java計算a+aa+aaa+aaaa的和

public class Test {
    public static void main(String[] args) {
        /*
         * 求a+aa+aaa+aaaa的和
         */
        int a = 1;
        int n = 3;
        int num = 0;
        int sum = 0;
        // 循環是輸出 a aa aaa  
        //所以還要用sum相加
        for (int j = 0; j < n; j++) {
            num = num * 10 + a; // 1 11 111
            System.out.print(num + "\t");
            sum += num;
        }
        System.out.println();
        System.out.println(sum);
    }

}
//執行結果：
1    11    111    
123