数独ソルバーで JFormattedTextField が適切にクリアされないのはなぜですか?

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

public class SudokuTextBox extends JPanel {

    private JTextField textField;

    public SudokuTextBox() {
        textField = new JTextField();
        textField.setHorizontalAlignment(JTextField.CENTER);
        textField.setInputVerifier(new InputVerifier() {
            @Override
            public boolean verify(JComponent input) {
                String text = ((JTextField) input).getText();
                return text.matches("[1-9]");
            }
        });

        this.add(textField, BorderLayout.CENTER);
        this.setPreferredSize(new Dimension(32, 32));
    }

    public String getText() {
        return textField.getText();
    }

    public void setText(String text) {
        textField.setText(text);
    }
}