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docs: iluwatar#590 refactor and add explanation for business delegate (iluwatar#1686)
Type: docs and refactoring Co-authored-by: Subhrodip Mohanta <hello@subho.xyz>
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‎business-delegate/README.md

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---
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## Intent
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The Business Delegate pattern adds an abstraction layer between
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presentation and business tiers. By using the pattern we gain loose coupling
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between the tiers and encapsulate knowledge about how to locate, connect to,
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and interact with the business objects that make up the application.
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## Explanation
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Real world example
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> A mobile phone application promises to stream any movie in existence to your phone. It captures
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> the user's search string and passes this on to the business delegate. The business delegate
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> selects the most suitable video streaming service and plays the video from there.
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In Plain Words
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> Business delegate adds an abstraction layer between the presentation and business tiers.
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Wikipedia says
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> Business delegate is a Java EE design pattern. This pattern is directing to reduce the coupling
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> in between business services and the connected presentation tier, and to hide the implementation
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> details of services (including lookup and accessibility of EJB architecture). Business delegates
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> acts as an adaptor to invoke business objects from the presentation tier.
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**Programmatic Example**
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First, we have an abstraction for video streaming services and a couple of implementations.
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```java
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public interface VideoStreamingService {
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void doProcessing();
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}
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@Slf4j
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public class NetflixService implements VideoStreamingService {
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@Override
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public void doProcessing() {
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LOGGER.info("NetflixService is now processing");
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}
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}
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@Slf4j
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public class YouTubeService implements VideoStreamingService {
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@Override
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public void doProcessing() {
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LOGGER.info("YouTubeService is now processing");
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}
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}
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```
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Then we have a lookup service that decides which video streaming service is used.
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```java
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@Setter
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public class BusinessLookup {
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private NetflixService netflixService;
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private YouTubeService youTubeService;
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public VideoStreamingService getBusinessService(String movie) {
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if (movie.toLowerCase(Locale.ROOT).contains("die hard")) {
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return netflixService;
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} else {
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return youTubeService;
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}
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}
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}
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```
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The business delegate uses a business lookup to route movie playback requests to a suitable
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video streaming service.
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```java
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@Setter
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public class BusinessDelegate {
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private BusinessLookup lookupService;
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public void playbackMovie(String movie) {
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VideoStreamingService videoStreamingService = lookupService.getBusinessService(movie);
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videoStreamingService.doProcessing();
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}
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}
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```
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The mobile client utilizes business delegate to call the business tier.
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```java
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public class MobileClient {
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private final BusinessDelegate businessDelegate;
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public MobileClient(BusinessDelegate businessDelegate) {
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this.businessDelegate = businessDelegate;
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}
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public void playbackMovie(String movie) {
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businessDelegate.playbackMovie(movie);
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}
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}
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```
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Finally, we can show the full example in action.
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```java
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public static void main(String[] args) {
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// prepare the objects
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var businessDelegate = new BusinessDelegate();
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var businessLookup = new BusinessLookup();
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businessLookup.setNetflixService(new NetflixService());
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businessLookup.setYouTubeService(new YouTubeService());
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businessDelegate.setLookupService(businessLookup);
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// create the client and use the business delegate
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var client = new MobileClient(businessDelegate);
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client.playbackMovie("Die Hard 2");
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client.playbackMovie("Maradona: The Greatest Ever");
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}
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```
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Here is the console output.
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```
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21:15:33.790 [main] INFO com.iluwatar.business.delegate.NetflixService - NetflixService is now processing
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21:15:33.794 [main] INFO com.iluwatar.business.delegate.YouTubeService - YouTubeService is now processing
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```
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## Class diagram
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![alt text](./etc/business-delegate.png "Business Delegate")
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![alt text](./etc/business-delegate.urm.png "Business Delegate")
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## Related patterns
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* [Service locator pattern](https://java-design-patterns.com/patterns/service-locator/)
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## Applicability
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Use the Business Delegate pattern when
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* you want loose coupling between presentation and business tiers
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* you want to orchestrate calls to multiple business services
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* you want to encapsulate service lookups and service calls
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* You want loose coupling between presentation and business tiers
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* You want to orchestrate calls to multiple business services
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* You want to encapsulate service lookups and service calls
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## Tutorials
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* [Business Delegate Pattern at TutorialsPoint](https://www.tutorialspoint.com/design_pattern/business_delegate_pattern.htm)
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## Credits
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* [J2EE Design Patterns](https://www.amazon.com/gp/product/0596004273/ref=as_li_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596004273&linkCode=as2&tag=javadesignpat-20&linkId=48d37c67fb3d845b802fa9b619ad8f31)
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* [Core J2EE Patterns: Best Practices and Design Strategies](https://www.amazon.com/gp/product/0130648841/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=0130648841&linkId=a0100de2b28c71ede8db1757fb2b5947)

‎business-delegate/etc/business-delegate.png

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‎business-delegate/etc/business-delegate.ucls

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‎business-delegate/etc/business-delegate.urm.png

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‎business-delegate/etc/business-delegate.urm.puml

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@@ -5,53 +5,42 @@ package com.iluwatar.business.delegate {
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+ main(args : String[]) {static}
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}
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class BusinessDelegate {
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- businessService : BusinessService
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- lookupService : BusinessLookup
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- serviceType : ServiceType
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+ BusinessDelegate()
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+ doTask()
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+ setLookupService(businessLookup : BusinessLookup)
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+ setServiceType(serviceType : ServiceType)
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+ playbackMovie(movie : String)
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+ setLookupService(lookupService : BusinessLookup)
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}
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class BusinessLookup {
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- ejbService : EjbService
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- jmsService : JmsService
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- netflixService : NetflixService
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- youTubeService : YouTubeService
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+ BusinessLookup()
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+ getBusinessService(serviceType : ServiceType) : BusinessService
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+ setEjbService(ejbService : EjbService)
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+ setJmsService(jmsService : JmsService)
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+ getBusinessService(movie : String) : VideoStreamingService
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+ setNetflixService(netflixService : NetflixService)
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+ setYouTubeService(youTubeService : YouTubeService)
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}
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interface BusinessService {
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+ doProcessing() {abstract}
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}
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class Client {
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class MobileClient {
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- businessDelegate : BusinessDelegate
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+ Client(businessDelegate : BusinessDelegate)
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+ doTask()
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+ MobileClient(businessDelegate : BusinessDelegate)
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+ playbackMovie(movie : String)
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}
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class EjbService {
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class NetflixService {
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- LOGGER : Logger {static}
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+ EjbService()
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+ NetflixService()
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+ doProcessing()
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}
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class JmsService {
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interface VideoStreamingService {
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+ doProcessing() {abstract}
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}
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class YouTubeService {
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- LOGGER : Logger {static}
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+ JmsService()
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+ YouTubeService()
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+ doProcessing()
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}
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enum ServiceType {
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+ EJB {static}
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+ JMS {static}
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+ valueOf(name : String) : ServiceType {static}
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+ values() : ServiceType[] {static}
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}
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}
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BusinessLookup --> "-ejbService" EjbService
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BusinessDelegate --> "-serviceType" ServiceType
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Client --> "-businessDelegate" BusinessDelegate
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BusinessDelegate --> "-businessService" BusinessService
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BusinessLookup --> "-netflixService" NetflixService
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BusinessLookup --> "-youTubeService" YouTubeService
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MobileClient --> "-businessDelegate" BusinessDelegate
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BusinessDelegate --> "-lookupService" BusinessLookup
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BusinessLookup --> "-jmsService" JmsService
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EjbService ..|> BusinessService
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JmsService ..|> BusinessService
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NetflixService ..|> VideoStreamingService
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YouTubeService ..|> VideoStreamingService
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@enduml

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