与Java类有关的问题

Question

我遇到了一个家庭作业的麻烦，创建一个调用另一个类的方法的类。给我们下一课：

/**
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;
  }
}

然后给出以下提示：

实现一个类Portfolio。这个类有两个对象,checking和savings,类型为BankAccount。实现四种方法：

• 公用无效存款（双倍金额，字符串帐户）
• 公开无效提取（双倍金额，字符串帐户）
• 公有作废转账（双倍金额，字符串账户）
• 公共双getBalance（字符串帐户）

这里的帐户字符串是“s”或“c”。对于存款或取款，它表示哪个帐户受到影响。对于转账，它表示取钱的帐户；这笔钱会自动转到另一个账户。“

下面是我所做的：

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();
    }
   }
}

但这样我就连存款的方法都用不上了。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");

    }

}

我得到这个错误：

线程“main”java.lang.NullPointerException在Portfolio.deposite(Portfolio.java:14)在PortfolioTester.main（PortfolioTester.java:6)

我想我有点误会了类是怎么工作。谁能给我指出正确的方向吗？

Answer 1

您之所以会出现这些错误，是因为您必须初始化存款方法中使用的BankAccount类。我建议在portfolio类中放置一个构造函数（类通常应该有初始化类属性的构造函数）。在组合构造函数中，可以初始化银行帐户类“检查”和“储蓄”。

记住对于非静态方法，您必须有一个类的实例才能调用它的方法。