Just make the parameters class variables and they will be visible to
both your calculation method and your rendering method.

import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

class test7 extends JPanel implements Runnable {
    static final Color[] COLORS =
     {Color.RED, Color.BLUE, Color.GREEN, Color.WHITE};
    volatile int colorIndex;

    public test7() {
        setPreferredSize(new Dimension(400,300));
    }

    public void paintComponent(Graphics g) {
        g.setColor(COLORS[colorIndex]);
        g.fillRect(0,0,getWidth(),getHeight());
    }

    public void run() {
        while (true) {
            ++colorIndex;
            colorIndex %= 4;
            repaint();
            try {
                Thread.sleep(1000);
            } catch (InterruptedException ie) { }
        }
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                JFrame f = new JFrame();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

                test7 t7 = new test7();
                f.add(t7);
                f.pack();
                f.setVisible(true);
                new Thread(t7).start();
            }
        });
    }
}

The frame to hold your display component need be no more complicated
than the one I show here.  If you need menus, add those to the frame but
 have the display component implement ActionListener.  You could
separate the rendering method from the display class but there is
probably no real need.  If you want one display class and multiple
calculating methods, just override that method.

The volatile modifier is needed on the colorIndex variable because it
will be read in more than one thread.  If you are going to do more
sophisticated animation, consider using a javax.swing.Timer so that the
calculations can be done on the EDT (unless they take a long time).

Take a look at the Asteroids game I wrote that has many of these
concepts in action.

http://rabbitbrush.frazmtn.com/

The source code is there on the applet page.

Look at the AnalogClock code too.

have a look at http://mindprod.com/products.html#SCREWS 
too see some sample code.