Iterator && Iterable Collection && Map

Java集合类库将集合的接口与实现分离。同样的接口，可以有不同的实现。

Java集合类的基本接口是Collection接口。而Collection接口必须实现Iterable接口。

以下图表示集合框架的接口，java.lang以及java.util两个包里的。红色字体部分是OCJP考纲要求的接口。其他部分可以从左向右看，比如Collection的Subinterfaces有List，Set以及Queue等。

Iterator接口

Iterator接口包含三个方法：

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public interface Iterator<E>{
E next();
boolean hasNext();
void remove();
}

以下例子是利用了Iterator接口的着三个方法，实现遍历ArrayList<String>类型。

一开始迭代器在所有元素的左边，调用next()之后，迭代器移到第一个和第二个元素之间，next()方法返回迭代器刚刚经过的元素。
hasNext()若返回True，则表明接下来还有元素，迭代器不在尾部。
remove()方法必须和next方法一起使用，功能是去除刚刚next方法返回的元素。

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package com.xujin;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Iterator;
public class Test{
public static void main(String...arg){
Collection<String> a = new ArrayList<String>();
a.add("Bob");
a.add("Alice");
a.add("Lisy");
Iterator<String> iter = a.iterator();
while(iter.hasNext()){
String ele = iter.next();
System.out.print(ele + " ");//Bob Alice Lisy
}
System.out.println();
System.out.println(a);//[Bob, Alice, Lisy]
Iterator<String> iter2 = a.iterator();
iter2.next();
iter2.remove();
System.out.println(a);//[Alice, Lisy]
}
}

Iterable接口

Iterable接口仅包含一个方法：

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public interface Iterable<E>{
Iterator<E> iterator();
}

for-each循环可以与任何实现了Iterable接口的对象一起工作。

而Collection接口扩展了Iterable接口，故标准类库中的任何集合都可以使用for-each循环。

Collection接口

此接口的方法

public interface Collection<E>{......}


Modifier and Type	Method and Description
`boolean`	`add(E e)` Ensures that this collection contains the specified element (optional operation).
`boolean`	`addAll(Collection<? extends E> c)` Adds all of the elements in the specified collection to this collection (optional operation).
`void`	`clear()` Removes all of the elements from this collection (optional operation).
`boolean`	`contains(Object o)` Returns `true` if this collection contains the specified element.
`boolean`	`containsAll(Collection<?> c)` Returns `true` if this collection contains all of the elements in the specified collection.
`boolean`	`equals(Object o)` Compares the specified object with this collection for equality.
`int`	`hashCode()` Returns the hash code value for this collection.
`boolean`	`isEmpty()` Returns `true` if this collection contains no elements.
`Iterator<E>`	`iterator()` Returns an iterator over the elements in this collection.
`boolean`	`remove(Object o)` Removes a single instance of the specified element from this collection, if it is present (optional operation).
`boolean`	`removeAll(Collection<?> c)` Removes all of this collection's elements that are also contained in the specified collection (optional operation).
`boolean`	`retainAll(Collection<?> c)` Retains only the elements in this collection that are contained in the specified collection (optional operation).
`int`	`size()` Returns the number of elements in this collection.
`Object[]`	`toArray()` Returns an array containing all of the elements in this collection.
`<T> T[]`	`toArray(T[] a)` Returns an array containing all of the elements in this collection; the runtime type of the returned array is that of the specified array.

因为其中有一个返回值为Iterator<E>类型的iterator()方法，所以，Collection接口必须实现Iterator接口

实现Collection接口的每一个类都要实现以上众多方法，但开发者自己实现很麻烦。所以java提供了AbstractCollection类来编写具体的类。

以下类都实现了Collection接口：

AbstractCollection, AbstractList, AbstractQueue, AbstractSequentialList, AbstractSet, ArrayBlockingQueue, ArrayDeque, ArrayList, AttributeList, BeanContextServicesSupport, BeanContextSupport, ConcurrentLinkedDeque,ConcurrentLinkedQueue, ConcurrentSkipListSet, CopyOnWriteArrayList, CopyOnWriteArraySet, DelayQueue, EnumSet, HashSet, JobStateReasons, LinkedBlockingDeque, LinkedBlockingQueue, LinkedHashSet, LinkedList,LinkedTransferQueue, PriorityBlockingQueue, PriorityQueue, RoleList, RoleUnresolvedList, Stack, SynchronousQueue, TreeSet, Vector

Collection接口有三个常用的子接口，分别是List，Set，Queue。