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"+count+"位的數！");?
????????}
????????else{
????????????System.out.println("輸入有誤！");
????????}
???}

這是你的代碼，發現問題的所在了么？

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"?+?count?+?"位的數！");
????????}?else?{
????????????System.out.println("輸入有誤！");
????????}
????}
}

認真對比下代碼的不同之處

你符號打錯了。“輸入后面”分號是英文的 ?

else 的輸入有誤那句話最后一個分號是中文分號,僅此而已.......