import java.util.Base64;import static java.lang.Math.*;class BitStream {    byte[] b;    // 需要补上几个字节,1个或者2个    int add;    // 长度,编码之后包含多少个有效字符    int length;    public BitStream(byte[] b) {        this.b = b;        length = (int) ceil(b.length / 3.0 * 4);        add = (3 - b.length % 3) % 3;    }    // 获取第x个bit,其中byte的最高位为第一个bit,次高位为第二个bit...也就是说此流为大头序    int getBit(int x) {        if (x >= b.length * 8)            return 0;        return (b[x / 8] & (1 << (7 - x % 8))) == 0 ? 0 : 1;    }    // 获取第x个字符,其中6x+0为最高位,6x+1为次高位...也就是此流为大头序    int get(int x) {        int ans = 0;        for (int i = 0; i < 6; i++) {            ans = ans << 1 | getBit(x * 6 + i);        }        return ans;    }}public class Base64Test {    static String encode(byte[] b) {        // 首先构造字符映射        String map = "";        for (char c = ‘A‘; c <= ‘Z‘; c++) {            map += c;        }        for (char c = ‘a‘; c <= ‘z‘; c++) {            map += c;        }        for (char c = ‘0‘; c <= ‘9‘; c++) {            map += c;        }        map += "+/";        String ans = "";        BitStream cin = new BitStream(b);        for (int i = 0; i < cin.length; i++) {            ans += map.charAt(cin.get(i));        }        // 添加了几个字符就add几个字符        ans += "==".substring(0, cin.add);        return ans;    }    public static void main(String[] args) {        byte[] bytes = "魏印福ad".getBytes();        String s = Base64.getEncoder().encodeToString(bytes);        System.out.println(s);        System.out.println(encode(bytes));    }}