Swing's JComboBox component provides editable functionality that allows end-users to input custom values not present in the initial dataset. When setEditable(true) is invoked, the component renders a text field alongside the dropdown arrow, acepting keyboard input.

To capture and persist user-defined entries, attach an ActionListener that responds to edit events. The listener should verify whether the input exists in the current data model before appending to prevent duplicates.

import javax.swing.*;
import java.awt.event.*;
import java.util.HashSet;
import java.util.Set;

public class DynamicComboBoxDemo {
    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> {
            JFrame window = new JFrame("Extensible Selector");
            
            String[] initialValues = {"Red", "Green", "Blue"};
            JComboBox<String> colorPicker = new JComboBox<>(initialValues);
            colorPicker.setEditable(true);
            
            Set<String> knownValues = new HashSet<>();
            for (String val : initialValues) {
                knownValues.add(val);
            }
            
            colorPicker.addActionListener(e -> {
                if ("comboBoxEdited".equals(e.getActionCommand())) {
                    String text = (String) colorPicker.getSelectedItem();
                    if (text != null && !text.trim().isEmpty() && !knownValues.contains(text)) {
                        colorPicker.addItem(text);
                        knownValues.add(text);
                    }
                }
            });
            
            window.add(colorPicker);
            window.pack();
            window.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
            window.setVisible(true);
        });
    }
}

The ActionListener approach leverages the component's built-in event system rather then monitoring keystrokes direct. The HashSet provides efficient O(1) lookup when checking for existing entries, improving performance with large datasets. For thread safety, the GUI construction executes within SwingUtilities.invokeLater() to ensure proper initialization on the Event Dispatch Thread.

This implementation allows seamless integration of user-generated content into the selection model while maintaining data integrity through duplicate detection.