从另一个类在 JTextArea 中键入文本

Question

I have a problem with updating a textArea from another class. I need a textArea to show a text while pressing a button. So when I press a buton I make a method actionPerformed() in ParceListener to print a text in a textArea which is located in MainFormAppearance class. But it doesn't do that. Could you please help me?

public class Main {

public static void main(String[] args) {
    JFrame.setDefaultLookAndFeelDecorated(true);
    JFrame jFrame = new JFrame("Title");

    MainFormAppearance demo = new MainFormAppearance();
    jFrame.setContentPane(demo.createContentPanel());

    jFrame.setDefaultCloseOperation(jFrame.EXIT_ON_CLOSE);
    jFrame.setSize(400,300);
    jFrame.setVisible(true);

}
}

MainFormAppearance

package com.company;

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

public class MainFormAppearance {

public JPanel totalGui;
public JTextArea frame;
public JLabel blueLabel;
public JButton parceButton;
public JButton mailButton;
public ParceListener parceListener;

public JPanel createContentPanel(){

    totalGui = new JPanel();
    frame = new JTextArea();
    blueLabel = new JLabel("Some program");
    parceButton = new JButton("Button 1");
    mailButton = new JButton("Button 2");
    parceListener = new ParceListener();
    totalGui.setLayout(null);

    //set program window
    blueLabel.setLocation(10,10);
    blueLabel.setSize(400,20);
    blueLabel.setHorizontalAlignment(SwingConstants.CENTER);
    blueLabel.setForeground(Color.blue);
    totalGui.add(blueLabel);

    //set Button 1
    parceButton.setLocation(270, 50);
    parceButton.setSize(100,30);
    totalGui.add(parceButton);
    //Pressing the Button 1
    parceButton.addActionListener(parceListener);

    //set Button 2
    mailButton.setLocation(270, 100);
    mailButton.setSize(100, 30);
    totalGui.add(mailButton);

    frame.setLocation(20, 115);
    frame.setSize(200, 15);
    totalGui.add(frame);

    totalGui.setOpaque(true);
    return totalGui;
}

public void setTextArea(String myString){
    frame.append(myString);
}


}

ParceListener

import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;

public class ParceListener implements ActionListener {

public String text = "some text";
MainFormAppearance mainFormAppearance = new MainFormAppearance();

public void actionPerformed(ActionEvent e) {
    mainFormAppearance.setTextArea(text);

}

}

It shows NullPointerException at frame.append(myString); in MainFormAppearance class.

Answer 1

Calling MainFormAppearance mainFormAppearance = new MainFormAppearance(); in your ParceListener is creating a new instance of MainFormAppearance which has nothing to do with what is actually been presented on the screen.

You need some way to return information back to the main UI from ParceListener .

This is best accomplished using an Observer Pattern, where ParceListener generates notifications/events when something changes. It shouldn't care about "who" is interested, only that they are.

Let's start with a simple interface...

public interface ParceObserver {
    public void parceChanged(String text);
}

MainFormAppearance can now implement this interface and make what ever updates it needs.

public class MainFormAppearance implements ParceObserver {
    //...

    public void parceChanged(String text) {
        frame.append("\n" + text);
    }
}

Then thing here is, ParceListener , doesn't care what happens after it's posted the notification.

Now, you just need to pass an instance of ParceObserver to ParceListener

parceListener = new ParceListener(this);

And update ParceListener to make use of it...

public class ParceListener implements ActionListener {

    private ParceObserver observer;

    public String text = "some text";

    public ParceListener(ParceObserver observer) {
        this.observer = observer;
    }

    public void actionPerformed(ActionEvent e) {
        if (observer == null) {
           return null;
        }
        observer.parceChanged(text);
    }

}

Now it's nicely de-coupled and re-usable.

And, if someone tells you to just pass a reference of the JTextArea or MainFormAppearance to ParceListener , please don't listen to them. It's inappropriate, tightly couples your code and exposes the components to the risk of been modified in ways you never intended them to be