为什么JOptionPane中的JScrollPane无法显示其所有内容？

Question

我试图将组件添加到JPanel，然后将该面板放入JScrollPane，然后将JScrollPane放入JOptionPane。问题：仅添加了19行组件。有一个用于确定组件行数的for循环，如果将条件计数器更改为19或更少，则将显示所有组件。这是问题的SSCCEimport java.awt.Dimension;import javax.swing.GroupLayout;import javax.swing.JButton;import javax.swing.JLabel;import javax.swing.JOptionPane;import javax.swing.JPanel;import javax.swing.JScrollPane;import javax.swing.JTextField;import javax.swing.SwingConstants;public class DynamicGroupLayout extends JPanel{    JButton addRecordButton;    JTextField[] FieldsArray;    JLabel[] LabelsArray;    GroupLayout layout;    JScrollPane scrollPane;    public DynamicGroupLayout()    {        addRecordButton = new JButton("add record");        layout = new GroupLayout(this);        this.setLayout(layout);        layout.setAutoCreateGaps(true);        layout.setAutoCreateContainerGaps(true);        scrollPane = new JScrollPane(this);        scrollPane.getVerticalScrollBar().setUnitIncrement(16);        scrollPane.setPreferredSize(this.getPreferredSize());        setTextFields();        showDialog();    }    @Override    public Dimension getPreferredSize()    {        return new Dimension(400, 500);    }    private void setTextFields()    {        int num = 30;  //If 19 or less, all components shown.        String[] labelsNames =        {            "1", "2", "3", "4", "5", "6", "7", "8", "9", "10",            "11", "12", "13", "14", "15", "16", "17", "18", "19", "20",            "21", "22", "23", "24", "25", "26", "27", "28", "29", "30"        };