java 在观察者模式中使用泛型T的实例

被观察者

public class observable<t> {

  list<observer> observers = new arraylist<observer>();

  boolean changed = 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.
   */
  public void addobserver(observer observer) {
    if (observer == null) {
      throw new nullpointerexception("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}.
   */
  protected void clearchanged() {
    changed = false;
  }

  /**
   * returns the number of observers registered to this {@code observable}.
   *
   * @return the number of observers.
   */
  public int countobservers() {
    return observers.size();
  }

  /**
   * removes the specified observer from the list of observers. passing null
   * won't do anything.
   *
   * @param observer
   *      the observer to remove.
   */
  public synchronized void deleteobserver(java.util.observer observer) {
    observers.remove(observer);
  }

  /**
   * removes all observers from the list of observers.
   */
  public synchronized void deleteobservers() {
    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.
   */
  public boolean haschanged() {
    return changed;
  }

  /**
   * 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()}.
   * <p>
   * equivalent to calling {@code notifyobservers(null)}.
   */
  public void notifyobservers() {
    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()}.
   */
  public void notifyobservers(t data) {
    int size = 0;
    observer[] arrays = null;
    synchronized (this) {
      if (haschanged()) {
        clearchanged();
        size = observers.size();
        arrays = new observer[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}.
   */
  protected void setchanged() {
    changed = true;
  }
}

观察者

public interface observer<t> {
  public void update(observable<t> observable, t data);
}

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