title: Java clears screen when calling paint method - how to avoid that?
body: <p>I'm trying to draw two lines in a Canvas in Java, calling two methods separately, but when I draw the second line, the first one disapears (Java clears the screen). How can I avoid that? I want to see the two lines. I've seen paint tutorials (how to make a program like the Paint on Windows) where the user uses the mouse to draw lines and when one line is drawn, the other do not disappear. They just call the paint method and it does not clear the screen. </p>
  
  <p>I'll be grateful if anyone can help me.
  Thanks.</p>
  
  <p>View Class</p>
  
  <pre><code>import java.awt.BorderLayout;
  import java.awt.Canvas;
  import java.awt.Color;
  import java.awt.Graphics;
  
  import javax.swing.JFrame;
  
  public class CircuitTracePlotView extends JFrame {
  
  
      private CircuitTracePlot circuitTracePlot;
  
      public  CircuitTracePlotView() {
  
  
          this.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
          this.getContentPane().add(circuitTracePlot = new CircuitTracePlot(), BorderLayout.CENTER);
          this.pack();
          this.setSize(250,250);
          this.setLocationRelativeTo(null);
  
          this.setVisible(true);
          circuitTracePlot.drawLine();
          circuitTracePlot.drawOval();
      }
  
  
  }
  
  class CircuitTracePlot extends Canvas {
  
      private final static short LINE = 1;
      private final static short OVAL = 2;
      private int paintType;
  
      private int x1;
      private int y1;
      private int x2;
      private int y2;
  
      public CircuitTracePlot() {
          this.setSize(250,250);
          this.setBackground(Color.WHITE);
  
      }
  
      private void setPaintType(int paintType) {
          this.paintType = paintType;
      }
  
      private int getPaintType() {
          return this.paintType;
      }
  
      public void drawLine() {
          this.setPaintType(LINE);
          this.paint(this.getGraphics());
      }
  
      public void drawOval() {
          this.setPaintType(OVAL);
          this.paint(this.getGraphics());
      }
  
      public void repaint() {
          this.update(this.getGraphics());
      }
  
      public void update(Graphics g) {
          this.paint(g);
      }
  
      public void paint(Graphics g) {
          switch (paintType) {
          case LINE:
              this.getGraphics().drawLine(10, 10, 30, 30);            
          case OVAL:
              this.getGraphics().drawLine(10, 20, 30, 30);
          }
  
  
      }
  
  
  }
  </code></pre>
  
  <p>Main class</p>
  
  <pre><code>import javax.swing.SwingUtilities;
  
  import view.CircuitTracePlotView;
  
  public class Main {
  
      public static void main(String[] args) {
          SwingUtilities.invokeLater(new Runnable() {
              public void run() {
                  CircuitTracePlotView cr = new CircuitTracePlotView();
              }
          });
      }
  }
  </code></pre>