0xff) << 32 |
                                (b[beginPos + 2] & 0xff) << 40 |
                                (b[beginPos + 1] & 0xff) << 48 |
                                (b[beginPos] & 0xff) << 56;
        }
        
        private static String intToString(int n) {
                byte[] b = intToByte(n);
                String s = "";
                try {
                        s = new String(b, "8859_1");
                } catch (UnsupportedEncodingException e) {
                        e.printStackTrace();
                }
                
                return new String(s);
        }
        
        private static String longToString(long n) {
                byte[] b = longToByte(n);
                String s = "";
                try {
                        s = new String(b, "8859_1");
                } catch (UnsupportedEncodingException e) {
                        e.printStackTrace();
                }
                
                return new String(s);
        }
        
        private static int stringToInt(String str) {
                byte[] b = new byte[4];
                try {
                        b = str.getBytes("8859_1");
                } catch (UnsupportedEncodingException e) {
                        e.printStackTrace();
                }
                return byteToInt(b, 0);
        }
        
        private static int stringToLong(String str) {
                byte[] b = new byte[8];
                try {
                        b = str.getBytes("8859_1");
                } catch (UnsupportedEncodingException e) {
                        e.printStackTrace();
                }
                return byteToLong(b, 0);
        }
}