The code below shows you how to add an ActionListener to a JComboBox component. In the snippet we add a listener by calling the addActionListener() method and give an instance of ActionListener listener as an anonymous class (a class without a specified name) as the parameter.

The ActionListener interface contract said that we must implement the actionPerformed(ActionEvent e) method. This is the place where the event will be handled by our program.

package org.kodejava.swing;

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

public class ComboBoxActionListener extends JFrame {
    public ComboBoxActionListener() {
        initialize();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(
                () -> new ComboBoxActionListener().setVisible(true));
    }

    private void initialize() {
        setSize(300, 300);
        setLayout(new FlowLayout(FlowLayout.LEFT));
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);

        String[] names = new String[]{
                "John", "Paul", "George", "Ringo"
        };
        JComboBox<String> comboBox = new JComboBox<>(names);
        comboBox.setEditable(true);

        // Create an ActionListener for the JComboBox component.
        comboBox.addActionListener(event -> {
            // Get the source of the component, which is our combo
            // box.
            JComboBox comboBox1 = (JComboBox) event.getSource();

            // Print the selected items and the action command.
            Object selected = comboBox1.getSelectedItem();
            System.out.println("Selected Item  = " + selected);
            String command = event.getActionCommand();
            System.out.println("Action Command = " + command);

            // Detect whether the action command is "comboBoxEdited"
            // or "comboBoxChanged"
            if ("comboBoxEdited".equals(command)) {
                System.out.println("User has typed a string in " +
                        "the combo box.");
            } else if ("comboBoxChanged".equals(command)) {
                System.out.println("User has selected an item " +
                        "from the combo box.");
            }
        });
        getContentPane().add(comboBox);
    }
}

This example demonstrate how to add an items into a specific position in the combo box list or at the end of the list using the insertItemAt(Object o, int index) and addItem(Object o) method. In the code we also learn how to remove one or all items from the list using removeItemAt(int index) method and removeAllItems() method of JComboBox.

package org.kodejava.swing;

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

public class ComboBoxAddRemoveItem extends JFrame {
    public ComboBoxAddRemoveItem() {
        initialize();
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(() -> new ComboBoxAddRemoveItem().setVisible(true));
    }

    private void initialize() {
        setSize(500, 500);
        setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        setLayout(new FlowLayout(FlowLayout.LEFT));

        String[] items = new String[]{"Two", "Four", "Six", "Eight"};
        final JComboBox<String> comboBox = new JComboBox<>(items);

        getContentPane().add(comboBox);

        JButton button1 = new JButton("Add One");
        button1.addActionListener(e -> {
            // Add item "One" at the beginning of the list.
            comboBox.insertItemAt("One", 0);
        });

        JButton button2 = new JButton("Add Five and Nine");
        button2.addActionListener(e -> {
            // Add item Five on the third index and Nine at the end of the
            // list.
            comboBox.insertItemAt("Five", 3);
            comboBox.addItem("Nine");
        });

        getContentPane().add(button1);
        getContentPane().add(button2);

        JButton remove1 = new JButton("Remove Eight and Last");
        remove1.addActionListener(e -> {
            // Remove the Eight item and the last item from the list.
            comboBox.removeItemAt(5);
            comboBox.removeItemAt(comboBox.getItemCount() - 1);
        });

        JButton remove2 = new JButton("Remove All");
        remove2.addActionListener(e -> comboBox.removeAllItems());

        getContentPane().add(remove1);
        getContentPane().add(remove2);
    }
}