爲什麼我一直得到一個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 

代碼：

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

你有循環內聲明cardNum7所以不會外循環中得到體現。用途：

//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); 

Answer 2

cardNum7聲明內的DO-而塊。它沒有特定的代碼塊之外的存在，所以你不能在這裏訪問：

} while (cardNum7 != 0); 

你可以在做，而之前宣佈cardNum7使其while語句訪問。