Android Base64处理数据

public class Base64 {
    private static final char[] legalChars = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"  
            .toCharArray();  
 
    public static String encode(byte[] data) {  
        int start = 0;  
        int len = data.length;  
        StringBuffer buf = new StringBuffer(data.length * 3 / 2);  
 
        int end = len - 3;  
        int i = start;  
        int n = 0;  
 
        while (i <= end) {  
            int d = ((((int) data[i]) & 0x0ff) << 16)  
                    | ((((int) data[i + 1]) & 0x0ff) << 8)  
                    | (((int) data[i + 2]) & 0x0ff);  
 
            buf.append(legalChars[(d >> 18) & 63]);  
            buf.append(legalChars[(d >> 12) & 63]);  
            buf.append(legalChars[(d >> 6) & 63]);  
            buf.append(legalChars[d & 63]);  
 
            i += 3;  
 
            if (n++ >= 14) {  
                n = 0;  
                buf.append(" ");  
            }  
        }  
 
        if (i == start + len - 2) {  
            int d = ((((int) data[i]) & 0x0ff) << 16)  
                    | ((((int) data[i + 1]) & 255) << 8);  
 
            buf.append(legalChars[(d >> 18) & 63]);  
            buf.append(legalChars[(d >> 12) & 63]);  
            buf.append(legalChars[(d >> 6) & 63]);  
            buf.append("=");  
        } else if (i == start + len - 1) {  
            int d = (((int) data[i]) & 0x0ff) << 16;  
 
            buf.append(legalChars[(d >> 18) & 63]);  
            buf.append(legalChars[(d >> 12) & 63]);  
            buf.append("==");  
        }  
 
        return buf.toString();  
    }  
 
    private static int decode(char c) {  
        if (c >= 'A' && c <= 'Z')  
            return ((int) c) - 65;  
        else if (c >= 'a' && c <= 'z')  
            return ((int) c) - 97 + 26;  
        else if (c >= '0' && c <= '9')  
            return ((int) c) - 48 + 26 + 26;  
        else  
            switch (c) {  
            case '+':  
                return 62;  
            case '/':  
                return 63;  
            case '=':  
                return 0;  
            default:  
                throw new RuntimeException("unexpected code: " + c);  
            }  
    }  
 
    public static byte[] decode(String s) {  
 
        ByteArrayOutputStream bos = new ByteArrayOutputStream();  
        try {  
            decode(s, bos);  
        } catch (IOException e) {  
            throw new RuntimeException();  
        }  
        byte[] decodedBytes = bos.toByteArray();  
        try {  
            bos.close();  
            bos = null;  
        } catch (IOException ex) {  
            System.err.println("Error while decoding BASE64: " + ex.toString());  
        }  
        return decodedBytes;  
    }  
 
    private static void decode(String s, OutputStream os) throws IOException {  
        int i = 0;  
 
        int len = s.length();  
 
        while (true) {  
            while (i < len && s.charAt(i) <= ' ')  
                i++;  
 
            if (i == len)  
                break;  
 
            int tri = (decode(s.charAt(i)) << 18)  
                    + (decode(s.charAt(i + 1)) << 12)  
                    + (decode(s.charAt(i + 2)) << 6)  
                    + (decode(s.charAt(i + 3)));  
 
            os.write((tri >> 16) & 255);  
            if (s.charAt(i + 2) == '=')  
                break;  
            os.write((tri >> 8) & 255);  
            if (s.charAt(i + 3) == '=')  
                break;  
            os.write(tri & 255);  
 
            i += 4;  
        }  
    }  
}