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Robot.java

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import josx.platform.rcx.*;

public class Robot
{
    public void go_forward()
    {
      Motor.A.forward();
      Motor.C.forward();
    }

    public void go_backward()
    {
      Motor.A.backward();
      Motor.C.backward();
    }

    public void stop()
    {
      Motor.A.stop();
      Motor.C.stop();
    }

    public void pause(int delay)
    {
      try { Thread.sleep(delay); }
      catch (InterruptedException ie) { }
    }
}

RobotLR.java

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import josx.platform.rcx.*;

public class RobotLR extends Robot
{
    Sensor touchSensor = Sensor.S1;

    public RobotLR ()
    {
      touchSensor.addSensorListener(new TouchBackListener(this));
            // As soon as the robot object is created
            // the SensorListener will be added to the touchsensor.

    }

    public void go_right()
    {
      Motor.A.forward();
      Motor.C.backward();
    }

    public void go_left()
    {
      Motor.A.backward();
      Motor.C.forward();
    }
}

RobotLine.java

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import josx.platform.rcx.*;

public class RobotLine extends RobotLR {

    Sensor  lightSensor = Sensor.S2;

    public RobotLine ()
    {
            // As soon as the robot object is created
      lightSensor.activate();
            // the lamp of the lightsensor will be switched on
      lightSensor.addSensorListener(new CourtListener(this));
            // and the SensorListener will be added to the lightsensor.
    }

    public void go() {
      TextLCD.print("LINE");

      this.go_forward();

      while (true) {  }
    }

    public void correct_line() {
      this.go_backward();
      this.pause(150);
      this.go_left();
      this.pause(80);
      this.go_forward();
    }
}

TouchBackListener.java

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import josx.platform.rcx.Sensor;
import josx.platform.rcx.SensorListener;
 
public class TouchBackListener implements SensorListener
{
    private RobotLR touchRobot;

    public TouchBackListener(RobotLR touchRobot) {
      
      this.touchRobot = touchRobot;
    }

    public void stateChanged(Sensor s, int oldValue, int newValue)
    {
      if (s.readBooleanValue()) 
      {
        touchRobot.go_backward();
        touchRobot.pause(300);
        touchRobot.go_forward();

      }
    }
}

CourtListener.java

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import josx.platform.rcx.Sensor;
import josx.platform.rcx.SensorListener;

public class CourtListener implements SensorListener {

    private RobotLine lineRobot;
    private int       threshold;

    public CourtListener(RobotLine lineRobot) {
      this.lineRobot  = lineRobot;
      this.threshold = 0;
      this.lineRobot.pause(300);
            // wait 300ms for reasonable sensor values.
    }

    public void stateChanged(Sensor s, int oldValue, int newValue) {

      if (threshold == 0) threshold = newValue;
            // If there is no threshold available,
            // it would be initialized once.
      if (newValue > threshold + 10) {
        lineRobot.correct_line();
      }     // If current lightsensor value is 10% over the threshold,
            // the robot has to correct his course.
    }
}

Test.java

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import josx.platform.rcx.*;

/**
 * Sample introduction to techniques of OOP.
 * @author T.Rinkens 25 Jul 2002
 *
 * DESCRIPTION
 *   Program that makes a simple robot follow the black line
 *   on the contructopedia test pad.
 *   Before pressing 'Run'-Button, the robot must be placed
 *   on the black line.
 */

class Test
{
    static RobotLine robbi;

    public static void main(String[] args)
    {
      robbi = new RobotLine();

      robbi.go();

    } // main
	
}// class