Java 类的问题

Question

我在完成家庭作业时遇到了麻烦，我创建了一个调用另一个类的方法的类。我们得到以下类：

/**
A bank account has a balance that can be changed by 
deposits and withdrawals.
*/
public class BankAccount
{  
   private double balance;

/**
Constructs a bank account with a zero balance.
*/
public BankAccount()
{   
  balance = 0;
}

/**
  Constructs a bank account with a given balance.
  @param initialBalance the initial balance
*/
public BankAccount(double initialBalance)
{   
  balance = initialBalance;
}

/**
  Deposits money into this account.
  @param amount the amount to deposit
*/
     public void deposit(double amount)
{  
    balance = balance + amount;
}

 /**
  Makes a withdrawal from this account, or charges a penalty if
  sufficient funds are not available.
  @param amount the amount of the withdrawal
  */
 public void withdraw(double amount)
{   
   final double PENALTY = 10;
   if (amount > balance)
  {
     balance = balance - PENALTY;
  }
  else
  {
      balance = balance - amount;
  }
}

/**
   Adds interest to this account.
   @param rate the interest rate in percent
*/
public void addInterest(double rate)
{   
   double amount = balance * rate / 100;
   balance = balance + amount;
}

/**
   Gets the current balance of this account.
   @return the current balance
*/
public double getBalance()
{   
   return balance;
  }
}

然后我收到以下提示：

“实现一个Portfolio类。这个类有两个对象，checking和savings，类型为BankAccount。实现四个方法：

• 公共无效存款（双倍金额，字符串账户）
• public void 取款（双倍金额，字符串账户）
• public void transfer(double amount, String account)
• public double getBalance(String account)

这里的帐户字符串是“S”或“C”。对于存款或取款，它表明哪个账户受到影响。对于转帐，它表示从哪个帐户取钱；这笔钱会自动转入另一个帐户。”

这就是我所做的：

public class Portfolio 
{
 BankAccount checking;
 BankAccount savings; 

 public void deposit(double x, String y)
 {
    if (y.equals("C"))
    {
        checking.deposit(x);
    }
    else if (y.equals("S"))
    {
        savings.deposit(x);
    }
}


public void withdraw(double x, String y)
{
    if (y.equals("C"))
    {
        checking.withdraw(x);
    }
    else if (y.equals("S"))
    {
        savings.withdraw(x);
    }
}


//public void transfer(double amount, String account)
//{
//  add later
//}

public double getBalance(String account)
{
    if (account.equals("C"))
    {
        return checking.getBalance();
    }
    else
    {
        return savings.getBalance();
    }
   }
}

但是我什至无法使用存款方式。当我运行这个程序时...

public class PortfolioTester
{
    public static void main(String [] args)
    {
        Portfolio money = new Portfolio();
         money.deposit(700, "S");

    }

}

我收到此错误：

线程“main”中的异常 java.lang.NullPointerException 在 Portfolio.deposit（Portfolio.java:14） 在 PortfolioTester.main(PortfolioTester.java:6)

我想我从根本上误解了课程的运作方式。有人能指出我正确的方向吗？

Answer 1

您遇到这些错误是因为您必须初始化 deposit 方法中使用的 BankAccount 类。我建议在Portfolio 类中放置一个构造函数（类通常应该具有初始化类属性的构造函数）。在Portfolio 构造函数中，您可以初始化BankAccount 类“checking”和“savings”。

请记住，对于非静态方法，您必须有一个类的实例才能调用其方法。

Answer 2

问题是您在Portfolio 类中创建了一个BankAccount，但还没有初始化它们。就像这样做：

int myInt;
System.out.println(myInt);

会发生什么？您将无法编译它，因为它尚未初始化。但是当你的变量在类中，在其他类中时，编译器就不能很容易地判断它是否被初始化了。您必须为您的 Portfolio 类创建一个构造函数并初始化 BankAccount 对象，如下所示：

public class Portfolio
{
    BankAccount savings;
    BankAccount checking;

    public Portfolio()
    {
        savings = new x();  //Replace x with what you think needs to be there
        checking = new x(); //Replace x with what you think needs to be there
    }
}