public?class?HelloWorld{?
????public?static?void?main(String[]?args){?
????????int?num?=?999;?
????????int?count?=?0;?
????????if(num>=0?&&?num<=999999999){
????????????while(num!=0){
????????????????count++;
????????????????num/=10;
????????????}?
????????????System.out.println("它是個(gè)"+count+"位的數(shù)！");?
????????}
????????else{
????????????System.out.println("輸入有誤！");
????????}
???}

這是你的代碼，發(fā)現(xiàn)問(wèn)題的所在了么？

public?class?HelloWorld{
????public?static?void?main(String[]?args){
????????int?num?=?999;
????????int?count?=?0;
????????if?(num?>=?0?&&?num?<=?999999999)?{
????????????while?(num?!=?0)?{
????????????????count++;
????????????????num?/=?10;
????????????}
????????????System.out.println("它是個(gè)"?+?count?+?"位的數(shù)！");
????????}?else?{
????????????System.out.println("輸入有誤！");
????????}
????}
}

認(rèn)真對(duì)比下代碼的不同之處

你符號(hào)打錯(cuò)了。“輸入后面”分號(hào)是英文的 ?

else 的輸入有誤那句話(huà)最后一個(gè)分號(hào)是中文分號(hào),僅此而已.......