java 在观察者模式中使用泛型T的实例

 public class Observable { List observers = new ArrayList(); 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()}. * 
 * 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; } }

 public interface Observer { public void update(Observable observable, T data); }