Observer

Observer && being observed, e.g. ProductList && e-commerce companies.

Reason to adopt this model: de-coupling.

Java中的观察者模式要同时存在观察者和被观察者，观察者可以是多个。

被观察者需要继承java.util.Observerable类. E.g. ProductList:

public class ProductList extends Observable {
  private List<String> productList = null;
  private static ProductList instance; // only one
  private ProductList() {} // singleton

  /* singleton:
    1. private constructor
    2. getInstance() method to create the only object
    3. "new" inside getInstance()

    @return the only ProductList instance
  */
  public static ProductList getInstance() {
  	if (instance == null) {
  	  instance = new ProductList();
  	  instance.productList = new ArrayList<String>();
  	}
  	return instance;
  }

  /*
    Add observer (e-commerce interface)
  */
  public void addProductListObserver(Observer observer) {
  	this.addObserver(observer);
  }

  /*
    add product
  */
  public void addProduct(String newProduct) {
  	productList.add(newProduct);
  	System.out.println("productList add:" + newProduct);
  	this.setChanged(); // observerd changed
  	this.notifyObservers(newProduct); // notify observers, add new products
  }
}

观察者需要实现java.util.Observer接口的update方法:

public class JDObserver implements Observer {
  @Override
  public void update(Observerable o, Object product) {
  	String newProduct = (String) product;
  	System.err.println("发送新产品" + newProduct + "同步到京东商城");
  }
}

public class TBObserver implements Observer {
  @Override
  public void update(Observerable o, Object product) {
  	String newProduct = (String) product;
  	System.err.println("发送新产品" + newProduct + "同步到淘宝");
  }
}

main:

public static void main(String[] args) {
  ProductList observable = ProductList.getInstance(); // singleton new object
  TBObserver tbObserver = new TBObserver();
  JDObserver jdObserver = new JDObserver();
  observable.addObserver(jdObserver);
  observable.addObserver(tbObserver);
  observable.addProduct("新增产品1");
}

output:

(TO BE MODIFIED)