Why BoxLayout is taking extra space?

I am working on a Client-Server Chatting Application in which I am trying to create Speech Bubble like we have in WhatsApp

I want to show sent messages on the right side of the frame and received messages on the left side of the frame, I am trying to achieve this using BoxLayout but it is taking extra space (like in picture below)

This is what I have done till now

a1.setLayout(new BoxLayout(a1, BoxLayout.Y_AXIS));
JPanel p1 = new JPanel(new FlowLayout(FlowLayout.RIGHT));
JPanel p2 = new JPanel(new FlowLayout(FlowLayout.LEFT));

The extra space you see between the two messages is because of BoxLayout which I am using
What to do in this case? and any alternatives how can we achieve this by not using BoxLayout?

So the FlowLayout respected the preferred height, but it also respected the preferred width.

Instead you need a layout that will:

1. respect the height of the component you add to the BoxLayout
2. fills the width of the available space.

You can use a BorderLayout for this:

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

public class SSCCE extends JPanel
{
    public SSCCE()
    {
        setLayout( new BorderLayout() );

        Box vertical = Box.createVerticalBox();
        add(vertical, BorderLayout.PAGE_START);

        for (int i = 0; i < 10; i++)
        {
            JPanel left = new JPanel( new BorderLayout() );
            left.add(new JLabel("left message " + i), BorderLayout.LINE_START);
            vertical.add( left );
            JPanel right = new JPanel( new BorderLayout() );
            right.add(new JLabel("right message " + i), BorderLayout.LINE_END);
            vertical.add( right );
        }
    }

    private static void createAndShowGUI()
    {
        JFrame frame = new JFrame("SSCCE");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.add( new JScrollPane(new SSCCE()) );
        frame.pack();
        frame.setLocationByPlatform( true );
        frame.setVisible( true );
    }

    public static void main(String[] args) throws Exception
    {
        java.awt.EventQueue.invokeLater( () -> createAndShowGUI() );
    }
}