In this post, I show you how to create a combo box using a JComboBox component in swing-based applications.

JComboBox is a component that combines a button or editable field and a drop-down list. The user can select a value from the drop-down list, which appears at the user's request. If you make the combo box editable, then the combo box includes an editable field into which the user can type a value.

Java Swing ComboBox Example

In below example, we have a combo box and a label. The combo box contains a list of strings denoting names of Linux distributions. The selected item from the combo box is displayed in the label. The combo box uses its ItemListener to detect changes.

package net.sourcecodeexamples.swingexample.components2;

import javax.swing.GroupLayout;
import javax.swing.JComboBox;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;

import java.awt.EventQueue;
import java.awt.event.ItemEvent;
import java.awt.event.ItemListener;

import static javax.swing.GroupLayout.Alignment.BASELINE;

public class ComboBoxExample extends JFrame
implements ItemListener {

    private static final long serialVersionUID = 1 L;
    private JLabel display;
    private JComboBox < String > box;
    private String[] distros;

    private void initializeUI() {

        distros = new String[] {
            "Ubuntu",
            "Redhat",
            "Arch",
            "Debian",
            "Mint"
        };

        box = new JComboBox < > (distros);
        box.addItemListener(this);

        display = new JLabel("Ubuntu");

        createLayout(box, display);

        setTitle("JComboBox");
        setSize(300, 200);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
    }

    private void createLayout(JComponent...arg) {

        JPanel pane = (JPanel) getContentPane();
        GroupLayout gl = new GroupLayout(pane);
        pane.setLayout(gl);

        gl.setAutoCreateContainerGaps(true);
        gl.setAutoCreateGaps(true);

        gl.setHorizontalGroup(gl.createSequentialGroup()
            .addComponent(arg[0])
            .addComponent(arg[1])
        );

        gl.setVerticalGroup(gl.createParallelGroup(BASELINE)
            .addComponent(arg[0])
            .addComponent(arg[1])
        );

        pack();
    }

    @Override
    public void itemStateChanged(ItemEvent e) {

        if (e.getStateChange() == ItemEvent.SELECTED) {
            display.setText(e.getItem().toString());
        }
    }

    public static void main(String[] args) {

        EventQueue.invokeLater(() - > {

            ComboBoxExample comboBoxExample = new ComboBoxExample();
            comboBoxExample.initializeUI();
            comboBoxExample.setVisible(true);
        });
    }
}

Let's understand above Java program.

The JComboBox will hold these string array: ```java` distros = new String[] {"Ubuntu", "Redhat", "Arch", "Debian", "Mint"};

The display area is a simple JLabel. It displays the item that is initially shown in the combo box:
```java
display = new JLabel("Ubuntu");

The constructor of the JComboBox takes a string array of Linux distributions. We plug a listener to the created object:

box = new JComboBox<>(distros);
box.addItemListener(this);

Vertically, the two components will be aligned to the baseline of their text:

gl.setVerticalGroup(gl.createParallelGroup(BASELINE)
        .addComponent(arg[0])
        .addComponent(arg[1])
);

The itemStateChanged() is invoked when an item has been selected or deselected by the user. We check for ItemEvent.SELECTED state and set the combo box's selected item to the label.

@Override
public void itemStateChanged(ItemEvent e) {
    
    if (e.getStateChange() == ItemEvent.SELECTED) {
        display.setText(e.getItem().toString());
    }
}