Observer Pattern with built-in Observable

The Observer Pattern defines a one-to-many dependency between objects so that when one object changes state, all of its dependents are notified and updated automatically

Use unified observer interface with update method to notify observers and perform actions

Java has built-in support in several of its APIs. The most general is the Observer interface and the Observable class in the java.util package. These are quite similar to our Subject and Observer interface, but give you a lot of functionality out of the box. You can also implement either a push or pull style of update to your observers

First, because Observable is a class, you have to subclass it. That means you can’t add on the Observable behavior to an existing class that already extends another superclass. This limits its reuse potential
If you look at the Observable API, the setChanged() method is protected. So what? Well, this means you can’t call setChanged() unless you’ve subclassed Observable. This means you can’t even create an instance of the Observable class and compose it with your own objects, you have to subclass.

Implementations

import java.util.Observable;
import java.util.Observer;
	
public class WeatherData extends Observable {
	private float temperature;
	private float humidity;
	private float pressure;
	
	public WeatherData() { }
	
	public void measurementsChanged() {
		setChanged();
		notifyObservers();
	}
	
	public void setMeasurements(float temperature, float humidity, float pressure) {
		this.temperature = temperature;
		this.humidity = humidity;
		this.pressure = pressure;
		measurementsChanged();
	}
	
	public float getTemperature() {
		return temperature;
	}
	
	public float getHumidity() {
		return humidity;
	}
	
	public float getPressure() {
		return pressure;
	}
}

import java.util.Observable;
import java.util.Observer;
	
public class CurrentConditionsDisplay implements Observer, DisplayElement {
	Observable observable;
	private float temperature;
	private float humidity;
	
	public CurrentConditionsDisplay(Observable observable) {
		this.observable = observable;
		observable.addObserver(this);
	}
	
	public void update(Observable obs, Object arg) {
		if (obs instanceof WeatherData) {
			WeatherData weatherData = (WeatherData)obs;
			this.temperature = weatherData.getTemperature();
			this.humidity = weatherData.getHumidity();
			display();
		}
	}
	
	public void display() {
		System.out.println("Current conditions: " + temperature 
			+ "F degrees and " + humidity + "% humidity");
	}
}