為什么我不斷收到缺少符號的編譯錯誤？

Question

錯誤：

CreditCard.java:180: error: cannot find symbol
        } while (cardNum7 != 0);
                 ^
  symbol:   variable cardNum7
  location: class CreditCard
1 error
Process javac exited with code 1

碼：

do {
    //Declarations
    long cardNum7 = 0;
    //testing
    System.out.print("Enter a card number (type 0 to end program): ");
    cardNum7 = keyboard.nextLong();
    System.out.println("Your credit card is " + isValid(cardNum7));
} while (cardNum7 != 0);

Answer 1

cardNum7在do-while塊內聲明。 它在該特定代碼塊之外不存在，因此您不能在這里訪問它：

} while (cardNum7 != 0);

您可以在do-while之前聲明cardNum7，以使while語句可以訪問它。

Answer 2

您在循環內聲明了cardNum7 ，因此不會在循環外反映出來。 采用：

//Declarations
long cardNum7 = 0;
do {
    //testing
    System.out.print("Enter a card number (type 0 to end program): ");
    cardNum7 = keyboard.nextLong();
    System.out.println("Your credit card is " + isValid(cardNum7));
} while (cardNum7 != 0);