A "simple" demonstration of using a `DocumentFilter` with a `ComboBoxEditor`

FilteredComoboBoxEditor.java

    import java.awt.Component;
    import java.awt.EventQueue;
    import java.awt.GridBagLayout;
    import java.awt.KeyboardFocusManager;
    import java.awt.Toolkit;
    import java.awt.event.ActionListener;
    import javax.swing.ComboBoxEditor;
    import javax.swing.DefaultComboBoxModel;
    import javax.swing.JComboBox;
    import javax.swing.JFrame;
    import javax.swing.JPanel;
    import javax.swing.JTextField;
    import javax.swing.LookAndFeel;
    import javax.swing.UIManager;
    import javax.swing.text.AbstractDocument;
    import javax.swing.text.AttributeSet;
    import javax.swing.text.BadLocationException;
    import javax.swing.text.DocumentFilter;

    public class Main {
        public static void main(String[] args) {
            new Main();
        }

        public Main() {
            EventQueue.invokeLater(new Runnable() {
                @Override
                public void run() {
                    JFrame frame = new JFrame();
                    frame.add(new TestPane());
                    frame.pack();
                    frame.setLocationRelativeTo(null);
                    frame.setVisible(true);
                }
            });
        }

        public class TestPane extends JPanel {

            public TestPane() {
                setLayout(new GridBagLayout());

                DefaultComboBoxModel<String> model = new DefaultComboBoxModel<>();
                model.addElement("Test");

                JComboBox<String> combobox = new JComboBox<>(model);
                combobox.setEditor(new FilteredComboBoxEditor());
                combobox.setEditable(true);

                add(combobox);
            }

        }

        public class FilteredComboBoxEditor implements ComboBoxEditor {

            private JTextField textField;

            public FilteredComboBoxEditor() {
                textField = new JTextField(10);
                ((AbstractDocument) textField.getDocument()).setDocumentFilter(new DocumentFilter() {

                    private int maxLength = 10;

                    protected String filter(String text) {
                        StringBuilder sb = new StringBuilder(text.length());
                        for (char c : text.toCharArray()) {
                            if (Character.isWhitespace(c) || c == '.' || c == '&') {
                                continue;
                            }
                            sb.append(c);
                        }
                        return sb.toString();
                    }

                    @Override
                    public void insertString(DocumentFilter.FilterBypass fb, int offset, String text, AttributeSet attr) throws BadLocationException {
                        String filteredText = filter(text);
                        if (filteredText.isEmpty()) {
                            provideErrorFeedback();
                            return;
                        }
                        if ((fb.getDocument().getLength() + filteredText.length()) < maxLength) {
                            provideErrorFeedback();
                            return;
                        }
                        if (!filteredText.equals(text)) {
                            provideErrorFeedback();
                        }
                        super.insertString(fb, offset, filteredText, attr);
                    }

                    @Override
                    public void replace(DocumentFilter.FilterBypass fb, int offset, int length, String text, AttributeSet attrs) throws BadLocationException {
                        text = filter(text);
                        int targetLength = fb.getDocument().getLength() - length;
                        if ((targetLength + text.length()) >= maxLength) {
                            // Some or all the text is going to be rejected
                            provideErrorFeedback();
                            int remainingLength = maxLength - targetLength;
                            if (remainingLength <= 0) {
                                return;
                            }
                            text = text.substring(0, remainingLength);
                        }
                        super.replace(fb, offset, length, text, attrs);
                    }

                    protected void provideErrorFeedback() {
                        LookAndFeel laf = UIManager.getLookAndFeel();

                        if (laf == null) {
                            Toolkit.getDefaultToolkit().beep();
                        } else {
                            KeyboardFocusManager fm = KeyboardFocusManager.getCurrentKeyboardFocusManager();
                            Component component = fm.getFocusOwner();
                            laf.provideErrorFeedback(component);
                        }
                    }
                });
            }

            @Override
            public Component getEditorComponent() {
                return textField;
            }

            @Override
            public void setItem(Object anObject) {
                textField.setText(anObject == null ? null : anObject.toString());
            }

            @Override
            public Object getItem() {
                return textField.getText();
            }

            @Override
            public void selectAll() {
                textField.selectAll();
            }

            @Override
            public void addActionListener(ActionListener l) {
                textField.addActionListener(l);
            }

            @Override
            public void removeActionListener(ActionListener l) {
                textField.removeActionListener(l);
            }

        }
    }