BoxLayout is a layout manager in Java that arranges components in a single column or row. It is useful when you need to arrange components in a specific order, such as a menu or toolbar.

Here's an example of how to use BoxLayout to create a horizontal toolbar with buttons and separators:

import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class BoxLayoutExample {
    public static void main(String[] args) {
        JFrame frame = new JFrame("BoxLayout Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        // Create a panel with a horizontal BoxLayout
        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel, BoxLayout.X_AXIS));

        // Add buttons and separators to the panel
        panel.add(new JButton("New"));
        panel.add(Box.createHorizontalStrut(10)); // Add a 10-pixel spacer
        panel.add(new JButton("Open"));
        panel.add(Box.createHorizontalGlue()); // Add a flexible space
        panel.add(new JButton("Save"));
        panel.add(Box.createHorizontalStrut(10)); // Add another spacer
        panel.add(new JButton("Exit"));

        // Add the panel to the frame
        frame.add(panel);

        // Set the size of the frame and show it
        frame.pack();
        frame.setVisible(true);
    }
}

In this example, we create a JPanel with a horizontal BoxLayout and add JButton components and separators to it. We use Box.createHorizontalStrut to add fixed-size spacers between the buttons, and Box.createHorizontalGlue to add a flexible space that will expand or contract as the frame is resized. Finally, we add the panel to the JFrame and show it.