依赖于抽象不对了？

[caoi]

UI Swing turorial online.
Related page:
http://java.sun.com/docs/books/tutorial/uiswing/layout/spring.html
source code:
http://java.sun.com/docs/books/tutorial/uiswing/layout/examples/SpringDemo2.java

My question is about this line:
SpringLayout layout = new SpringLayout();

As we know, class SpringLayout is implemented the interface LayoutManger, so this SpringLayout-to-LayoutManager is is-a relation. But when I replace it by:
LayoutManager layout = new SpringLayout();
Problem occurs, the IDE indicates that methods cannot be found.

Jeff Yan说过依赖于抽象的依赖反转原则DIP，这里怎么没有效果呢？我错哪里了？

[shoyonder]

这个不是说依赖倒转原则，好像应该叫做迪米特法则巴。如果真的是父类子类关系，肯定没有问题。

[caoi]

to shoyonder(笑天)
迪米特法则(law of Detemetr)说的是一个对象对其他对象的了解度应该是最少。

而DIP说的依赖于抽象（比如接口）进行编程。比如ArrayList的建立：
List al = new ArrayList();而不是ArrayList al = new ArrayList();

[caoi]

此问题在www.javaworld.com.tw上已经有了完整的回复。
http://www.javaworld.com.tw/jute/post/view?bid=29&id=167399&tpg=1&ppg=1&sty=1#167498