The State Pattern allows an object to alter its behavior when its internal state changes. The object will appear to change its class.

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public class Vector extends AbstractList implements List

The Facade Pattern provides a unified interface to a set of interfaces in subsystem. Facade defines a higher-level interface that makes the subsystem easier to use.

The Decorator Pattern attaches additional responsibilities to an object dynamically. Decorators provide a flexible alternative to subclassing for extending functionality.

The Template Method Pattern defines the skeleton of an algorithm in a method, deferring some steps to subclasses. Template Method lets subclass redefine certain steps of an algorithm without changing the algorithm's structure.

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.