An Observer design pattern implementation example based on Ruby's Observable API

Price.java

import java.util.Random;

public class Price {
  private int value;

  public Price(int value) {
    this.value = value;
  }

  public int getValue() {
    return value;
  }

  public boolean equals(Price otherPrice) {
    if(otherPrice != null) {
      return this.getValue() == otherPrice.getValue();
    } else {
      return false;
    }
  }

  public static Price fetch() {
    return new Price(new Random().nextInt(300));
  }
}

Ticker.java

import java.util.Observable;

public class Ticker extends Observable implements Runnable {
  public void run() {
    Price lastPrice = null;
    while(true) {
      Price price = Price.fetch();
      System.out.format("\nCurrent price: %d", price.getValue());
      if(!price.equals(lastPrice)) {
        this.setChanged();
        lastPrice = price;
        notifyObservers(lastPrice);
        try {
          Thread.sleep(1000);
        } catch( InterruptedException e ) {
          break;
        }
      }
    }
  }
}

TickerRunner.java

public class TickerRunner {
  public static void main(String[] args) throws InterruptedException {
    Ticker ticker = new Ticker();
    new WarnLow(ticker, 80);
    new WarnHigh(ticker, 120);
    ticker.run();
  }
}

Warner.java

import java.util.Observer;

public abstract class Warner implements Observer {
  protected int limit;
  protected Price price;
  protected Ticker ticker;

  Warner(Ticker ticker, int limit) {
    this.ticker = ticker;
    this.limit = limit;

    ticker.addObserver(this);
  }
}

WarnHigh.java

import java.util.Observer;
import java.util.Observable;

public class WarnHigh extends Warner {
  public WarnHigh(Ticker ticker, int limit) {
    super(ticker, limit);
  }

  public void update(Observable observable, Object obj) {
    if (obj instanceof Price) {
      price = (Price) obj;
      if(price.getValue() > this.limit) {
        System.out.format("\n+++ Price above %d: %d", limit, price.getValue() );
      }
    }
  }
}

WarnLow.java

import java.util.Observer;
import java.util.Observable;

public class WarnLow extends Warner {
    public WarnLow(Ticker ticker, int limit) {
    super(ticker, limit);
  }

  public void update(Observable observable, Object obj) {
    if (obj instanceof Price) {
      price = (Price) obj;
      if(price.getValue() < this.limit) {
        System.out.format("\n--- Price bellow %d: %d", limit, price.getValue() );
      }
    }
  }
}