Iterator w języku Java

Iterator to behawioralny wzorzec projektowy pozwalający sekwencyjnie przechodzić od elementu do elementu jakiegoś zbioru bez konieczności eksponowania jego formy.

Dzięki Iteratorowi klienci mogą przeglądać kolejne elementy różnych kolekcji w podobny sposób, za pośrednictwem jednego interfejsu.

Dowiedz się więcej o Iterator

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Popularność:

Przykłady użycia: Wzorzec Iterator jest bardzo rozpowszechniony w kodzie Java. Wiele frameworków i bibliotek pozwala za jego pomocą poruszać się po elementach ich kolekcji.

Oto kilka przykładów użycia wzorca w głównych bibliotekach Java:

Wszystkie implementacje java.util.Iterator (oraz java.util.Scanner).
Wszystkie implementacje java.util.Enumeration.

Identyfikacja: Iterator łatwo rozpoznać po obecności metod nawigacyjnych (takich jak następny, poprzedni i innych). Kod klienta stosujący iteratory może nie mieć bezpośredniego dostępu do badanej kolekcji.

Przechodzenie po kolejnych profilach użytkowników platformy społecznościowej

W poniższym przykładzie wzorzec Iterator wykorzystano do przechodzenia po kolejnych profilach użytkowników zdalnej platformy społecznościowej bez ujawniania szczegółów komunikacji kodowi klienta.

iterators

iterators/ProfileIterator.java: Definiuje interfejs profilu użytkownika

package refactoring_guru.iterator.example.iterators;

import refactoring_guru.iterator.example.profile.Profile;

public interface ProfileIterator {
    boolean hasNext();

    Profile getNext();

    void reset();
}

iterators/FacebookIterator.java: Implementuje przeglądanie profili Facebook

package refactoring_guru.iterator.example.iterators;

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

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

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: Implementuje przeglądanie profili LinkedIn

package refactoring_guru.iterator.example.iterators;

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

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

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

social_networks/SocialNetwork.java: Definiuje wspólny interfejs platform społecznościowych

package refactoring_guru.iterator.example.social_networks;

import refactoring_guru.iterator.example.iterators.ProfileIterator;

public interface SocialNetwork {
    ProfileIterator createFriendsIterator(String profileEmail);

    ProfileIterator createCoworkersIterator(String profileEmail);
}

social_networks/Facebook.java: Facebook

package refactoring_guru.iterator.example.social_networks;

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

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

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: LinkedIn

package refactoring_guru.iterator.example.social_networks;

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

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

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/Profile.java: Profile użytkowników

package refactoring_guru.iterator.example.profile;

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

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

spammer/SocialSpammer.java: Aplikacja wysyłająca wiadomości

package refactoring_guru.iterator.example.spammer;

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

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: Kod klienta

package refactoring_guru.iterator.example;

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

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

/**
 * Demo class. Everything comes together here.
 */
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;
    }
}

OutputDemo.txt: Wynik działania

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].'