Question

在AccountApplet类的actionPerformed方法中，我试图让setBalance在调用时更新余额，我不知道为什么，但我调用它进行存款的方式不会更新余额。这是它的正确输出应该是什么样子。注意当我跑我的时，1000保持1000而不是余额+存款输入

我还收到以下错误

AccountApplet2.java:155: error: non-static method getId() cannot be referenced from a static context
    aitf.setText("" + Account.getId());
                             ^
AccountApplet2.java:156: error: non-static method getBalance() cannot be referenced from a static context
    abtf.setText("" + fmt.format(Account.getBalance()));

这是我的帐户小程序类

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


public class AccountApplet2 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(Double.parseDouble(dptf.getText())); // doesnt update the 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);
        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) 
      {
        // Something went wrong - handle your error here
        status.setText(" for withdraw");
      }

      refreshFields();
    }
  }


  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

方法等等的帐户类

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;



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

  Account (int id, double balance)
  {
    id      = 1234;
    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;


  }




}

Answer 1

您正在整个程序中创建新的Account对象，并发现更改一个Account对象对其他对象没有影响。解决此问题的关键是在GUI中创建一个帐户对象，单独对其进行更改，并在GUI中显示这些更改。要查看我的意思，请在代码中搜索new Account。在你的程序中应该只有其中一个，它不应该在任何动作监听器代码中。

实际上，我会在程序的变量声明部分创建我的Account对象并完成它：

public class AccountApplet2 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 *******

另请注意，这已被破坏：

Account (int id, double balance) {
    id = 1234;
    this.balance = balance;
}

您完全忽略了传递给参数的id。更好的是

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

关于新的“非静态方法无法从静态上下文引用”错误，请勿尝试在Account类上调用实例（非静态）方法。而是在帐户变量上调用它们（根据我上面的建议创建它之后）。

例如，改变

int id = Account.getId();

到

int id = account.getId();

但只有在您根据我的建议宣布并初始化帐户类型的帐户变量后才会再次使用。

请阅读Java and OOP Tutorial了解有关类和变量（或实例）的关键概念的更多信息＆gt;

通过从另一个类调用方法来更新信息

1 个答案: