迭代器模式（學習筆記）

　　1. 意圖

　　提供一種方法順序訪問一個聚合對象中的各個元素，而又不須要暴露該對象內部表示

　　2. 動機

　　有時候，會使用不一樣的算法來遍歷集合中的元素，不斷地向集合中添加遍歷算法會模糊其高效存儲數據的主要職責。此外，有些算法多是根據特定應用訂製的，將其加入到泛型集合類中會顯得很是奇怪。另外一方面，使用多種集合的客戶端代碼可能並不關心存儲數據的方式。不過，因爲集合提供不一樣的元素訪問方式，代碼將不得不與特定的集合類耦合。迭代器模式的主要思想是將集合的遍歷行爲抽取爲單獨的迭代器對象。除實現自身算法外， 迭代器還封裝了遍歷操做的全部細節， 例如當前位置和末尾剩餘元素的數量。 所以， 多個迭代器能夠在相互獨立的狀況下同時訪問集合。迭代器一般會提供一個獲取集合元素的基本方法。 客戶端可不斷調用該方法直至它不返回任何內容， 這意味着迭代器已經遍歷了全部元素。全部迭代器必須實現相同的接口。 這樣一來， 只要有合適的迭代器， 客戶端代碼就能兼容任何類型的集合或遍歷算法。 若是你須要採用特殊方式來遍歷集合， 只需建立一個新的迭代器類便可， 無需對集合或客戶端進行修改。

　　3. 適用性

• 訪問一個聚合對象的內容而無須暴露它的內部表示
• 支持對聚合對象的多種遍歷
• 爲遍歷不一樣的聚合結構提供一個統一的接口（即支持多態迭代）

　　4. 結構

  

 

　　5. 效果

　　1）單一職責原則。經過將體積龐大的遍歷算法代碼抽取爲獨立的類，可對客戶端代碼和集合進行整理

　　2） 開閉原則。 能夠實現新型的集合和迭代器並將其傳遞給現有代碼， 無需修改現有代碼

　　3）能夠並行遍歷同一集合， 由於每一個迭代器對象都包含其自身的遍歷狀態

　　4）若是你的程序只與簡單的集合進行交互， 應用該模式可能會矯枉過正

　　6. 代碼實現

　　iterators/ProfileIterator.java: 定義檔案接口

package iterator.iterators;

import iterator.profile.Profile;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:24
 */
public interface ProfileIterator {
    boolean hasNext();

    Profile getNext();

    void reset();
}

　　iterators/FacebookIterator.java: 在 Facebook 檔案上實現迭代

package iterator.iterators;

import iterator.profile.Profile;
import iterator.social_networks.Facebook;

import java.util.ArrayList;
import java.util.List;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:23
 */
public class FacebookIterator implements ProfileIterator{
    private Facebook facebook;
    private String type;
    private String email;
    private int currentPosition = 0;
    private List<String> emails = new ArrayList<>();
    private List<Profile> profiles = new ArrayList<>();

    public FacebookIterator(Facebook facebook, String type, String email) {
        this.facebook = facebook;
        this.type = type;
        this.email = email;
    }

    private void lazyLoad() {
        if (emails.size() == 0) {
            List<String> profiles = facebook.requestProfileFriendsFromFacebook(this.email, this.type);
            for (String profile : profiles) {
                this.emails.add(profile);
                this.profiles.add(null);
            }
        }
    }

    @Override
    public boolean hasNext() {
        lazyLoad();
        return currentPosition < emails.size();
    }

    @Override
    public Profile getNext() {
        if (!hasNext()) {
            return null;
        }

        String friendEmail = emails.get(currentPosition);
        Profile friendProfile = profiles.get(currentPosition);
        if (friendProfile == null) {
            friendProfile = facebook.requestProfileFromFacebook(friendEmail);
            profiles.set(currentPosition, friendProfile);
        }
        currentPosition++;
        return friendProfile;
    }

    @Override
    public void reset() {
        currentPosition = 0;
    }
}

　　iterators/LinkedInIterator.java: 在領英檔案上實現迭代

package iterator.iterators;

import iterator.profile.Profile;
import iterator.social_networks.LinkedIn;

import java.util.ArrayList;
import java.util.List;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:24
 */
public class LinkedInIterator implements ProfileIterator{
    private LinkedIn linkedIn;
    private String type;
    private String email;
    private int currentPosition = 0;
    private List<String> emails = new ArrayList<>();
    private List<Profile> contacts = new ArrayList<>();

    public LinkedInIterator(LinkedIn linkedIn, String type, String email) {
        this.linkedIn = linkedIn;
        this.type = type;
        this.email = email;
    }

    private void lazyLoad() {
        if (emails.size() == 0) {
            List<String> profiles = linkedIn.requestRelatedContactsFromLinkedInAPI(this.email, this.type);
            for (String profile : profiles) {
                this.emails.add(profile);
                this.contacts.add(null);
            }
        }
    }

    @Override
    public boolean hasNext() {
        lazyLoad();
        return currentPosition < emails.size();
    }

    @Override
    public Profile getNext() {
        if (!hasNext()) {
            return null;
        }

        String friendEmail = emails.get(currentPosition);
        Profile friendContact = contacts.get(currentPosition);
        if (friendContact == null) {
            friendContact = linkedIn.requestContactInfoFromLinkedInAPI(friendEmail);
            contacts.set(currentPosition, friendContact);
        }
        currentPosition++;
        return friendContact;
    }

    @Override
    public void reset() {
        currentPosition = 0;
    }
}

　　social_networks/SocialNetwork.java: 定義通用的社交網絡接口

package iterator.social_networks;

import iterator.iterators.ProfileIterator;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:31
 */
public interface SocialNetwork {
    ProfileIterator createFriendsIterator(String profileEmail);

    ProfileIterator createCoworkersIterator(String profileEmail);
}

　　social_networks/Facebook.java: Facebook

package iterator.social_networks;

import iterator.iterators.FacebookIterator;
import iterator.iterators.ProfileIterator;
import iterator.profile.Profile;

import java.util.ArrayList;
import java.util.List;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:30
 */
public class Facebook implements SocialNetwork{
    private List<Profile> profiles;

    public Facebook(List<Profile> cache) {
        if (cache != null) {
            this.profiles = cache;
        } else {
            this.profiles = new ArrayList<>();
        }
    }

    public Profile requestProfileFromFacebook(String profileEmail) {
        // Here would be a POST request to one of the Facebook API endpoints.
        // Instead, we emulates long network connection, which you would expect
        // in the real life...
        simulateNetworkLatency();
        System.out.println("Facebook: Loading profile '" + profileEmail + "' over the network...");

        // ...and return test data.
        return findProfile(profileEmail);
    }

    public List<String> requestProfileFriendsFromFacebook(String profileEmail, String contactType) {
        // Here would be a POST request to one of the Facebook API endpoints.
        // Instead, we emulates long network connection, which you would expect
        // in the real life...
        simulateNetworkLatency();
        System.out.println("Facebook: Loading '" + contactType + "' list of '" + profileEmail + "' over the network...");

        // ...and return test data.
        Profile profile = findProfile(profileEmail);
        if (profile != null) {
            return profile.getContacts(contactType);
        }
        return null;
    }

    private Profile findProfile(String profileEmail) {
        for (Profile profile : profiles) {
            if (profile.getEmail().equals(profileEmail)) {
                return profile;
            }
        }
        return null;
    }

    private void simulateNetworkLatency() {
        try {
            Thread.sleep(2500);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
    }

    @Override
    public ProfileIterator createFriendsIterator(String profileEmail) {
        return new FacebookIterator(this, "friends", profileEmail);
    }

    @Override
    public ProfileIterator createCoworkersIterator(String profileEmail) {
        return new FacebookIterator(this, "coworkers", profileEmail);
    }

}

　　social_networks/LinkedIn.java: 領英

package iterator.social_networks;

import iterator.iterators.LinkedInIterator;
import iterator.iterators.ProfileIterator;
import iterator.profile.Profile;

import java.util.ArrayList;
import java.util.List;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:32
 */
public class LinkedIn implements SocialNetwork{
    private List<Profile> contacts;

    public LinkedIn(List<Profile> cache) {
        if (cache != null) {
            this.contacts = cache;
        } else {
            this.contacts = new ArrayList<>();
        }
    }

    public Profile requestContactInfoFromLinkedInAPI(String profileEmail) {
        // Here would be a POST request to one of the LinkedIn API endpoints.
        // Instead, we emulates long network connection, which you would expect
        // in the real life...
        simulateNetworkLatency();
        System.out.println("LinkedIn: Loading profile '" + profileEmail + "' over the network...");

        // ...and return test data.
        return findContact(profileEmail);
    }

    public List<String> requestRelatedContactsFromLinkedInAPI(String profileEmail, String contactType) {
        // Here would be a POST request to one of the LinkedIn API endpoints.
        // Instead, we emulates long network connection, which you would expect
        // in the real life.
        simulateNetworkLatency();
        System.out.println("LinkedIn: Loading '" + contactType + "' list of '" + profileEmail + "' over the network...");

        // ...and return test data.
        Profile profile = findContact(profileEmail);
        if (profile != null) {
            return profile.getContacts(contactType);
        }
        return null;
    }

    private Profile findContact(String profileEmail) {
        for (Profile profile : contacts) {
            if (profile.getEmail().equals(profileEmail)) {
                return profile;
            }
        }
        return null;
    }

    private void simulateNetworkLatency() {
        try {
            Thread.sleep(2500);
        } catch (InterruptedException ex) {
            ex.printStackTrace();
        }
    }

    @Override
    public ProfileIterator createFriendsIterator(String profileEmail) {
        return new LinkedInIterator(this, "friends", profileEmail);
    }

    @Override
    public ProfileIterator createCoworkersIterator(String profileEmail) {
        return new LinkedInIterator(this, "coworkers", profileEmail);
    }
}

　　profile/Profile.java: 社交檔案

package iterator.profile;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:25
 */
public class Profile {
    private String name;
    private String email;
    private Map<String, List<String>> contacts = new HashMap<>();

    public Profile(String email, String name, String... contacts) {
        this.email = email;
        this.name = name;

        // Parse contact list from a set of "friend:email@gmail.com" pairs.
        for (String contact : contacts) {
            String[] parts = contact.split(":");
            String contactType = "friend", contactEmail;
            if (parts.length == 1) {
                contactEmail = parts[0];
            }
            else {
                contactType = parts[0];
                contactEmail = parts[1];
            }
            if (!this.contacts.containsKey(contactType)) {
                this.contacts.put(contactType, new ArrayList<>());
            }
            this.contacts.get(contactType).add(contactEmail);
        }
    }

    public String getEmail() {
        return email;
    }

    public String getName() {
        return name;
    }

    public List<String> getContacts(String contactType) {
        if (!this.contacts.containsKey(contactType)) {
            this.contacts.put(contactType, new ArrayList<>());
        }
        return contacts.get(contactType);
    }
}

　　spammer/SocialSpammer.java: 消息發送應用

package iterator.spammer;

import iterator.iterators.ProfileIterator;
import iterator.profile.Profile;
import iterator.social_networks.SocialNetwork;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:33
 */
public class SocialSpammer {
    public SocialNetwork network;
    public ProfileIterator iterator;

    public SocialSpammer(SocialNetwork network) {
        this.network = network;
    }

    public void sendSpamToFriends(String profileEmail, String message) {
        System.out.println("\nIterating over friends...\n");
        iterator = network.createFriendsIterator(profileEmail);
        while (iterator.hasNext()) {
            Profile profile = iterator.getNext();
            sendMessage(profile.getEmail(), message);
        }
    }

    public void sendSpamToCoworkers(String profileEmail, String message) {
        System.out.println("\nIterating over coworkers...\n");
        iterator = network.createCoworkersIterator(profileEmail);
        while (iterator.hasNext()) {
            Profile profile = iterator.getNext();
            sendMessage(profile.getEmail(), message);
        }
    }

    public void sendMessage(String email, String message) {
        System.out.println("Sent message to: '" + email + "'. Message body: '" + message + "'");
    }
}

　　Demo.java: 客戶端代碼

package iterator;

import iterator.profile.Profile;
import iterator.social_networks.Facebook;
import iterator.social_networks.LinkedIn;
import iterator.social_networks.SocialNetwork;
import iterator.spammer.SocialSpammer;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

/**
 * @author GaoMing
 * @date 2021/7/19 - 19:34
 */
public class Demo {
    public static Scanner scanner = new Scanner(System.in);

    public static void main(String[] args) {
        System.out.println("Please specify social network to target spam tool (default:Facebook):");
        System.out.println("1. Facebook");
        System.out.println("2. LinkedIn");
        String choice = scanner.nextLine();

        SocialNetwork network;
        if (choice.equals("2")) {
            network = new LinkedIn(createTestProfiles());
        }
        else {
            network = new Facebook(createTestProfiles());
        }

        SocialSpammer spammer = new SocialSpammer(network);
        spammer.sendSpamToFriends("anna.smith@bing.com",
                "Hey! This is Anna's friend Josh. Can you do me a favor and like this post [link]?");
        spammer.sendSpamToCoworkers("anna.smith@bing.com",
                "Hey! This is Anna's boss Jason. Anna told me you would be interested in [link].");
    }

    public static List<Profile> createTestProfiles() {
        List<Profile> data = new ArrayList<Profile>();
        data.add(new Profile("anna.smith@bing.com", "Anna Smith", "friends:mad_max@ya.com", "friends:catwoman@yahoo.com", "coworkers:sam@amazon.com"));
        data.add(new Profile("mad_max@ya.com", "Maximilian", "friends:anna.smith@bing.com", "coworkers:sam@amazon.com"));
        data.add(new Profile("bill@microsoft.eu", "Billie", "coworkers:avanger@ukr.net"));
        data.add(new Profile("avanger@ukr.net", "John Day", "coworkers:bill@microsoft.eu"));
        data.add(new Profile("sam@amazon.com", "Sam Kitting", "coworkers:anna.smith@bing.com", "coworkers:mad_max@ya.com", "friends:catwoman@yahoo.com"));
        data.add(new Profile("catwoman@yahoo.com", "Liza", "friends:anna.smith@bing.com", "friends:sam@amazon.com"));
        return data;
    }
}

　　執行結果

Please specify social network to target spam tool (default:Facebook):
1. Facebook
2. LinkedIn
> 1

Iterating over friends...

Facebook: Loading 'friends' list of 'anna.smith@bing.com' over the network...
Facebook: Loading profile 'mad_max@ya.com' over the network...
Sent message to: 'mad_max@ya.com'. Message body: 'Hey! This is Anna's friend Josh. Can you do me a favor and like this post [link]?'
Facebook: Loading profile 'catwoman@yahoo.com' over the network...
Sent message to: 'catwoman@yahoo.com'. Message body: 'Hey! This is Anna's friend Josh. Can you do me a favor and like this post [link]?'

Iterating over coworkers...

Facebook: Loading 'coworkers' list of 'anna.smith@bing.com' over the network...
Facebook: Loading profile 'sam@amazon.com' over the network...
Sent message to: 'sam@amazon.com'. Message body: 'Hey! This is Anna's boss Jason. Anna told me you would be interested in [link].'

 

　　7. 與其餘模式的關係

• 可使用迭代器模式來遍歷組合模式樹
• 能夠同時使用工廠方法模式和迭代器來讓子類集合返回不一樣類型的迭代器， 並使得迭代器與集合相匹配
• 能夠同時使用備忘錄模式和迭代器來獲取當前迭代器的狀態， 而且在須要的時候進行回滾
• 能夠同時使用訪問者模式和迭代器來遍歷複雜數據結構，並對其中的元素執行所需操做，即便這些元素所屬的類徹底不一樣

　　8. 已知應用　　

• java.util.Iterator的全部實現 （還有 java.util.Scanner）
• java.util.Enumeration的全部實現

　　識別方法：迭代器能夠經過導航方法（例如 next和 previous等）來輕鬆識別。使用迭代器的客戶端代碼可能沒有其所遍歷的集合的直接訪問權限