设计模式-迭代器模式

1. 所有的实现迭代器的类都直接或间接的继承实现了 Iterable接口.并在类的内部实现了 Iterator 类

/**
 * Implementing this interface allows an object to be the target of the enhanced
 * {@code for} statement (sometimes called the "for-each loop" statement).
 *
 * @param <T> the type of elements returned by the iterator
 *
 * @since 1.5
 * @jls 14.14.2 The enhanced {@code for} statement
 */
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.  Actions are performed in the order of iteration, if that
     * order is specified.  Exceptions thrown by the action are relayed to the
     * caller.
     * <p>
     * The behavior of this method is unspecified if the action performs
     * side-effects that modify the underlying source of elements, unless an
     * overriding class has specified a concurrent modification policy.
     *
     * @implSpec
     * <p>The default implementation behaves as if:
     * <pre>{@code
     *     for (T t : this)
     *         action.accept(t);
     * }</pre>
     *
     * @param action The action to be performed for each element
     * @throws NullPointerException if the specified action is null
     * @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}.
     *
     * @implSpec
     * The default implementation creates an
     * <em><a href="../util/Spliterator.html#binding">early-binding</a></em>
     * spliterator from the iterable's {@code Iterator}.  The spliterator
     * inherits the <em>fail-fast</em> properties of the iterable's iterator.
     *
     * @implNote
     * The default implementation should usually be overridden.  The
     * spliterator returned by the default implementation has poor splitting
     * capabilities, is unsized, and does not report any spliterator
     * characteristics. Implementing classes can nearly always provide a
     * better implementation.
     *
     * @return a {@code Spliterator} over the elements described by this
     * {@code Iterable}.
     * @since 1.8
     */
    default Spliterator<T> spliterator() {
        return Spliterators.spliteratorUnknownSize(iterator(), 0);
    }
}

并在内部实现Iterator接口

/*
 * Copyright (c) 1997, 2018, Oracle and/or its affiliates. All rights reserved.
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 */

package java.util;

import java.util.function.Consumer;

/**
 * An iterator over a collection.  {@code Iterator} takes the place of
 * {@link 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}/java.base/java/util/package-summary.html#CollectionsFramework">
 * Java Collections Framework</a>.
 *
 * @apiNote
 * An {@link Enumeration} can be converted into an {@code Iterator} by
 * using the {@link Enumeration#asIterator} method.
 *
 * @param <E> the type of elements returned by this iterator
 *
 * @author  Josh Bloch
 * @see Collection
 * @see ListIterator
 * @see Iterable
 * @since 1.2
 */
public interface Iterator<E> {
    /**
     * Returns {@code true} if the iteration has more elements.
     * (In other words, returns {@code true} if {@link #next} would
     * return an element rather than throwing an exception.)
     *
     * @return {@code true} if the iteration has more elements
     */
    boolean hasNext();

    /**
     * Returns the next element in the iteration.
     *
     * @return the next element in the iteration
     * @throws NoSuchElementException if the iteration has no more elements
     */
    E next();

    /**
     * Removes from the underlying collection the last element returned
     * by this iterator (optional operation).  This method can be called
     * only once per call to {@link #next}.
     * <p>
     * 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, unless an overriding class has specified a
     * concurrent modification policy.
     * <p>
     * The behavior of an iterator is unspecified if this method is called
     * after a call to the {@link #forEachRemaining forEachRemaining} method.
     *
     * @implSpec
     * The default implementation throws an instance of
     * {@link UnsupportedOperationException} and performs no other action.
     *
     * @throws UnsupportedOperationException if the {@code remove}
     *         operation is not supported by this iterator
     *
     * @throws IllegalStateException if the {@code next} method has not
     *         yet been called, or the {@code remove} method has already
     *         been called after the last call to the {@code next}
     *         method
     */
    default void remove() {
        throw new UnsupportedOperationException("remove");
    }

    /**
     * Performs the given action for each remaining element until all elements
     * have been processed or the action throws an exception.  Actions are
     * performed in the order of iteration, if that order is specified.
     * Exceptions thrown by the action are relayed to the caller.
     * <p>
     * The behavior of an iterator is unspecified if the action modifies the
     * collection in any way (even by calling the {@link #remove remove} method
     * or other mutator methods of {@code Iterator} subtypes),
     * unless an overriding class has specified a concurrent modification policy.
     * <p>
     * Subsequent behavior of an iterator is unspecified if the action throws an
     * exception.
     *
     * @implSpec
     * <p>The default implementation behaves as if:
     * <pre>{@code
     *     while (hasNext())
     *         action.accept(next());
     * }</pre>
     *
     * @param action The action to be performed for each element
     * @throws NullPointerException if the specified action is null
     * @since 1.8
     */
    default void forEachRemaining(Consumer<? super E> action) {
        Objects.requireNonNull(action);
        while (hasNext())
            action.accept(next());
    }
}

部分List 的继承关系图可以清晰的看到

具体的实战案例，将对User对象遍历内部的属性进行遍历。在Itreator接口中实现 hasNext方法判断是否有一个元素。用next方法获取下一个元素。

package org.example;


import java.util.Iterator;
import java.util.NoSuchElementException;

public class User implements Iterable<String> {


    @Override
    public String toString() {
        return "User{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }

    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    private String name;

    private Integer age;

    @Override
    public Iterator<String> iterator() {
        return new UserIterator();
    }

    class UserIterator implements Iterator<String> {
        public int count = 2 ;
        @Override
        public boolean hasNext() {
            return count > 0;
        }

        //拿到下一个元素
        @Override
        public String next() {
            count--;
           if(count == 1) {
               return User.this.name;
           }
           if(count == 0) {
               return User.this.age.toString();
           }

           throw new NoSuchElementException();

        }
    }
}

2. 使用迭代器模式 编写方法实现对demo.user文件的添加。（特别使用于demo.user 文件较大的情况下）

核心代码逻辑，使用 IO流读取文件并进行文件的分割操作，并实现将属性封装成java的对象。实现对文件。UserFile作为一个可迭代的对象。里面实现一个内部类继承迭代器类，实现 迭代器的具体方法

package org.example;

import java.io.File;
import java.nio.file.Files;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.stream.Collectors;

public class UserFile implements Iterable<User> {

    private final File file;

    @Override
    public Iterator<User> iterator() {
        return new UserIterator();
    }

   public UserFile(File file) {
         this.file = file;
    }

    class UserIterator implements Iterator<User> {

       List<User> userList = loadUserFromFile( );

       int cursor = 0;

       private List<User> loadUserFromFile( ) {
           try {
               // 组装 user 对象
             return Files.readAllLines(file.toPath()).stream().map(line->{
                   String midStr = line.substring(1,line.length()-1);
                   String[] split = midStr.split(",");
                   return new User(split[0], Integer.parseInt(split[1]));
               }).collect(Collectors.toList()) ;

           }catch (Exception e) {
               e.printStackTrace();
               return null;
           }
       }
         //   判断 下一个元素是否存在
        @Override
        public boolean hasNext() {
            return cursor!=userList.size();
        }
        // 返回下一个元素 
        @Override
        public User next() {
   
           if(cursor >= userList.size()) {
              throw new NoSuchElementException();
           }

            return userList.get(cursor ++);
        }
    }
}

执行结果: 可以看到具体的代码实现结果