Loop

public class Solution {
    private static final String characters = "0123456789abcdef";
    private static final char[] digits = characters.toCharArray(); 
    
    private Stack<Integer> stack = new Stack<>();
    public String toHex(int num) {
        if (num == 0) return "0";
        
        stack.clear();
        while (num != 0) {
            stack.push(getDigit(num));
            num = num >>> 4;
            System.out.println(num);
            if (stack.size() > 8) break;
        }
        
        StringBuilder buffer = new StringBuilder();
        while (!stack.empty()) {
            buffer.append(digits[stack.pop()]);
        }
        
        return buffer.toString();
    }
    
    private int getDigit(int num) {
        int result = num & 0xF;
        return result;
    }
}