迭代器模式

1.迭代器模式定义 迭代器模式（Iterator），提供一种方法顺序访问一个聚合对象中的各种元素，而又不暴露该对象的内部表示。 简单理解：对容器内元素遍历 2.迭代器模式角色划分 迭代器角色（Iterator）：定义遍历元素所需要的方法，一般来说会有这么三个方法：取得下一个元素next()，判断是否遍历结束的方法hasNext()，移除当前对象的方法remove()。 具体迭代器角色（Concrete Iterator）：实现迭代器接口中定义的方法，完成集合的迭代。 容器角色（Aggregate）：一般是一个接口，提供一个iterator()方法，例如java中的Collection接口、List接口、Set接口等。 具体容器角色（Concrete Aggregate）：就是抽象容器的具体实现类，比如List接口的有序列表实现ArrayList，List接口的链表实现LinkList，Set接口的哈希列表的实现HashSet等。

3.具体实例 大公司收购小公司（有员工信息） 大公司：保存员工信息，采用List集合保存 小公司：保存员工信息，采用数组保存 两个公司用不同数据类型（List 、Array）保存数据，现在读取他们 常规的写法：

public class Client {
    public static void main(String[] args) {
        AppleCompany appleCompany = new AppleCompany();
        List<UserBean> users = appleCompany.getUserList();
        for (UserBean userBean : users) {
            System.out.println("用户信息：" + userBean.toString());
        }
        SmallCompany smallCompany = new SmallCompany();
        UserBean[] userArray = smallCompany.getUserArray();
        for ( UserBean userBean : userArray ) {
            System.out.println("用户信息：" + userBean.toString());
        }
    }
    public static void save(List<UserBean> users){
        //保存数据库代码
 
    }
    public static void save(UserBean[] users){

    }
}
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这样写会将遍历方法以及信息暴露出来

迭代器模式写法：

public class UserBean {
    private String userId;
    private String userName;
    private String userSex;

    public UserBean(String userId, String userName, String userSex) {
        this.userId = userId;
        this.userName = userName;
        this.userSex = userSex;
    }

    public String getUserId() {
        return userId;
    }

    public void setUserId(String userId) {
        this.userId = userId;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getUserSex() {
        return userSex;
    }

    public void setUserSex(String userSex) {
        this.userSex = userSex;
    }

    @Override
    public String toString() {
        return "UserBean{" +
                "userId='" + userId + '\'' +
                ", userName='" + userName + '\'' +
                ", userSex='" + userSex + '\'' +
                '}';
    }
}
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/**
 * 容器接口
 * 存储数据
 * Created by Xionghu on 2017/7/6.
 * Desc:
 */

public interface Aggregate<T> {
    /**
     * 获取迭代器
     * @return
     */
    public Iterator<T> iterator();
}
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public class AppleAggregate implements Aggregate<UserBean> {

    private List<UserBean> userList;

    public AppleAggregate() {
        userList = new ArrayList<UserBean>();
        userList.add(new UserBean("0001","zhangsan","男"));
        userList.add(new UserBean("0002","lisi","男"));
        userList.add(new UserBean("0003","Michel","男"));
        userList.add(new UserBean("0004","Goode","男"));
    }

    @Override
    public Iterator<UserBean> iterator() {
        return new AppleIterator(userList);
    }
}
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public class SmallAggregate implements Aggregate<UserBean> {

    private UserBean[] userArray;

    public SmallAggregate() {
        this.userArray =new UserBean[6];
        this.userArray[0] = new UserBean("001","Boy1","男");
        this.userArray[1] = new UserBean("002","Boy2","男");
        this.userArray[2] = new UserBean("003","Boy3","男");
        this.userArray[3] = new UserBean("004","Boy4","男");
        this.userArray[4] = new UserBean("005","Boy5","男");
        this.userArray[5] = new UserBean("006","Boy6","男");
    }

    @Override
    public Iterator<UserBean> iterator() {
        return new SmallIterator(userArray);
    }
}
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/**
 * Created by Xionghu on 2017/7/6.
 * Desc: 用来遍历数据
 */

public interface Iterator<T> {

    /**
     * 是否有下一个元素
     *
     * @return
     */
    public boolean hasNext();

    /**
     * 下一个元素
     *
     * @return
     */
    public T next();

    /**
     * 删除元素
     */
    public void remove();
}
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/**
 * Created by Xionghu on 2017/7/6.
 * Desc:
 * 苹果公司员工信息迭代器
 */

public class AppleIterator implements Iterator<UserBean> {

    private List<UserBean> userList;

    private int index = 0;

    public AppleIterator(List<UserBean> userList) {
        this.userList = userList;
    }

    @Override
    public boolean hasNext() {
        if(userList == null){
            return false;
        }
        return index < userList.size();
    }

    @Override
    public UserBean next() {
        if (userList == null) {
            return null;
        }
        return userList.get(index++);
    }

    @Override
    public void remove() {

    }
}
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/**
* Created by Xionghu on 2017/7/6.
* Desc:小公司员工信息迭代器
*/

public class SmallIterator implements Iterator<UserBean> {

   private UserBean[] userList;
   private int index = 0;

   public SmallIterator( UserBean[] userList) {
       this.userList = userList;
   }
   @Override
   public boolean hasNext() {
       if(userList == null){
           return false;
       }
       return index < userList.length;
   }

   @Override
   public UserBean next() {
       if (userList == null) {
           return null;
       }
       return userList[index++];
   }

   @Override
   public void remove() {

   }
}
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public class Client {
    public static void main(String[] args) {
        AppleAggregate appleAggregate = new AppleAggregate();
        Iterator<UserBean> iterator = appleAggregate.iterator();
        save(iterator);

        SmallAggregate smallAggregate = new SmallAggregate();
        Iterator<UserBean> iterator2 = smallAggregate.iterator();
        save(iterator2);
    }

    /**
     * 不需要关注细节，所有遍历细节（比如数据类型）已经封装好了，没有对外暴露
     * @param iterator
     */
    public static void save(Iterator<UserBean> iterator) {
        while (iterator.hasNext()) {
            UserBean userBean = iterator.next();
            System.out.println("用户信息：" + userBean.toString());
        }
    }
}
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结果输出： 用户信息：UserBean{userId='0001', userName='zhangsan', userSex='男'} 用户信息：UserBean{userId='0002', userName='lisi', userSex='男'} 用户信息：UserBean{userId='0003', userName='Michel', userSex='男'} 用户信息：UserBean{userId='0004', userName='Goode', userSex='男'} 用户信息：UserBean{userId='001', userName='Boy1', userSex='男'} 用户信息：UserBean{userId='002', userName='Boy2', userSex='男'} 用户信息：UserBean{userId='003', userName='Boy3', userSex='男'} 用户信息：UserBean{userId='004', userName='Boy4', userSex='男'} 用户信息：UserBean{userId='005', userName='Boy5', userSex='男'} 用户信息：UserBean{userId='006', userName='Boy6', userSex='男'}

可以看到迭代器模式有点： 不需要关注细节，所有遍历细节（比如数据类型）已经封装好了，没有对外暴露 4.UML图

5.分析Android里面有哪些地方用到了迭代器模式 Java里面集合Collection(List、Set、Map等)都用到了迭代器模式

public interface Iterable<T> {
    /**
     * Returns an iterator over elements of type {@code T}.
     *
     * @return an Iterator.
     */
    Iterator<T> iterator();

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

    E next();

    default void remove() {
        throw new UnsupportedOperationException("remove");
    }

    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}
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ArrayList 中 Itr类

  private class Itr implements Iterator<E> {
        // The "limit" of this iterator. This is the size of the list at the time the
        // iterator was created. Adding & removing elements will invalidate the iteration
        // anyway (and cause next() to throw) so saving this value will guarantee that the
        // value of hasNext() remains stable and won't flap between true and false when elements
        // are added and removed from the list.
        protected int limit = ArrayList.this.size;

        int cursor;       // index of next element to return
        int lastRet = -1; // index of last element returned; -1 if no such
        int expectedModCount = modCount;

        public boolean hasNext() {
            return cursor < limit;
        }

        @SuppressWarnings("unchecked")
        public E next() {
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
            int i = cursor;
            if (i >= limit)
                throw new NoSuchElementException();
            Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length)
                throw new ConcurrentModificationException();
            cursor = i + 1;
            return (E) elementData[lastRet = i];
        }

        public void remove() {
            if (lastRet < 0)
                throw new IllegalStateException();
            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();

            try {
                ArrayList.this.remove(lastRet);
                cursor = lastRet;
                lastRet = -1;
                expectedModCount = modCount;
                limit--;
            } catch (IndexOutOfBoundsException ex) {
                throw new ConcurrentModificationException();
            }
        }

        @Override
        @SuppressWarnings("unchecked")
        public void forEachRemaining(Consumer<? super E> consumer) {
            Objects.requireNonNull(consumer);
            final int size = ArrayList.this.size;
            int i = cursor;
            if (i >= size) {
                return;
            }
            final Object[] elementData = ArrayList.this.elementData;
            if (i >= elementData.length) {
                throw new ConcurrentModificationException();
            }
            while (i != size && modCount == expectedModCount) {
                consumer.accept((E) elementData[i++]);
            }
            // update once at end of iteration to reduce heap write traffic
            cursor = i;
            lastRet = i - 1;

            if (modCount != expectedModCount)
                throw new ConcurrentModificationException();
        }
    }
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Iterable：相当于容器接口 ArrayList 相当于具体的容器 iterator：迭代器接口 Itr：相当于具体的迭代器

其他HashSet、HashMap类似

使用场景：遍历数据的