How to solve misalignment in boxlayout?

Question 1

enter image description here

only in due all my respect to @Andrew Thompson :-) and my curiosity

import java.awt.BorderLayout;
import java.awt.Component;
import java.awt.EventQueue;
import java.awt.Rectangle;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BoxLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.ScrollPaneConstants;

public class ChatPanel {

    private JFrame jf = new JFrame("Testing msgpanel");
    private JScrollPane scroll = new JScrollPane();
    private JPanel jp;
    private ChatPaneMsgBox mb;
    private ChatPaneMsgBox mb2;
    private JPanel cp = new JPanel();
    private JButton jb = new JButton("  ME !!! ");
    private JButton jb1 = new JButton("  YOU !!! ");

    public ChatPanel() {
        jp = new JPanel();
        jp.setLayout(new BoxLayout(jp, BoxLayout.Y_AXIS));
        mb = new ChatPaneMsgBox("<html><body style='width: 200px; padding: 5px;'>"
                + "<h1>Do U C Me?</h1>"
                + "Here is a long string that will wrap.  "
                + "The effect we want is a multi-line label.", true);
        addmsg(mb);
        mb2 = new ChatPaneMsgBox("Hello 2", false);
        addmsg(mb2);
        scroll.setViewportView(jp);
        scroll.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_AS_NEEDED);
        scroll.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_NEVER);
        jb.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                mb = new ChatPaneMsgBox("<html><body style='width: 200px; padding: 5px;'>"
                        + "<h1>Do U C Me?</h1>"
                        + "Here is a long string that will wrap.  "
                        + "The effect we want is a multi-line label.", true);
                addmsg(mb);
                Rectangle rect = jp.getBounds();
                Rectangle r2 = scroll.getViewport().getVisibleRect();
                jp.scrollRectToVisible(new Rectangle((int) rect.getWidth(), 
                        (int) rect.getHeight(), (int) r2.getWidth(), (int) r2.getHeight()));
            }
        });
        jb1.addActionListener(new ActionListener() {
            @Override
            public void actionPerformed(ActionEvent e) {
                mb2 = new ChatPaneMsgBox("Hello 2", false);
                addmsg(mb2);
                Rectangle rect = jp.getBounds();
                Rectangle r2 = scroll.getViewport().getVisibleRect();
                jp.scrollRectToVisible(new Rectangle((int) rect.getWidth(), 
                        (int) rect.getHeight(), (int) r2.getWidth(), (int) r2.getHeight()));
            }
        });
        cp.add(jb);
        cp.add(jb1);
        jf.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        jf.add(scroll);
        jf.add(cp, BorderLayout.SOUTH);
        jf.setSize(400, 400);
        jf.setVisible(true);
    }

    private void addmsg(Component co) {
        jp.add(co);
        jp.revalidate();
        jp.repaint();
    }

    public static void main(String args[]) {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                new ChatPanel();
            }
        });
    }
}

.

import java.awt.Color;
import javax.swing.BoxLayout;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingConstants;

class ChatPaneMsgBox extends JPanel {

    boolean ij = true;
    private String msg = "";
    private JLabel jl;

    public ChatPaneMsgBox() {
    }

    public ChatPaneMsgBox(String str, boolean bj) {
        ij = bj;
        msg = str;
        msgpnl();
    }

    private void msgpnl() {
        this.setLayout(new BoxLayout(this, BoxLayout.PAGE_AXIS));
        if (ij) {
            jl = new JLabel(msg);

            jl.setOpaque(true);
            jl.setBorder(new TextBubbleBorder(Color.LIGHT_GRAY, 1, 10, 7, ij));

            jl.setBackground(new Color(123, 104, 238));
            jl.setAlignmentX(0);
        } else {
            jl = new JLabel(msg, SwingConstants.RIGHT);

            jl.setOpaque(true);
            jl.setBorder(new TextBubbleBorder(Color.LIGHT_GRAY, 1, 10, 7, ij));

            jl.setBackground(new Color(224, 102, 255));
            jl.setAlignmentX(1);
        }
        this.add(jl);
        System.out.print("\nThis executed");
    }
}

.

import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Component;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Insets;
import java.awt.Polygon;
import java.awt.Rectangle;
import java.awt.RenderingHints;
import java.awt.geom.Area;
import java.awt.geom.RoundRectangle2D;
import javax.swing.border.AbstractBorder;

class TextBubbleBorder extends AbstractBorder {

    private Color color;
    private int thickness = 4;
    private int radii = 8;
    private int pointerSize = 7;
    private Insets insets = null;
    private BasicStroke stroke = null;
    private int strokePad;
    private int pointerPad = 4;
    private boolean left = true;
    RenderingHints hints;

    TextBubbleBorder(Color color) {
        new TextBubbleBorder(color, 4, 8, 7);
    }

    TextBubbleBorder(Color color, int thickness, int radii, int pointerSize) {
        this.thickness = thickness;
        this.radii = radii;
        this.pointerSize = pointerSize;
        this.color = color;
        stroke = new BasicStroke(thickness);
        strokePad = thickness / 2;
        hints = new RenderingHints(RenderingHints.KEY_ANTIALIASING, 
                RenderingHints.VALUE_ANTIALIAS_ON);
        int pad = radii + strokePad;
        int bottomPad = pad + pointerSize + strokePad;
        insets = new Insets(pad, pad, bottomPad, pad);
    }

    TextBubbleBorder(Color color, int thickness, int radii, int pointerSize, boolean left) {
        this(color, thickness, radii, pointerSize);
        this.left = left;
    }

    @Override
    public Insets getBorderInsets(Component c) {
        return insets;
    }

    @Override
    public Insets getBorderInsets(Component c, Insets insets) {
        return getBorderInsets(c);
    }

    @Override
    public void paintBorder(Component c, Graphics g, int x, int y, int width, int height) {
        Graphics2D g2 = (Graphics2D) g;
        int bottomLineY = height - thickness - pointerSize;
        RoundRectangle2D.Double bubble = new RoundRectangle2D.Double(
                0 + strokePad,
                0 + strokePad,
                width - thickness,
                bottomLineY,
                radii,
                radii);
        Polygon pointer = new Polygon();
        if (left) {
            // left point
            pointer.addPoint(
                    strokePad + radii + pointerPad,
                    bottomLineY);
            // right point
            pointer.addPoint(
                    strokePad + radii + pointerPad + pointerSize,
                    bottomLineY);
            // bottom point
            pointer.addPoint(
                    strokePad + radii + pointerPad + (pointerSize / 2),
                    height - strokePad);
        } else {
            // left point
            pointer.addPoint(
                    width - (strokePad + radii + pointerPad),
                    bottomLineY);
            // right point
            pointer.addPoint(
                    width - (strokePad + radii + pointerPad + pointerSize),
                    bottomLineY);
            // bottom point
            pointer.addPoint(
                    width - (strokePad + radii + pointerPad + (pointerSize / 2)),
                    height - strokePad);
        }
        Area area = new Area(bubble);
        area.add(new Area(pointer));
        g2.setRenderingHints(hints);
        // Paint the BG color of the parent, everywhere outside the clip
        // of the text bubble.
        Component parent = c.getParent();
        if (parent != null) {
            Color bg = parent.getBackground();
            Rectangle rect = new Rectangle(0, 0, width, height);
            Area borderRegion = new Area(rect);
            borderRegion.subtract(area);
            g2.setClip(borderRegion);
            g2.setColor(bg);
            g2.fillRect(0, 0, width, height);
            g2.setClip(null);
        }
        g2.setColor(color);
        g2.setStroke(stroke);
        g2.draw(area);
    }
}

Question 2

Interchange values of jl.setAlignmentX(0) 0 to 1 and 1 to 0 .in msgbox or ChatPaneMsgBox classes which are given here. Like

      if (ij) {
              this.setLayout(new FlowLayout(FlowLayout.LEFT));
              //rest code
              jl.setAlignmentX(1); 
              //rest code here
              }
       else   {
               this.setLayout(new FlowLayout(FlowLayout.Right));
              //rest code
              jl.setAlignmentX(0); 
              //rest code here
              }

even i think now jl.setAlignmentX(); doesn't matter anymore.

Thanks a lot to @mKorbel & @kleopatra for their help & suggestions. :-)

Question 3

The GridbagLayoutManager does a good job of helping to keep rectangular layouts easy to manage in terms of width and auto sizing. The programming mechanisms of GridbagLayout are data intensive and this causes many people to run away from it.

Back in 1996, I created the Packer class that is on Java.net under the Packer project. It wraps GridbagLayoutManager in a subclass which creates and uses GridbagLayout data items to make things work.

To create a simple layout like this, you just need to do something like the following.

class MsgObject extends JPanel {
    public MsgObject(String text) {
        Packer pk = new Packer( this );
        pk.pack( new JLabel("<html><body>"+text+"</body></html>") ).fillBoth();
    }
}
class TheirMsgObject extends MsgObject {
    public TheirMsgObject( string text ) {
        super(text);
        setBackground( new Color( 255, 220, 230 ) );
    }
}
class MyMsgObject extends MsgObject {
    public MyMsgObject( string text ) {
        super(text);
        setBackground( new Color( 220, 230, 255 ) );
    }
}
class MsgPane extends JPanel {
    int items = -1;
    Packer mpk;
    BlankSpace = blank;
    class BlankSpace extends JPanel {
        public Dimension PreferredSize() {
            return new Dimension( 100, 2 );
        }
    }
    public void MsgPane() {
        mpk = new Packer( this);
        blank = new BlankSpace();
    }

    private void addMessage( MsgObject msg, int col ) {
        mpk.pack( msg ).gridx(0).gridy(++items).gridw(col).fillx(1);
    }
    public void addMessage( TheirMsgObject msg ) {
        addMessage( msg, 0 );
    }

    public void addMessage( MyMsgObject msg ) {
        addMessage( msg, 1 );
        mpk.pack( blank ).gridx(0).gridy(items);
    }
}

Then, you can create the appropriate MsgObject and just add it to the container and it will appear there. You can add appropriate code for scrolling on add etc.