ArrayList在Swing监听器中变为空的问题求助

我正在为学校编写程序，遇到如下问题：尝试通过按钮从另一个类获取ArrayList，但该ArrayList进入监听器后变为空；即使在监听器外的其他函数中使用，它仍然是空的。我尝试将getter放在不同位置，但均未解决问题。

代码示例 #

import javax.swing.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.util.ArrayList;

public class addPanel {
    private JPanel panel1;
    private JTextField textField1;
    private JButton OKButton;
    private JButton registreerButton;
    private JTextField textField2;
    private String voorNaam;
    private String achterNaam;

    public addPanel() {

        textField1.addKeyListener(new KeyAdapter() {
            @Override
            public void keyReleased(KeyEvent e) {
                super.keyReleased(e);
                voorNaam = textField1.getText().toLowerCase();
            }
        });
        textField2.addKeyListener(new KeyAdapter() {
            @Override
            public void keyReleased(KeyEvent e) {
                super.keyReleased(e);
                achterNaam = textField2.getText().toLowerCase();
            }
        });
        OKButton.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {

                System.out.println(gebruikas); // 输出为空
                returnInputGebruiker();
            }
        });
    }
    Gebruikers g = new Gebruikers();
    ArrayList<Gebruiker> gebruikas = g.getGebruikers();
    
    public void worksFine() {
        System.out.println(gebruikas);// 单独调用时能正常输出内容
    }

    public void returnInputGebruiker(){

        System.out.println(gebruikas);// 输出为空
    }

    public void run(JFrame parent) {
        JFrame frame = new JFrame();
        frame.setContentPane(panel1);
        frame.setTitle("persoon details");
        frame.pack();
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
        if (parent != null)
            frame.setLocationRelativeTo(parent);
        frame.setVisible(true);
    }
}

内容的提问来源于stack exchange，提问作者Juanita Pardo Isidoro Rasgado