如何使用另一个类的按钮更改 JLabel？

Question

I have two classes and a text file database, The JLabel in the first class, let's call it class1 automatically set it self to the number in the database. Now, in class2 I have this little JFrame and a text field and of course a button, the value I put in the text field overwrites the one in the database, but here's the problem. The label in the first class wont update while running, but if I restart it it will show me the value that I want.

How do I update it while the program is running? I've tried to change the label in the buttonActionperformed in the other class but it gives me a NullPointException every time.

How do I fix this?

THE MAIN CLASS ( JUST THE JFRAME )

package BankrollG;

import java.awt.Graphics;
import javax.swing.JFrame;

public class BGbrain {

    BGbody body = new BGbody();
    JFrame Frame = new JFrame();

    public BGbrain() {
        Frame.setSize(body.width, body.height);
        Frame.setTitle(body.title);
        Frame.setResizable(false);
        Frame.setDefaultCloseOperation(Frame.EXIT_ON_CLOSE);
        Frame.add(body.panel);
        Frame.setLocationRelativeTo(null);
        Frame.setFocusable(true);

        Frame.setVisible(true);
    }

    public static void main(String[] args ) {
        new BGbrain();
    }
}

Then you got the class with the components:

private JLabel bankroll_label

    public BGbody(){        
        panel.setLayout(null);
        windowComponents();           
    }

    public void windowComponents() {
        // Bankroll database access
        try {
           
            FileReader fr = new FileReader("Bankroll.txt");
            BufferedReader br = new BufferedReader(fr);
            set_bankroll = br.readLine();
                
        } catch(IOException e) {
            System.out.println("FEL MED LÄSNING AV DATABAS /// BGBODY");
        }
    
    }
}

THEN you got the JFrame class that I created with the netbeans function

private void AddcurrencyActionPerformed(java.awt.event.ActionEvent evt) {
                    
    String CBR = txt_bankroll.getText();

    try {
        FileWriter fw = new FileWriter("Bankroll.txt");
        PrintWriter pw = new PrintWriter(fw);

        pw.println(CBR);
        pw.close();
    } catch(IOException e) {
        System.out.println("FEL MED INSKRIVNINGEN I DATABASEN");
    }
}         

Now, everything goes as plan, but I can't update my JLabel "bankroll_label" from the button class because it just returns nullpointsexceptions. The data is there, because the JLabel reads from the database but it wont update when changes has been made from the button class. So a getter setter method wont work because the value IS there but it wont update the JLabel.

I hope this made it easier to understand my problem. It's ALOT more code, that dont have to do with this, I hope I simplified it at least some.

Answer 1

Your question is a specific example of a basic problem in programming in Java -- how to transfer information between classes. There are several ways to do this, one of the most elegant being giving to use a "model" class that holds your program's logic code and key data, having one class change the model's state by changing a text String that it holds. Then using a listener or observer pattern, have the model notify the other class that it has been changed so the other class can extract the new information, its new String from the model. While this is likely the best solution, it may be a bit of overkill and likely is above your current level of coding, so for you, I'm not going to recommend this.

Instead, I'm going to recommend a simpler less elegant solution, that you instead have one class call a setter method of the other to push the new String into it.

One problem we have as volunteer answerers here is that your question is hard to answer because it lacks critical code, making it hard for us to guess why your code is misbehaving, why specifically you're running into a NullPointerException (or NPE) when you try to run it. So all I can do is guess, but guess I will try nevertheless.

For simplicity's sake, let's call one class the, the one that holds the JLabel, the LabelClass and the other class the ButtonTextFieldClass .

One possible reason is that you've got a NullPointerException is because your ButtonTextFieldClass may have a LabelClass variable, but never initialized the variable, something like so:

// this guy is null because it is never initialized
private LabelClass labelClass; 

A simple solution could be to try to initialize it like so:

private LabelClass labelClass = new LabelClass();

But this won't work because while it does create and assign a LabelClass instance, it's likely not the LabelClass instance that is visualized in the running GUI.

A better solution is to give the ButtonTextFieldClass a setter method that allows other classes to set the ButtonTextFieldClass with the proper LabelClass instance.

eg,

public void setLabelClass(LabelClass labelClass) {
   this.labelClass = labelClass;
}

This way the code that sets up both classes can pass the visualized LabelClass to the first class, and it can call methods on it.

A simple example of LabelClass could look like so:

class LabelClass extends JPanel {
   private JLabel label = new JLabel("");

   public LabelClass() {
      setBorder(BorderFactory.createTitledBorder("Label Panel"));
      add(label);
   }

   public void setLabelText(String text) {
      label.setText(text);
   }
}

I have it extend JPanel because this way it gives me the freedom of placing it into a JFrame or JDialog or other JPanel as I see fit. Note that I've made the JLabel private and have given the class a public setter method, setLabelText(String text) , that allows outside classes the ability to set the JLabel's text.

The ButtonTextFieldClass could look something like:

class ButtonTextFieldClass extends JPanel {
   private JTextField textField = new JTextField(10);
   private JButton button = new JButton(new ButtonAction("Send Text"));
   private LabelClass labelClass;

   public ButtonTextFieldClass() {
      setBorder(BorderFactory.createTitledBorder("Button TextField Panel"));
      add(textField);
      add(button);
   }

   // here we allow other classes to set instances of our LabelClass  
   public void setLabelClass(LabelClass labelClass) {
      this.labelClass = labelClass;
   }

   // ....

I've also given the button an AbstractAction in place of an ActionListener since it is like a super ActionListener on steroids. Inside of it, I'd get the text from the JTextField and then call the LabelClass's setter method (if the variable is not null) to set the label's text:

  public void actionPerformed(ActionEvent e) {
     String text = textField.getText(); 
     if (labelClass != null) {
        labelClass.setLabelText(text);
     }
  }

Then to set everything up, in another class I'd create instances of both LabelClass and ButtonTextFieldClass , and then "hook them up" by calling the setter method:

  LabelClass labelClass = new LabelClass();
  ButtonTextFieldClass buttonTextFieldClass = new ButtonTextFieldClass();
  buttonTextFieldClass.setLabelClass(labelClass); // set our reference

The whole thing could look like so:

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

public class TransferData {

   private static void createAndShowGui() {
      LabelClass labelClass = new LabelClass();
      ButtonTextFieldClass buttonTextFieldClass = new ButtonTextFieldClass();
      buttonTextFieldClass.setLabelClass(labelClass); // set our reference

      JPanel mainPanel = new JPanel(new GridLayout(0, 1));
      mainPanel.add(buttonTextFieldClass);
      mainPanel.add(labelClass);

      JFrame frame = new JFrame("TransferData");
      frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
      frame.getContentPane().add(mainPanel);
      frame.pack();
      frame.setLocationByPlatform(true);
      frame.setVisible(true);
   }

   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         public void run() {
            createAndShowGui();
         }
      });
   }
}

class LabelClass extends JPanel {
   private JLabel label = new JLabel("");

   public LabelClass() {
      setBorder(BorderFactory.createTitledBorder("Label Panel"));
      add(label);
   }

   public void setLabelText(String text) {
      label.setText(text);
   }
}

class ButtonTextFieldClass extends JPanel {
   private JTextField textField = new JTextField(10);
   private JButton button = new JButton(new ButtonAction("Send Text"));

   // one possible solution -- give this class a variable 
   // of the LabelClass -- but don't initialize the variable 
   // here, but rather do it in a setter
   private LabelClass labelClass;

   public ButtonTextFieldClass() {
      setBorder(BorderFactory.createTitledBorder("Button TextField Panel"));
      add(textField);
      add(button);
   }

   // here we allow other classes to set instances of our LabelClass  
   public void setLabelClass(LabelClass labelClass) {
      this.labelClass = labelClass;
   }

   // an AbstractAction is like a "super" ActionListener
   private class ButtonAction extends AbstractAction {
      public ButtonAction(String name) {
         super(name); // set the button's text and actionCommand
         int mnemonic = (int) name.charAt(0); // get first char
         putValue(MNEMONIC_KEY, mnemonic); // set mnemonic
      }

      @Override
      public void actionPerformed(ActionEvent e) {
         String text = textField.getText(); 
         if (labelClass != null) {
            labelClass.setLabelText(text);
         }
      }
   }
}

For simplicity's sake, I've displayed both JPanels within the same GUI, but it could work just as well if one JPanel were in one JFrame and the other within a JDialog.