方法需要从user返回字符串.仅需要进行故障排除

Question

我是Java编程的初学者,我尽我所能,但我似乎无法解决我的问题.

我面临的问题是getCardID()方法保持返回null值,getTokenBalance()似乎工作正常,或者我需要特殊的代码来返回字符串值getCardID()？

任何帮助都非常感谢.

这是Prepaidcard.java班级:

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;
    }
}

这是用于测试的类 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());

而其他行没有错误,所以我没有添加它们.

您可以在此处查看我的运行时错误.

Answer 1

在构造函数中,您声明了一个新的局部变量:

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

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

根据您的IDE,您可以启用可以告诉您未初始化的私有成员的警告.