Convert integer source code to binary to hexadecimal

There is a private method, where tounsignedstring (int I, int shift) is
Public static string tohexstring (int I ){
Return tounsignedstring (I, 4 );
}
Public static string tobinarystring (int I ){
Return tounsignedstring (I, 1 );
}
Two methods are provided.

final static char[] digits = {        '0' , '1' , '2' , '3' , '4' , '5' ,        '6' , '7' , '8' , '9' , 'a' , 'b' ,        'c' , 'd' , 'e' , 'f' , 'g' , 'h' ,        'i' , 'j' , 'k' , 'l' , 'm' , 'n' ,        'o' , 'p' , 'q' , 'r' , 's' , 't' ,        'u' , 'v' , 'w' , 'x' , 'y' , 'z'    }; private static String toUnsignedString(int i, int shift) {        char[] buf = new char[32];        int charPos = 32;        int radix = 1 << shift;        int mask = radix - 1;        do {            buf[--charPos] = digits[i & mask];            i >>>= shift;        } while (i != 0);        return new String(buf, charPos, (32 - charPos));    }

For example, convert 559 to hexadecimal:

559 binary bits 10 0010 1111;

Tounsignedstring (I, 4 );
1. Take the hexadecimal character corresponding to the 4-digit calculation;
2. Move four places to the right and add 0 to the front;
Repeat 1, 2;
----------
Mask = 15, corresponding to 1111;

10 0010 1111 & Mask = 1111-> 15-> digits [15]-> F;
Shift four digits right after 00 0010 0010 & Mask = 0010-> 2-> digits [2]-> 2;
Shift four digits right after 00 0000 0010 & Mask = 0010-> 2-> digits [2]-> 2;

----> 22f

Similarly, if it is converted to an octal format, it is: tounsignedstring (I, 3 );

I feel that all my algorithms are weak.

Convert integer source code to binary to hexadecimal