The Observer Pattern defines as one-to-many dependcy between objects so that when one object changes state, all if its depends are notified and updated automatically.

public interface List extends Collection

给定数列 1, 1, 1, 3, 5, 9, 17, …，从第 4 项开始，每项都是前 3 项的和。求第 20190324 项的最后 4 位数字。

public interface Map

The Strategy Pattern defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.

The Command Pattern encapsulates a request as an object, thereby letting you parameterize other objects with different requests, qucue or log requests, and support undoable operations.