方法需要從用戶返回字符串。需要排除故障

Question

我是Java編程的初學者，我盡我所能，但我似乎無法解決我的問題。

我面臨的問題是getCardID()方法保持返回空值，getTokenBalance()似乎工作正常，或者我需要特殊的代碼從getCardID()返回一個字符串值？

任何幫助是非常感激。

這是Prepaidcard.java類：

class PrepaidCard { 
    private String cardID; 
    private int tokenBalance; 

    public PrepaidCard(String id) { // My First Constructor 
     String cardID = id ; 
    } 

    public PrepaidCard(String id, int token) { // My Second Constructor 
     String cardID = id; 
     tokenBalance = token; 
    } 

    public void addToken(int token) { // Methods 
     tokenBalance =token+tokenBalance; 
    } 

    public void deductToken(int token) { // Methods 
     tokenBalance=tokenBalance-token; 
    } 

    public int getTokenBalance() { // Methods 
     return tokenBalance; 
    } 

    public String getCardID() { // Why does this method keep returning null value? 
     return cardID; 
    } 
} 

這是類用來測試prepaidcard.java

import java.util.*; 

class testprepaid { 
    public static void main(String [] args) { 
     Scanner sc = new Scanner(System.in); 

     System.out.print("Insert card1 id : "); 
     String newid = sc.nextLine(); // my scanner class that read user input in String 

     System.out.print("Insert card2 id : "); 
     String newid2 = sc.nextLine(); // my scanner class that read user input in String 

     System.out.print("Enter Card2 initial token : "); 
     int itoken = sc.nextInt(); 

     PrepaidCard card1 = new PrepaidCard(newid); // object created base on 1st & 2nd Constructor 
     PrepaidCard card2 = new PrepaidCard(newid2,itoken); 

     System.out.println("Card1 ID: " + card1.getCardID());  // the method return null when called 
     System.out.println("Card1 token balance : " + card1.getTokenBalance()); 
     System.out.println("Card2 ID: " + card2.getCardID());   // and this 
     System.out.println("Card2 token balance : " + card2.getTokenBalance()); 

與其他行有沒有錯誤，所以我沒有添加。

你可以看到我的運行時錯誤here。

Answer 1

在你的構造函數，你宣佈一個新的局部變量：

public PrepaidCard(String id, int token) {// My Second Constructor 

    String cardID = id; //This should be cardID = id; 
    tokenBalance = token; 
} 

根據您的IDE，您可以啓用警告，可以告訴你未初始化的私有成員。

Answer 2

String cardID = id ; 

應該是一個實例變量。

所以刪除字符串cardID的局部聲明 -

public PrepaidCard(String id){ //My First Constructor 
cardID = id ; 
}