随笔档案

1. Observable is a class
  First, because Observable is a class, you have to subclass it. That means you can't add on the Observable to an existing class that already extends another superclass. This limits its reuse potential.
Second, because there isn't an Observable interface, you can't even create your own implementation that plays well with Java's built-in Observer API. Nor do you have the option of swapping out the java.util implementation for another.
2. Observable protects crucial methods
  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. The design violates a second design principle here...favor composition over inheritance.