External Listener

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.GridLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import javax.swing.*;

public class MainGUI2 {
    private static final int ROWS = 24;
    private static final int COLS = 48;
    private static final Insets LIST_MARGINS = new Insets(2, 4, 0, 0);
    private JPanel mainPanel = new JPanel();
    private JTextArea listText = new JTextArea(ROWS, COLS);

    public MainGUI2() {
        listText.setEditable(false);
        listText.setFocusable(false);
        listText.setWrapStyleWord(true);
        listText.setLineWrap(true);
        listText.setMargin(LIST_MARGINS);
        JScrollPane scrollPane = new JScrollPane(listText);
        scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_NEVER);

        JPanel buttonPanel = new JPanel();
        buttonPanel.setLayout(new GridLayout(0, 1, 0, 1));
        buttonPanel.add(new JButton(new RefreshAction("Refresh", this)));

        JPanel eastPanel = new JPanel(new BorderLayout());
        eastPanel.add(buttonPanel, BorderLayout.PAGE_START);
        eastPanel.setBackground(Color.DARK_GRAY);

        mainPanel.setLayout(new BorderLayout());
        mainPanel.add(scrollPane, BorderLayout.CENTER);
        mainPanel.add(eastPanel, BorderLayout.LINE_END);
    }

    public JPanel getMainPanel() {
        return mainPanel;
    }

    public void appendText(String text) {
        listText.append(text);
    }

    private static void createAndShowGui() {
        MainGUI2 mainGui = new MainGUI2();

        JFrame frame = new JFrame("GUI");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.getContentPane().add(mainGui.getMainPanel());
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> createAndShowGui());
    }
}

@SuppressWarnings("serial")
class RefreshAction extends AbstractAction {
    private MainGUI2 main;

    public RefreshAction(String name, MainGUI2 main) {
        super(name);
        int mnemonic = (int) name.charAt(0);
        putValue(MNEMONIC_KEY, mnemonic);
        this.main = main;
    }

    @Override
    public void actionPerformed(ActionEvent e) {
        main.appendText("From Refresh Action!\n");
    }
}