java - 为什么我不断收到缺少符号的编译错误？

Question

错误：

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

代码：

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 块内声明。它不存在于该特定代码块之外，因此您无法在此处访问它：

} while (cardNum7 != 0);

您可以在 do-while 之前声明 cardNum7 以使其可供 while 语句访问。

Answer 2

您已cardNum7在循环内声明，因此它不会反映在循环外。利用：

//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);