java - 余额 -= 10，但减去 20？

Question

我正在尝试在类里面使用一种收费的方法。方法是这样的:

  public double chargeFee()
  {
    balance -= 10;
    return balance;
  }

但是，它会减去 20。我尝试重新编译它，但找不到导致问题的原因。

完整代码:

public class ManageAccounts
{
    public static void main(String[] args)
    {
     Account acct1, acct2; 


    //create account1 for Sally with $1000
    acct1 = new Account(1000, "Sally", 1111);
    acct2 = new Account(500, "Joe", 2222);//create account2 for Joe with $500

    System.out.println("Depositing $100 into Account 2222...");
    acct2.deposit(100.00);//deposit $100 to Joe's account
    System.out.println("New Balance for Account 2222: $" + acct2.getBalance());//print Joe's new balance (use getBalance())
    System.out.println();
    System.out.println("Withdrawing $50 from Account 1111...");
    acct1.withdraw(50);//withdraw $50 from Sally's account

    System.out.println("New Balance for Account 1111: $" + acct1.getBalance());//print Sally's new balance (use getBalance())
    System.out.println();
    acct1.chargeFee();
    acct2.chargeFee();//charge fees to both accounts
    System.out.println("Charging usage Fees...");
    System.out.println("Account balance after fees:");
    System.out.println("Account 1111: $" + acct1.chargeFee());
    System.out.println("Account 2222: $" + acct2.chargeFee());
    System.out.println();
    System.out.println("Changing name on Account 2222...");
    acct2.changeName("Joseph");//change the name on Joe's account to Joseph
    System.out.println();
    System.out.println("Printing account summaries...");
    System.out.println(acct1.toString());
    System.out.println(acct2.toString());//print summary for both accounts

    }
}



import java.text.NumberFormat;

public class Account
{
  private double balance;
  private String name;
  private long acctNum;
    NumberFormat money = NumberFormat.getCurrencyInstance();
  //----------------------------------------------
  //Constructor -- initializes balance, owner, and account number
  //----------------------------------------------
  public Account(double initBal, String owner, long number)
  {
    balance = initBal;
    name = owner;
    acctNum = number;
  }

  //----------------------------------------------
  // Checks to see if balance is sufficient for withdrawal.
  // If so, decrements balance by amount; if not, prints message.
  //----------------------------------------------
  public void withdraw(double amount)
  {
    if (balance >= amount)
       balance -= amount;
    else
       System.out.println("Insufficient funds");
  }

  //----------------------------------------------
  // Adds deposit amount to balance.
  //----------------------------------------------
  public void deposit(double amount)
  {
    balance += amount;
  }

  //----------------------------------------------
  // Returns balance.
  //----------------------------------------------
  public double getBalance()
  {
    return balance;
  }


  //----------------------------------------------
  // Returns a string containing the name, account number, and balance.
  //----------------------------------------------
  public String toString()
  {
    return ("Name: " + name + "\tAccount Number: " + acctNum + "\tBalance: " + money.format(balance));
  }

  //----------------------------------------------
  // Deducts $10 service fee
  //----------------------------------------------
  public double chargeFee()
  {
    balance -= 10;
     return balance;
  }

  //----------------------------------------------
  // Changes the name on the account 
  //----------------------------------------------
  public void changeName(String newName)                          
  {
    name = newName;
  }

}

一些示例输出:

Depositing $100 into Account 2222...
New Balance for Account 2222: $600.0

Withdrawing $50 from Account 1111...
New Balance for Account 1111: $950.0

Charging usage Fees...

Account balance after fees:

Account 1111: $930.0
Account 2222: $580.0

Changing name on Account 2222...

Printing account summaries...
Name: Sally Account Number: 1111    Balance: $930.00
Name: Joseph    Account Number: 2222    Balance: $580.00

Answer 1

您在两个帐户上调用了两次 chargeFee，一次是在您收取实际费用时，另一次是在您打印结果时。

为了将来引用，有一个名为 "Seperate command and query" 的指南, 或 CQS。你可能想阅读它。了解您的情况:

• 您正在使用 chargeFee 作为查询(即，您返回余额)
• 然后你使用chargeFee作为命令，扣钱。

更改 chargeFee 以返回一个 void(如果您想要方法链接，则返回 this)，您就不会不小心将其用作查询。

http://en.wikipedia.org/wiki/Command-query_separation