Draw a JPanel inside a JFrame [duplicate]

I would like to know how to draw a line inside a JPanel that is within a JFrame , at the push of a button. The layout of the button and JPanel within JFrame is exemplified in the image below:

Whenthebuttonispressedtheprogramshoulddrawalinefrompoint10.10topoint100,100ofJPanel.I'vealreadyseenexampleslikethecodebelowtodrawline.

publicvoiddesenha(Graphicsg){g.setColor(red);g.drawLine(10,10,100,100);}

TheproblemisthatIdonotknowwheretousethismethodsothatthelineisdrawninJPanel,andonlywhenthebuttonispressed.

Codes:MainClass:

publicclassTeste{publicstaticvoidmain(String[]args){Telat=newTela();t.setVisible(true);}}

ScreenClass:

publicclassTelaextendsjavax.swing.JFrame{publicTela(){initComponents();}@SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        botao = new javax.swing.JButton();
        viewport = new javax.swing.JPanel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        botao.setText("desenha");
        botao.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                botaoActionPerformed(evt);
            }
        });

        viewport.setBackground(new java.awt.Color(255, 255, 255));
        viewport.setBorder(javax.swing.BorderFactory.createLineBorder(new java.awt.Color(0, 0, 0), 3));

        javax.swing.GroupLayout viewportLayout = new javax.swing.GroupLayout(viewport);
        viewport.setLayout(viewportLayout);
        viewportLayout.setHorizontalGroup(
            viewportLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 361, Short.MAX_VALUE)
        );
        viewportLayout.setVerticalGroup(
            viewportLayout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 231, Short.MAX_VALUE)
        );

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGap(142, 142, 142)
                        .addComponent(botao))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(20, 20, 20)
                        .addComponent(viewport, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)))
                .addContainerGap(13, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(botao)
                .addGap(18, 18, 18)
                .addComponent(viewport, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                .addContainerGap())
        );

        pack();
    }// </editor-fold>                        

    private void botaoActionPerformed(java.awt.event.ActionEvent evt) {                                      

    }                                     


    // Variables declaration - do not modify                     
    public javax.swing.JButton botao;
    public static javax.swing.JPanel viewport;
    // End of variables declaration                   
}