Collection是JDK中集合层次结构中的最根本的接口。定义了集合类的基本方法。源码中的解释：
* The root interface in the <i>collection hierarchy</i>. A collection
* represents a group of objects, known as its <i>elements</i>. Some
* collections allow duplicate elements and others do not. Some are ordered
* and others unordered. The JDK does not provide any <i>direct</i>
* implementations of this interface: it provides implementations of more
* specific subinterfaces like <tt>Set</tt> and <tt>List</tt>. This interface
* is typically used to pass collections around and manipulate them where
* maximum generality is desired.
Collections是一个包装类。它包含有各种有关集合操作的静态多态方法,不能实例化，像一个Collection集合框架中的工具类。
* This class consists exclusively of static methods that operate on or return
* collections. It contains polymorphic algorithms that operate on
* collections, "wrappers", which return a new collection backed by a
* specified collection, and a few other odds and ends.