BasicInternalFrameUI ui = (BasicInternalFrameUI) internalFrame.getUI();
final Component northPane = ui.getNorthPane();

import java.awt.BorderLayout;
import java.awt.Color;
import javax.swing.plaf.basic.*;
import javax.swing.*;
import java.awt.Component;
import java.awt.Cursor;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Point;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.util.ArrayList;
import java.awt.event.*;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import java.awt.geom.Point2D;

public class MiDesktopPane extends JDesktopPane{
    private ArrayList<JInternalFrame> frames;
    private Peer inicioLinea,finLinea;
    private boolean [][] uniones;

    public MiDesktopPane(){

        setBackground(new Color(255,255,255,255));

        frames = new ArrayList<>();

        inicioLinea = finLinea = null;
        uniones = new boolean[15][15];

        //Se inicia la matriz para uniones de peers
        for(int i=0; i<15;i++){
            for(int j=0; j<15;j++){
                uniones[i][j] = false;
            }
        }

        this.revalidate();

        this.addMouseListener(new MouseAdapter() {
            @Override
            public void mouseClicked(MouseEvent e){
                //System.out.println("Clic de Desktop: "+e.getX()+" y "+e.getY());
                final Peer internalFrame = new Peer(e.getX(), e.getY());

                frames.add(internalFrame);
                add(internalFrame);

                internalFrame.addComponentListener(new ComponentAdapter() {
                    @Override
                    public void componentMoved(ComponentEvent e){
                        repaint();
                    }
                });

                BasicInternalFrameUI ui = (BasicInternalFrameUI) internalFrame.getUI();
                final Component northPane = ui.getNorthPane();

                northPane.addMouseListener(new MouseAdapter(){
                        @Override
                        public void mouseClicked(MouseEvent e2){
                            if(inicioLinea == null && finLinea == null){
                                inicioLinea = internalFrame;
                            }else{
                                if(finLinea == null && inicioLinea != null){
                                    finLinea = internalFrame;
                                    //Se llama a funcion agregar union
                                    addPeerComunication();
                                }
                            }
                            repaint();
                        }

                        /*Si no se pudiera mover el IF se usarian estos metodos para unirse entre ellos
                        @Override
                        public void mousePressed(MouseEvent evento)
                        {
                            Point2D x = evento.getPoint();
                            System.out.println("Presionado en barra");
                        }

                        @Override
                        public void mouseReleased(MouseEvent evento)
                        {
                            Point2D x = evento.getPoint();
                            System.out.println("Soltado en barra");
                        }
                        */
                        });

                //Funcion para que reaccion el cursor a la manita
                    northPane.addMouseMotionListener(new MouseMotionAdapter(){
                        @Override
                        public void mouseMoved(MouseEvent e3){
                            //System.out.println("Mouse movido: "+e3.getPoint());
                            //System.out.println(""+northPane.getBounds());
                            Point2D pun = (Point2D) e3.getPoint();
                            if(northPane.getBounds().contains(e3.getPoint())){
                                northPane.setCursor(new Cursor(Cursor.HAND_CURSOR));
                            }
                            repaint();
                        }

                        @Override
                        public void mouseDragged(MouseEvent e2){
                            Point2D pun = (Point2D) e2.getPoint();

                            repaint();
                        }
                    });

                //Se deshabilita drag de Internals (que es automático pero no detectable)
                /*
                JInternalFrame[] frames = getAllFrames();
                for (JInternalFrame frame: frames) {
                   //BasicInternalFrameUI ui = (BasicInternalFrameUI) frame.getUI();
                   //Component northPane = ui.getNorthPane();
                   MouseMotionListener[] motionListeners = (MouseMotionListener[]) northPane.getListeners(MouseMotionListener.class);
                    for (MouseMotionListener listener: motionListeners){
                        //Este remueve el drag automático y le ponemos uno creado por nosotros.
                        //northPane.removeMouseMotionListener(listener);
                    }
                }*/
                internalFrame.setVisible(true);
                repaint();
            }
        }
        );
    }

    public void addPeerComunication(){
        if(uniones[inicioLinea.getMiNoPeer()][finLinea.getMiNoPeer()] == false){
            if(inicioLinea.getMiNoPeer() != finLinea.getMiNoPeer()){
                int menor, mayor; menor = mayor = 0;
                //Aqui se reduce solo a la matriz triangular superior
                if(inicioLinea.getMiNoPeer() < finLinea.getMiNoPeer()){
                    menor = inicioLinea.getMiNoPeer();
                    mayor = finLinea.getMiNoPeer();
                }else{
                mayor = inicioLinea.getMiNoPeer();
                menor = finLinea.getMiNoPeer();}
                //Antes preguntar con JOptionPane si se desea aniadir union
                int valorRespuesta =
                        JOptionPane.showConfirmDialog(this,
                            "¿Desea unir el Peer "+menor+" con el Peer "+mayor, "Confirma la unión", JOptionPane.YES_NO_CANCEL_OPTION);
                if(valorRespuesta == JOptionPane.YES_OPTION){
                    uniones[menor][mayor] = true;
                    this.repaint();
                }
            }else{/*System.out.println("No se puede establecer comunicacion con uno mismo");*/}
            inicioLinea = finLinea = null;
        }
    }


    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;

        g2.setColor(Color.black);
        for(int i=0; i<15;i++){
            for(int j=i; j<15; j++){
                //Este If grande sirve para no mostrar lineas cuando se cierra
                //Uno o mas JInternalFrames a los que esta conectado
                //Incluso si se minimiza se cierra la linea
                if(uniones[i][j] &&
                        (!frames.get(i).isClosed() && !frames.get(j).isClosed())
                        && (!frames.get(i).isIcon() && !frames.get(j).isIcon())){
                    Point2D p1,p2;
                    p1 = new Point2D.Double(frames.get(i).getBounds().getCenterX(),frames.get(i).getBounds().getCenterY());
                    p2 = new Point2D.Double(frames.get(j).getBounds().getCenterX(),frames.get(j).getBounds().getCenterY());
                    g2.draw(new Line2D.Double(p1, p2));
                }else{repaint();}
            }
        }
    }
}