Objects communicating with each other

Question

Suppose I have a main class that has a Thread instance variable. What is the best way to notify the main class that something has happened in the thread?
Example:
I have a main class, a Producer, and a Consumer. Both the Producer and the Consumer extend Thread.
The main class is supposed to keep track of when the Producer produces something, and when a Consumer consumes something.
What is the best way for the Producer/Consumer to tell the main class that something has been produced/consumed?

Answer 1

example of Vistor parttern from Wikipedia
interface Visitor {
    void visit(Wheel wheel);
    void visit(Engine engine);
    void visit(Body body);
    void visitCar(Car car);
    void visitVehicle(Vehicle vehicle);
class Wheel {
    private String name;
    Wheel(String name) {
        this.name = name;
    String getName() {
        return this.name;
    public void accept(Visitor visitor) {
        visitor.visit(this);
class Engine {
    public void accept(Visitor visitor) {
        visitor.visit(this);
class Body {
    public void accept(Visitor visitor) {
        visitor.visit(this);
abstract class Vehicle {
    protected Engine  engine = new Engine();
    protected Body    body   = new Body();
    protected Wheel[] wheels;
    public Engine getEngine() {
        return this.engine;
    public Body getBody() {
        return this.body;
    public Wheel[] getWheels() {
        return this.wheels;
    public void accept(Visitor visitor) {
        visitor.visitVehicle(this);
class Car extends Vehicle {
    public Car() {
        super();
        this.wheels = new Wheel[]
          { new Wheel("front left"), new Wheel("front right"),
            new Wheel("back left") , new Wheel("back right")  };
    public void accept(Visitor visitor) {
         visitor.visitCar(this);
class PrintVisitor implements Visitor {
    public void visit(Wheel wheel) {
        System.out.println("Visiting "+ wheel.getName()
                            + " wheel");
    public void visit(Engine engine) {
        System.out.println("Visiting engine");
    public void visit(Body body) {
        System.out.println("Visiting body");
    public void visitVehicle(Vehicle vehicle) {
        System.out.println("Visiting vehicle");
    public void visitCar(Car car) {
        System.out.println("Visiting car");
        car.getEngine().accept(this);
        car.getBody().accept(this);
        for(Wheel wheel : car.getWheels()) {
            wheel.accept(this);
class DoVisitor implements Visitor {
    public void visit(Wheel wheel) {
        System.out.println("Steering my wheel");
    public void visit(Engine engine) {
        System.out.println("Starting my engine");
    public void visit(Body body) {
        System.out.println("Moving my body");
    public void visitCar(Car car) {
        System.out.println("Starting my car");
        car.getEngine().accept(this);
        car.getBody().accept(this);
        for(Wheel wheel : car.getWheels()) {
            wheel.accept(this);
    public void visitVehicle(Vehicle vehicle) {
        System.out.println("Starting my vehicle");
public class VisitorDemo {
    static public void main(String[] args){
        Car car = new Car();
        Visitor printVisitor = new PrintVisitor();
        Visitor doVisitor = new DoVisitor();
        car.accept(printVisitor);
        car.accept(doVisitor);
}