public class HelloWorld{
  public static void main(String[] args){
    int num = 999;
    int count = 0;
    while(num>=0 && num<=999999999){//为什么num>=0不正确，而num>0是正确的
        num = num/10;
        count++;
    }
    System.out.println("它是个"+count+"位的数！");
  }
}