java 在观察者模式中使用泛型T的实例

publicclassObservable {

  List observers =newArrayList();

  booleanchanged =false;

  /**

   * Adds the specified observer to the list of observers. If it is already

   * registered, it is not added a second time.

   *

   * @param observer

   *      the Observer to add.

   */

  publicvoidaddObserver(Observer observer) {

    if(observer ==null) {

      thrownewNullPointerException("observer == null");

    }

    synchronized(this) {

      if(!observers.contains(observer))

        observers.add(observer);

    }

  }

  /**

   * Clears the changed flag for this {@code Observable}. After calling

   * {@code clearChanged()}, {@code hasChanged()} will return {@code false}.

   */

  protectedvoidclearChanged() {

    changed =false;

  }

  /**

   * Returns the number of observers registered to this {@code Observable}.

   *

   * @return the number of observers.

   */

  publicintcountObservers() {

    returnobservers.size();

  }

  /**

   * Removes the specified observer from the list of observers. Passing null

   * won't do anything.

   *

   * @param observer

   *      the observer to remove.

   */

  publicsynchronizedvoiddeleteObserver(java.util.Observer observer) {

    observers.remove(observer);

  }

  /**

   * Removes all observers from the list of observers.

   */

  publicsynchronizedvoiddeleteObservers() {

    observers.clear();

  }

  /**

   * Returns the changed flag for this {@code Observable}.

   *

   * @return {@code true} when the changed flag for this {@code Observable} is

   *     set, {@code false} otherwise.

   */

  publicbooleanhasChanged() {

    returnchanged;

  }

  /**

   * If {@code hasChanged()} returns {@code true}, calls the {@code update()}

   * method for every observer in the list of observers using null as the

   * argument. Afterwards, calls {@code clearChanged()}.

   *

   * Equivalent to calling {@code notifyObservers(null)}.

   */

  publicvoidnotifyObservers() {

    notifyObservers(null);

  }

  /**

   * If {@code hasChanged()} returns {@code true}, calls the {@code update()}

   * method for every Observer in the list of observers using the specified

   * argument. Afterwards calls {@code clearChanged()}.

   *

   * @param data

   *      the argument passed to {@code update()}.

   */

  publicvoidnotifyObservers(T data) {

    intsize =0;

    Observer[] arrays =null;

    synchronized(this) {

      if(hasChanged()) {

        clearChanged();

        size = observers.size();

        arrays =newObserver[size];

        observers.toArray(arrays);

      }

    }

    if(arrays !=null) {

      for(Observer observer : arrays) {

        observer.update(this, data);

      }

    }

  }

  /**

   * Sets the changed flag for this {@code Observable}. After calling

   * {@code setChanged()}, {@code hasChanged()} will return {@code true}.

   */

  protectedvoidsetChanged() {

    changed =true;

  }

}

publicinterfaceObserver {

  publicvoidupdate(Observable observable, T data);

}