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();
}

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]  }
}  

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();
}

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));}}@Overridepublic 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falsepublic boolean hasNext() {return index != students.length;}@Overridepublic 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;}@Overridepublic String toString() {return "Student{" +"sid='" + sid + '\'' +", name='" + name + '\'' +'}';}
}