while(num!=o){

這里0寫成了o

while(num!=0){

System.out.println("它是個(gè)"+count+"位的數(shù)！")

這里缺;

System.out.println("它是個(gè)"+count+"位的數(shù)！");

括號是成對出現(xiàn)的，最后缺2個(gè)}

注意代碼縮進(jìn)，這樣更易閱讀

修改后的代碼

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!");
????}
??}
}