Iterable
Iterable是迭代器的意思，作用是为集合类提供for-each循环的支持。由于使用for循环需要通过位置获取元素，而这种获取方式仅有数据支持，其他数据结构，比如链表，只能通过查询获取数据，会降低效率。Iterable就可以让不同的集合类自己提供遍历的最佳方式
/**
* Implementing this interface allows an object to be the target of
* the "for-each loop" statement. See
*/
public interface Iterable<T> {
/**
* Returns an iterator over elements of type {@code T}.
* @return an Iterator.
*/
Iterator<T> iterator();
/**
* Performs the given action for each element of the {@code Iterable}
* until all elements have been processed or the action throws an
* exception. Unless otherwise specified by the implementing class,
* actions are performed in the order of iteration (if an iteration order
* is specified). Exceptions thrown by the action are relayed to the
* caller.
*
* @since 1.8
*/
default void forEach(Consumer<? super T> action) {
Objects.requireNonNull(action);
for (T t : this) {
action.accept(t);
}
}
/**
* Creates a {@link Spliterator} over the elements described by this
* {@code Iterable}.
* @since 1.8
*/
default Spliterator<T> spliterator() {
return Spliterators.spliteratorUnknownSize(iterator(), 0);
}
}
Collection
Collection是List、Queue和Set的超集，它直接继承于Iterable，也就是所有的Collection集合类都支持for-each循环。除此之外，Collection也是面向接口编程的典范，通过它可以在多种实现类间转换
/**
* 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.
**/
public interface Collection<E> extends Iterable<E> {
}
AbstractCollection
在Collection中定义的许多方法，根据现有的定义以及继承的Iterable，都可以在抽象类中实现，这样可以减少实现类需要实现的方法
/**
* This class provides a skeletal implementation of the <tt>Collection</tt>
* interface, to minimize the effort required to implement this interface. <p>
**/
public abstract class AbstractCollection<E> implements Collection<E> {
}
Collections
是一个包装类（工具类/帮助类）。它包含有各种有关集合操作的静态多态方法。此类不能实例化，就像一个工具类，用于对集合中元素进行排序、搜索以及线程安全等各种操作，服务于Java的Collection框架。
public class Collections { // Suppresses default constructor, ensuring non-instantiability. private Collections() { } }