Question

I got a JPanel where a triangle is painted.
When someone clicks on a button the triangle should be repainted with new parameters. The problem is the old triangle is still there and the new one is messed up with part of the textfield underneath.

public class Vermessung {   

private static void eingabe(){
    f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    f.setSize(screen.height/2, screen.height/4*3);
    JPanel jp = new JPanel();
    jp.setLayout(new BoxLayout(jp, BoxLayout.PAGE_AXIS));

    //Eingabebereich
    JPanel eingabebereich = new JPanel(new GridLayout(3, 1));
    JPanel abc = new JPanel(new GridLayout(4, 2));
    abc.add(new JLabel("Strecke"));
    abc.add(new JLabel("Gemessener Wert in [m]"));
    abc.add(new JLabel("a:"));
    abc.add(tfa);
    abc.add(new JLabel("b:"));
    abc.add(tfb);
    abc.add(new JLabel("c:"));
    abc.add(tfc);

    //AusgabeBereich
    JPanel ausgabe = new JPanel(new GridLayout(2, 3));
    ausgabe.add(new JLabel("p [m]"));
    ausgabe.add(new JLabel("q [m]"));
    ausgabe.add(new JLabel("h [m]"));
    ausgabe.add(P);
    ausgabe.add(Q);
    ausgabe.add(H);
    P.setEditable(false);
    Q.setEditable(false);
    H.setEditable(false);

    //Buttons mit Listenern
    JPanel buttons = new JPanel(new FlowLayout());
    JButton ok = new JButton("OK"); 
    JButton cancel = new JButton("beenden");

    ActionListener al = new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent arg0) {

            //Textfelder auslesen
            TextfelderAuslesen();

            //bei gueltiger Eingabe Höhe Berechnen
            if(berechenbar){
                berechnungPQ();
                berechnungH();
                P.setText(String.valueOf(p));
                Q.setText(String.valueOf(q));
                H.setText(String.valueOf(h));
                sketch.update(vec);
sketch.repaint();
            }else{

            }

        }
    };
    ok.addActionListener(al);

    ActionListener beenden = new ActionListener() {

        @Override
        public void actionPerformed(ActionEvent arg0) {
            System.exit(0);

        }
    };
    cancel.addActionListener(beenden);

    buttons.add(ok);
    buttons.add(cancel);

    //Fensteraufbau
    sketch.setPreferredSize(new Dimension(screen.height/2, screen.height/2));
    jp.add(sketch);
    eingabebereich.add(abc);
    eingabebereich.add(ausgabe);
    eingabebereich.add(buttons);
    eingabebereich.setPreferredSize(new Dimension(screen.height/4, screen.height/4));
    jp.add(eingabebereich);
    f.add(jp);
    f.setVisible(true);
}


}

public class Zeichnung extends JPanel{

public void paint(Graphics g){
    zeichneDreieck(g);
}

private void zeichneDreieck(Graphics g){
    berechneLaengen();
    g.setColor(new Color(255,0,0));
    g.drawLine(30, 30, ca, 30);
    g.drawString("c", ca/2, 20);
    g.drawLine(ca, 30, qa, ha);
    g.drawString("a", (ca-pa/2), ha/2);
    g.drawLine(qa, ha, 30, 30);
    g.drawString("b", (qa/2), ha/2);
    g.setColor(new Color(0,0,0));
    g.drawLine(qa, ha, qa, 30);
    g.drawString("h", qa+5, ha/2);
}
}

Answer 1

public void paintComponent(Graphics g){
    super.paintComponent(g);
    zeichneDreieck(g);
}

For JPanel, override paintComponent() instead. As the first line, call the super method.