产生异常错误信息

Question

在AccountApplet中我试图显示错误信息“Empty field not allowed for deposit”，一部分是通过getMessage方法完成的，另一半是不允许存款的，

但是在我的程序中，getMessage 方法产生“空字符串”，而不是“空字段”，我将如何进行更改？

getMessage 方面在 actionPerformed 方法中完成

这是 AccountApplet 类

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.border.*;
import java.text.NumberFormat;


public class AccountApplet extends JApplet implements ActionListener
{    

  //  For West
  public JLabel  ai       = new JLabel("Account ID ");
  public JTextField  aitf = new JTextField();
  public JLabel  ab       = new JLabel("Account Balance ");
  public JTextField  abtf = new JTextField();

  //  For East
  public JButton     dp   = new JButton ("Deposit");
  public JTextField  dptf = new JTextField();
  public JButton       wt = new JButton ("Withdraw");
  public JTextField  wttf = new JTextField();

  // For South
  public JLabel  status   = new JLabel("");

  Account account = new Account(1234,1000);  // ******** here *******  

  public void init()
  {
    this.setSize(400, 90);

    //----------------------
    //  Set up the Structure
    //----------------------

    Container      c = getContentPane();
    JPanel         b = new JPanel(new BorderLayout());
    JPanel      west = new JPanel(new GridLayout(2,2));
    JPanel      east = new JPanel(new BorderLayout());
    JPanel depo_with = new JPanel(new GridLayout(2,2));

    // Add BorderLayout to the container
    c.add(b);

    // Add everything to West
    b.add(west, BorderLayout.WEST);
    west.setBorder(new TitledBorder("Display Account Information"));
    west.add(ai);
    west.add(aitf);
    aitf.setEditable(false);
    west.add(ab);
    west.add(abtf);
    abtf.setEditable(false);

    // Add everything to EAST
    b.add(east, BorderLayout.EAST); 
    east.setBorder(new TitledBorder("Deposit or Withdrawl Funds"));    
    east.add(depo_with, BorderLayout.EAST);    
    depo_with.add(dptf);
    depo_with.add(dp);
    depo_with.add(wttf);
    depo_with.add(wt);   
    dp.addActionListener(this);
    wt.addActionListener(this);

    // Add everything to SOUTH
    b.add(status, BorderLayout.SOUTH);



    refreshFields();

  }  // End intit

//-------------------------------------------------------------------------------------------------------------------------------------------------------
  public void actionPerformed(ActionEvent e)
  {


    if (e.getSource() == dp)  //  Executes if deposit was clicked
    {
      try 
      {  
        getAmount(dptf);
        account.deposit(Double.parseDouble(dptf.getText()));
        account.setBalance(account.balance); 
        status.setText("Deposit processed");

        refreshFields();
      } 


      catch (NegativeAmountException nae) 
      {  
        status.setText(nae.getMessage() + " not allowed for deposit");
      }
      catch (EmptyFieldException efe) 
      {  
        status.setText(efe.getMessage() + " not allowed for deposit");
      }
      catch (Exception ex) 
      { 
        status.setText(ex.getMessage() + " not allowed for deposit");
      } 
    }   

    if (e.getSource() == wt)  //  Executes if withdraw was clicked
    {
      try 
      {  
        getAmount(wttf);
        account.withdraw(Double.parseDouble(wttf.getText()));
        account.setBalance(account.balance); 
        status.setText("Withdraw processed");

        refreshFields();
      } 

      catch (InsufficientFundsException ife)
      {
        status.setText(ife.getMessage() + " Insufficient funds");
      }
      catch (NegativeAmountException nae) 
      {  
        status.setText(nae.getMessage() + " not allowed for withdraw");
      }
      catch (EmptyFieldException efe) 
      {  
        status.setText(efe.getMessage() + " not allowed for withdraw");
      }
      catch (Exception ex) 
      { 
        status.setText(ex.getMessage() + " not allowed for withdraw");
      }    


    }    


  } // end ActionPerformed


  public void refreshFields()
  {
    NumberFormat fmt = NumberFormat.getCurrencyInstance();
    aitf.setText("" + account.getId() );
    abtf.setText("" + fmt.format(account.getBalance()));

    // diplays accound id and balance in left text fields
    //should be called when the applet is first displayed and after each valid transaction
  }

 public double getAmount(JTextField tf) throws EmptyFieldException,
                                               NumberFormatException,
                                               NegativeAmountException
 {
   double depo;

   try 
   {
     depo = Double.parseDouble(dptf.getText());  // read in one textfield and convert to a number
   } 
     catch (NumberFormatException nfe)  // catch NumberFormatException
   {
     throw nfe;  // catch throws NumberFormatException
   }



    return depo;
  }  //  End    



} // End Class

帐户类

import java.awt.*;
import java.awt.event.*;



public class Account
{
  int id         = 1234;
  double balance = 1000.00;

  Account (int id, double balance) 
  {
    this.id = id;
    this.balance = balance;
  }    

  public int getId()
  {

    return id; 
  }

  public double getBalance()
  {
    return balance;   
  }

  public void setBalance(double balance) throws NegativeAmountException
  {
    if ( balance < 0)
      throw new NegativeAmountException();
    this.balance = balance;
  }

  public void deposit(double amount) throws NegativeAmountException
  {
    if (amount < 0)
    throw new NegativeAmountException();
    balance += amount;
  }

  public void withdraw(double amount) throws NegativeAmountException,
                                             InsufficientFundsException
  {

    if (amount > balance )
    {
      throw new NegativeAmountException();
    }

    if (amount > balance )
    {
      throw new InsufficientFundsException();
    }

    balance -= amount;


  }




}

EmptyFieldException

public class EmptyFieldException extends Exception

{
  EmptyFieldException() 
  {
    super();
  }

}

InsufficientFundsException

public class InsufficientFundsException extends Exception
{
  InsufficientFundsException()
  {
    super();
  }
}

NegativeAmountException

public class NegativeAmountException extends Exception
{
  NegativeAmountException()
  {
    super();
  }
}

Answer 1

也许我不明白你的问题，但为什么不简单地用所需的输出设置文本呢？即，改变这个：

catch (EmptyFieldException efe) 
{  
    status.setText(efe.getMessage() + " not allowed for deposit");
}

到这里：

catch (EmptyFieldException efe) 
{  
    status.setText("An empty field is not allowed for deposit");
}

在你的其他 catch 块中也是如此。

---

您在 cmets 中问过为什么从不抛出 InsufficientFundsException，即使在抛出它的条件 if (amount > balance ) 为真时也是如此。

要学会解决这个问题，更重要的是一种伟大的调试技术，它可以帮助你学习解决未来的错误，你需要学会在脑海中遍历你的代码，看看你的脑海里有什么它正在运行。请仔细查看该撤回方法。想象它被调用的提款金额参数大于余额，会发生什么？以什么顺序？

public void withdraw(double amount) throws NegativeAmountException,
                                           InsufficientFundsException {
    if (amount > balance ) {
        throw new NegativeAmountException();
    }
    if (amount > balance ) {
        throw new InsufficientFundsException();
    }
    balance -= amount;
}

请注意，一旦抛出任何异常，该方法就会结束。那么当金额大于余额时会发生什么？什么异常只会被抛出，为什么？如果你按照我的建议去做，答案对你来说就很明显了。