这道题为什么能输出它是几位数，我好晕，求指教

public class HelloWorld{

public static void main(String[] args){

int num = 100;

int count = 0;

if (num >= 0  && num<=999999999){

while(num != 0){

count++;

num/=10;

}

System.out.println("它是个"+ count+"位的数！");

} else{

System.out.println("输入有误！");

}

}

}