Collection
概述
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.
集合的root接口；
collection代表一组对象，称对象为元素；
jdk没有提供直接的collection的实现，提供了子接口：Set、List；
public interface Collection<E> extends Iterable<E> {
Iterator<E> iterator();
}
Iterable
概述
Implementing this interface allows an object to be the target of the "for-each loop" statement.
实现Iterable，允许一个对象进行遍历；
public interface Iterable<T> {
Iterator<T> iterator();
}