https://www.cnblogs.com/softidea/p/5167676.html
ava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接口必须继承java.lang.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。其他部分可以从左向右看，比如Collection的Subinterfaces有List，Set以及Queue等。
image
package java.util;
/** * An iterator over a collection. Iterator takes the place of Enumeration in
* the Java collections framework. Iterators differ from enumerations in two
* ways: <ul>
* <li> Iterators allow the caller to **remove elements** from the
* underlying collection during the iteration with well-defined
* semantics.
* <li> Method names have been improved.
* </ul><p>
*
* This interface is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @author Josh Bloch
* @version %I%, %G%
* @see Collection
* @see ListIterator
* @see Enumeration
* @since 1.2 */
public interface Iterator<E> { /** * Returns <tt>true</tt> if the iteration has more elements. (In other
* words, returns <tt>true</tt> if <tt>next</tt> would return an element
* rather than throwing an exception.)
*
* @return <tt>true</tt> if the iterator has more elements. */
boolean hasNext(); /** * Returns **the next element** （每一次迭代，the next element就是index为0的元素）in the iteration.
*
* @return the next element in the iteration.
* @exception NoSuchElementException iteration has no more elements. */ E next(); /** *
* Removes from the underlying collection the last element returned by the
* iterator (optional operation). This method can be called only once per
* call to <tt>next</tt>. The behavior of an iterator is unspecified if
* the underlying collection is modified while the iteration is in
* progress in any way other than by calling this method.
*
* @exception UnsupportedOperationException if the <tt>remove</tt>
* operation is not supported by this Iterator.
* @exception IllegalStateException if the <tt>next</tt> method has not
* yet been called, or the <tt>remove</tt> method has already
* been called after the last call to the <tt>next</tt>
* method. */
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。
import java.util.ArrayList; import java.util.Collection;
import java.util.Iterator;
public class ForEachDemo {
public static void main(String... arg) {
Collection<String> a = new ArrayList<String>();
a.add("Bob");
a.add("Alice");
a.add("Lisy");
Iterator<String> iterator = a.iterator(); while (iterator.hasNext()) {
String ele = iterator.next();
System.out.println(ele);//Bob Alice Lisy
}
System.out.println(a);//[Bob, Alice, Lisy]
iterator = a.iterator();
iterator.next();
iterator.remove();
System.out.println(a);//[Alice, Lisy]
}
}
<pre>package java.lang; import java.util.Iterator; /** Implementing this interface allows an object to be the target of
- the "foreach" statement.
- @since 1.5 /
public interface Iterable<T> { /* * Returns an iterator over a set of elements of type T.- @return an Iterator. */ Iterator<T> iterator();
}</pre>
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for-each循环可以与任何实现了Iterable接口的对象一起工作。
而java.util.Collection接口继承java.lang.Iterable，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>{......}
<caption> </caption>
| Modifier and Type | Method and Description |
| boolean | **[add](file:///D:/jdk1.7/docs/api/java/util/Collection.html#add(E))**([E](file:///D:/jdk1.7/docs/api/java/util/Collection.html "type parameter in Collection") e)
Ensures that this collection contains the specified element (optional operation).
|
| boolean | **[addAll](file:///D:/jdk1.7/docs/api/java/util/Collection.html#addAll(java.util.Collection))**([Collection](file:///D:/jdk1.7/docs/api/java/util/Collection.html "interface in java.util")<? extends [E](file:///D:/jdk1.7/docs/api/java/util/Collection.html "type parameter in Collection")> c)
Adds all of the elements in the specified collection to this collection (optional operation).
|
| void | **[clear](file:///D:/jdk1.7/docs/api/java/util/Collection.html#clear())**()
Removes all of the elements from this collection (optional operation).
|
| boolean | **[contains](file:///D:/jdk1.7/docs/api/java/util/Collection.html#contains(java.lang.Object))**([Object](file:///D:/jdk1.7/docs/api/java/lang/Object.html "class in java.lang") o)
Returns <tt>true</tt> if this collection contains the specified element.
|
| boolean | **[containsAll](file:///D:/jdk1.7/docs/api/java/util/Collection.html#containsAll(java.util.Collection))**([Collection](file:///D:/jdk1.7/docs/api/java/util/Collection.html "interface in java.util")<?> c)
Returns <tt>true</tt> if this collection contains all of the elements in the specified collection.
|
| boolean | **[equals](file:///D:/jdk1.7/docs/api/java/util/Collection.html#equals(java.lang.Object))**([Object](file:///D:/jdk1.7/docs/api/java/lang/Object.html "class in java.lang") o)
Compares the specified object with this collection for equality.
|
| int | **[hashCode](file:///D:/jdk1.7/docs/api/java/util/Collection.html#hashCode())**()
Returns the hash code value for this collection.
|
| boolean | **[isEmpty](file:///D:/jdk1.7/docs/api/java/util/Collection.html#isEmpty())**()
Returns <tt>true</tt> if this collection contains no elements.
|
| [Iterator](file:///D:/jdk1.7/docs/api/java/util/Iterator.html "interface in java.util")<[E](file:///D:/jdk1.7/docs/api/java/util/Collection.html "type parameter in Collection")> | **[iterator](file:///D:/jdk1.7/docs/api/java/util/Collection.html#iterator())**()
Returns an iterator over the elements in this collection.
|
| boolean | **[remove](file:///D:/jdk1.7/docs/api/java/util/Collection.html#remove(java.lang.Object))**([Object](file:///D:/jdk1.7/docs/api/java/lang/Object.html "class in java.lang") o)
Removes a single instance of the specified element from this collection, if it is present (optional operation).
|
| boolean | **[removeAll](file:///D:/jdk1.7/docs/api/java/util/Collection.html#removeAll(java.util.Collection))**([Collection](file:///D:/jdk1.7/docs/api/java/util/Collection.html "interface in java.util")<?> c)
Removes all of this collection's elements that are also contained in the specified collection (optional operation).
|
| boolean | **[retainAll](file:///D:/jdk1.7/docs/api/java/util/Collection.html#retainAll(java.util.Collection))**([Collection](file:///D:/jdk1.7/docs/api/java/util/Collection.html "interface in java.util")<?> c)
Retains only the elements in this collection that are contained in the specified collection (optional operation).
|
| int | **[size](file:///D:/jdk1.7/docs/api/java/util/Collection.html#size())**()
Returns the number of elements in this collection.
|
| [Object](file:///D:/jdk1.7/docs/api/java/lang/Object.html "class in java.lang")[] | **[toArray](file:///D:/jdk1.7/docs/api/java/util/Collection.html#toArray())**()
Returns an array containing all of the elements in this collection.
|
| <T> T[] | **[toArray](file:///D:/jdk1.7/docs/api/java/util/Collection.html#toArray(T[]))**(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，java.util.Collection接口必须继承java.lang.Iterable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。
java.util
Interface Collection<E>
<dl>
<dt>All Superinterfaces:</dt>
<dd>Iterable<E></dd>
</dl>
<dl>
<dt>All Known Subinterfaces:</dt>
<dd>BeanContext, BeanContextServices, BlockingDeque<E>, BlockingQueue<E>, Deque<E>, List<E>, NavigableSet<E>, Queue<E>, Set<E>, SortedSet<E></dd>
</dl>
<dl>
<dt>All Known Implementing Classes:</dt>
<dd>AbstractCollection, AbstractList, AbstractQueue, AbstractSequentialList, AbstractSet, ArrayBlockingQueue, ArrayDeque, ArrayList, AttributeList, BeanContextServicesSupport, BeanContextSupport, ConcurrentLinkedQueue, ConcurrentSkipListSet, CopyOnWriteArrayList, CopyOnWriteArraySet, DelayQueue, EnumSet, HashSet, JobStateReasons, LinkedBlockingDeque, LinkedBlockingQueue, LinkedHashSet, LinkedList, PriorityBlockingQueue, PriorityQueue, RoleList, RoleUnresolvedList, Stack, SynchronousQueue, TreeSet, Vector</dd>
</dl>
Collection接口有三个常用的子接口，分别是List，Set，Queue。
http://blog.csdn.net/xujinsmile/article/details/8543544
为什么一定要去实现Iterable这个接口呢？ 为什么不直接实现Iterator接口呢？
看一下JDK中的集合类，比如List一族或者Set一族，
都是继承了Iterable接口，但并不直接继承Iterator接口。
仔细想一下这么做是有道理的。因为Iterator接口的核心方法next()或者hasNext()
是依赖于迭代器的当前迭代位置的。
如果Collection直接继承Iterator接口，势必导致集合对象中包含当前迭代位置的数据(指针)。
当集合在不同方法间被传递时，由于当前迭代位置不可预置，那么next()方法的结果会变成不可预知。
除非再为Iterator接口添加一个reset()方法，用来重置当前迭代位置。
但即时这样，Collection也只能同时存在一个当前迭代位置。
而Iterable则不然，每次调用都会返回一个从头开始计数的迭代器。
多个迭代器是互不干扰的。
http://www.cnblogs.com/highriver/archive/2011/07/27/2077913.html
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<pre>import java.util.Iterator; public class ForEachAPIDemo { public static void main(String[] args) throws Exception {
Students students = new Students(10); for (Student student : students) {
System.out.println(student.getSid() + ":" + student.getName());
}
}
} // 支持for each迭代循环的学生集合类
class Students implements Iterable<Student> { // 存储所有学生类的数组
private Student[] students; // 该构造函数可以生成指定大小的学生类变量数组，并初始化该学生类变量数组
public Students(int size) {
students = new Student[size]; for (int i = 0; i < size; i++) {
students[i] = new Student(String.valueOf(i), "学生" + String.valueOf(i));
}
}
@Override public Iterator<Student> iterator() { return new StudentIterator();
} // 实现Iterator接口的私有内部类，外界无法直接访问
private class StudentIterator implements Iterator<Student> { // 当前迭代元素的下标
private int index = 0; // 判断是否还有下一个元素，如果迭代到最后一个元素就返回false
public boolean hasNext() { return index != students.length;
}
@Override public Student next() { return students[index++];
} // 这里不支持，抛出不支持操作异常
public void remove() { throw new UnsupportedOperationException();
}
}
} class Student { private String sid; private String name; public Student(String sid, String name) {
setSid(sid);
setName(name);
} public String getSid() { return sid;
} public void setSid(String sid) { this.sid = sid;
} public String getName() { return name;
} public void setName(String name) { this.name = name;
}
@Override public String toString() { return "Student{" +
"sid='" + sid + '\'' +
", name='" + name + '\'' +
'}';
}
}</pre>
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