为怎么我注释的这段代码yu不减0的时候（或者为怎么yu还要加一个0），打印出来会报错,为怎么前面还要加个'0'?

import java.util.Scanner;
public class Case17 {
    public static void main(String [] args){
        Scanner in = new Scanner (System.in);
        System.out.println("请输入number的十进制数");
        int number = in.nextInt();
        int shang = number;
        String tex = "";
        int yu;
        while(shang!=0){
                yu = shang%16;
                shang = shang/16;
                char result;
                if(yu<9){
                    result = (char)('0'+(yu-0));//为怎么我注释的这段代码yu不减0的时候，打印出来会报错,为怎么前面还要加个'0'?
                }else{
                    result = (char)('A'+(yu-10));
                }
                tex = result + tex;
            }
        System.out.println(number+"的十六进制形式是："+tex);
    }
}