通过调用另一个类的方法来更新信息

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并没有更新余额。这是它的正确输出应该是什么样子。 请注意，当我运行我的时，1000 保持 1000，而不是输入的余额 + 存款

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

方法等所在的账户类

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 对象不会影响其他的 Account 对象。解决这个问题的关键是在 GUI 中创建 一个 Account 对象，单独对其进行更改，并在 GUI 中显示这些更改。要了解我的意思，请在您的代码中搜索new Account。您的程序中应该只有其中之一，并且不应在任何动作侦听器代码中。

事实上，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，这是损坏的：

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

您完全忽略了传递给参数的 id。会更好

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

---

关于您的新“无法从静态上下文引用非静态方法”错误，请不要尝试在 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();

但仅在您按照我上面的建议声明并初始化 Account 类型的帐户变量之后再说一次。

请阅读Java and OOP Tutorial 了解更多关于类和变量（或实例）的关键概念>