Instance of the toBinaryString of method of Integer in Java

During an interview, I was asked to write out the toBinaryString() method in the Integer class

In other words, write out the process of converting an Integer to a Binary

But I was surprised, in the JDK source code, found this very useful method, here to show you

Here's a test I did:

package com.b510.test;

public class TestF {
    public static void main(String[] args) {
        //output:1000
        System.out.println(toBinaryString(8));
        //printInfo();
    }

    
    private static void printInfo(){
        for(int i =0; i< 10; i++){
            System.out.println("i= " + i + "         "+(i & 1));
        }
        
    }

    public static String toBinaryString(int i) {
        return toUnsignedString(i, 1);
    }
    
    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 {
            //The mask is always 1, so when I is odd, the "I & mask" operation is 1
            //Otherwise returns: 0
            //System.out.println(i & mask); 
            buf[--charPos] = digits[i & mask];
            i >>>= shift;//Shift the assignment to the right and fill the empty bit on the left with 0
        //System.out.println(buf);
        //System.out.println(charPos);
        //System.out.println(i);
        } while (i != 0);
        return new String(buf, charPos, (32 - charPos));
    }

    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'
        };
}

In our code, we can actually simplify the digits array, because we'll only use the array: digits[0],digits[1]

So:

final static char[] digits = {
        '0' , '1' 
        };

The method USES the shift and & operations, which are key.