JUnit testing GUI class

I've looked over the stackoverflow and the internet and I couldn't find a clear answer that helped me.

I have an assignment and it includes the following class, which is a GUI. I have Junit tested the other classes but for this I didn't know how.

import java.awt.*;

public class CruiseDisplay extends Canvas {


private int recorded     = 0;     //recorded speed
private boolean cruiseOn = false; //cruise control state
private final static int botY = 200;
private Font small = new Font("Helvetica",Font.BOLD,14);
private Font big = new Font("Helvetica",Font.BOLD,18);

public CruiseDisplay() {
    super();
    setSize(150,260);
}


Image offscreen;
Dimension offscreensize;
Graphics offgraphics;

public void backdrop() {
    Dimension d = getSize();
    if ((offscreen == null) || (d.width != offscreensize.width)
                            || (d.height != offscreensize.height)) {
        offscreen = createImage(d.width, d.height);
        offscreensize = d;
        offgraphics = offscreen.getGraphics();
        offgraphics.setFont(small);
    }
    offgraphics.setColor(Color.black);
    offgraphics.fillRect(0, 0, getSize().width, getSize().height);
    offgraphics.setColor(Color.white);
    offgraphics.drawRect(5,10,getSize().width-15,getSize().height-40);
    offgraphics.setColor(Color.blue);
    offgraphics.fillRect(6,11,getSize().width-17,getSize().height-42);
 }

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

public void update(Graphics g) {
    backdrop();
    // display recorded speed
    offgraphics.setColor(Color.white);
    offgraphics.setFont(big);
    offgraphics.drawString("Cruise Control",10,35);
    offgraphics.setFont(small);
    drawRecorded(offgraphics,20,80,recorded);
    if (cruiseOn)
        offgraphics.drawString("Enabled",20,botY+15);
    else
        offgraphics.drawString("Disabled",20,botY+15);
    if (cruiseOn)
       offgraphics.setColor(Color.green);
    else
       offgraphics.setColor(Color.red);
    offgraphics.fillArc(90,botY,20,20,0,360);
    g.drawImage(offscreen, 0, 0, null);
}

public void drawRecorded(Graphics g, int x, int y, int speed) {
    g.drawString("Cruise Speed",x,y+10);
    g.drawRect(x+20,y+20,50,20);
    g.setFont(big);
    g.drawString(String.valueOf(speed+20),x+30,y+37);
    g.setFont(small);
}

public void enabled() {
    cruiseOn = true;
    repaint();
}

public void disabled() {
    cruiseOn = false;
    repaint();
}

public void record(int speed) {
    recorded=speed;
    repaint();
}
}

Can somebody help me please?