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  • Circular reference with entity manager factory

    - by CodesLikeA_Mokey
    Every time I try and start my app, I get this error on startup: SEVERE: Exception sending context initialized event to listener instance of class org.springframework.web.context.ContextLoaderListener org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor#0': Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'identityAccessAppConfig': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: com.package.identityaccess.identity.UserRepository com.package.identityaccess.IdentityAccessAppConfig.userRepository; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userRepository': Cannot create inner bean '(inner bean)' of type [org.springframework.orm.jpa.SharedEntityManagerCreator] while setting bean property 'entityManager'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name '(inner bean)#1': Cannot resolve reference to bean 'identityAccessEntityManagerFactory' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'identityAccessEntityManagerFactory': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:529) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:198) at org.springframework.context.support.AbstractApplicationContext.registerBeanPostProcessors(AbstractApplicationContext.java:741) at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:464) at org.springframework.web.context.ContextLoader.configureAndRefreshWebApplicationContext(ContextLoader.java:389) at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:294) at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:112) at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4797) at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5291) at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559) at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549) at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334) at java.util.concurrent.FutureTask.run(FutureTask.java:166) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) at java.lang.Thread.run(Thread.java:722) Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'identityAccessAppConfig': Injection of autowired dependencies failed; nested exception is org.springframework.beans.factory.BeanCreationException: Could not autowire field: com.package.identityaccess.identity.UserRepository com.package.identityaccess.IdentityAccessAppConfig.userRepository; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userRepository': Cannot create inner bean '(inner bean)' of type [org.springframework.orm.jpa.SharedEntityManagerCreator] while setting bean property 'entityManager'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name '(inner bean)#1': Cannot resolve reference to bean 'identityAccessEntityManagerFactory' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'identityAccessEntityManagerFactory': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:288) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1116) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:519) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194) at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:353) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1025) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:921) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:487) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:198) at org.springframework.beans.factory.support.DefaultListableBeanFactory.getBeansOfType(DefaultListableBeanFactory.java:438) at org.springframework.beans.factory.BeanFactoryUtils.beansOfTypeIncludingAncestors(BeanFactoryUtils.java:277) at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.detectPersistenceExceptionTranslators(PersistenceExceptionTranslationInterceptor.java:139) at org.springframework.dao.support.PersistenceExceptionTranslationInterceptor.<init>(PersistenceExceptionTranslationInterceptor.java:79) at org.springframework.dao.annotation.PersistenceExceptionTranslationAdvisor.<init>(PersistenceExceptionTranslationAdvisor.java:71) at org.springframework.dao.annotation.PersistenceExceptionTranslationPostProcessor.setBeanFactory(PersistenceExceptionTranslationPostProcessor.java:85) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeAwareMethods(AbstractAutowireCapableBeanFactory.java:1502) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1470) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:521) ... 20 more Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: com.package.identityaccess.identity.UserRepository com.package.identityaccess.IdentityAccessAppConfig.userRepository; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userRepository': Cannot create inner bean '(inner bean)' of type [org.springframework.orm.jpa.SharedEntityManagerCreator] while setting bean property 'entityManager'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name '(inner bean)#1': Cannot resolve reference to bean 'identityAccessEntityManagerFactory' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'identityAccessEntityManagerFactory': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:514) at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:87) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:285) ... 45 more Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'userRepository': Cannot create inner bean '(inner bean)' of type [org.springframework.orm.jpa.SharedEntityManagerCreator] while setting bean property 'entityManager'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name '(inner bean)#1': Cannot resolve reference to bean 'identityAccessEntityManagerFactory' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'identityAccessEntityManagerFactory': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveInnerBean(BeanDefinitionValueResolver.java:282) at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:126) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1387) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1128) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:519) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:295) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:223) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194) at org.springframework.beans.factory.support.DefaultListableBeanFactory.findAutowireCandidates(DefaultListableBeanFactory.java:912) at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:855) at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:770) at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:486) ... 47 more Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name '(inner bean)#1': Cannot resolve reference to bean 'identityAccessEntityManagerFactory' while setting constructor argument; nested exception is org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'identityAccessEntityManagerFactory': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:329) at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveValueIfNecessary(BeanDefinitionValueResolver.java:107) at org.springframework.beans.factory.support.ConstructorResolver.resolveConstructorArguments(ConstructorResolver.java:615) at org.springframework.beans.factory.support.ConstructorResolver.instantiateUsingFactoryMethod(ConstructorResolver.java:441) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.instantiateUsingFactoryMethod(AbstractAutowireCapableBeanFactory.java:1025) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:921) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:487) at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:458) at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveInnerBean(BeanDefinitionValueResolver.java:271) ... 60 more Caused by: org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 'identityAccessEntityManagerFactory': Requested bean is currently in creation: Is there an unresolvable circular reference? at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.beforeSingletonCreation(DefaultSingletonBeanRegistry.java:327) at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:217) at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:292) at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194) at org.springframework.beans.factory.support.BeanDefinitionValueResolver.resolveReference(BeanDefinitionValueResolver.java:323) I am new to configuring spring through java and have been spinning circles for days. I have my application which includes 2 separate modules. The identity-access module will handle user and access information and will have a datasource to an older database. The other module module2 will eventually have its own database. Here are the config files: application: @Configuration @EnableWebMvc @ComponentScan(basePackages = "com.package.myapp.web") @ImportResource({"classpath:com/package/appbase/appbase-context.xml"}) @Import(com.package.identityaccess.IdentityAccessAppConfig.class) public class IMSAppConfig extends WebMvcConfigurerAdapter { @Override public void addResourceHandlers(ResourceHandlerRegistry registry) { registry.addResourceHandler("/resources/**").addResourceLocations("ims/resources/"); } } identity-access: @Configuration @ComponentScan(basePackages = "com.package.identityaccess") @EnableJpaRepositories(entityManagerFactoryRef = "identityAccessEntityManagerFactory", value = "com.package.identityaccess") @EnableTransactionManagement public class IdentityAccessAppConfig { @Autowired UserRepository userRepository; @Bean public DataSource identityAccessDataSource() throws IOException, SQLException { return new SelfConfiguringBasicDataSource(System.getProperty("db_properties_path") + "/dev.oracle.properties", ""); } @Bean public Map<String, Object> identityAccessJpaProperties() { Map<String, Object> props = new HashMap<>(); props.put("eclipselink.weaving", "false"); props.put("eclipselink.target-database", OraclePlatform.class.getName()); return props; } @Bean public JpaVendorAdapter identityAccessJpaVendorAdapter() { EclipseLinkJpaVendorAdapter eclipseLinkJpaVendorAdapter = new EclipseLinkJpaVendorAdapter(); eclipseLinkJpaVendorAdapter.setDatabasePlatform(OraclePlatform.class.getName()); eclipseLinkJpaVendorAdapter.setGenerateDdl(false); eclipseLinkJpaVendorAdapter.setShowSql(true); return eclipseLinkJpaVendorAdapter; } @Bean public PlatformTransactionManager identityAccessTransactionManager() throws IOException, SQLException { JpaTransactionManager txManager = new JpaTransactionManager(); txManager.setEntityManagerFactory(identityAccessEntityManagerFactory().getObject()); return txManager; } @Bean public LocalContainerEntityManagerFactoryBean identityAccessEntityManagerFactory() throws IOException, SQLException { LocalContainerEntityManagerFactoryBean lef = new LocalContainerEntityManagerFactoryBean(); lef.setDataSource(identityAccessDataSource()); lef.setJpaPropertyMap(identityAccessJpaProperties()); lef.setJpaVendorAdapter(identityAccessJpaVendorAdapter()); lef.setPackagesToScan("com.package.identityaccess"); return lef; } @Bean public AuthenticationService authenticationService() { return new AuthenticationServiceImpl(userRepository); } } module2: @Configuration @ComponentScan(basePackages = "com.package.myapp.core") @EnableJpaRepositories("com.package.myapp.core.domain") @EnableTransactionManagement public class ModuleTwoAppConfig { @Bean public DataSource dataSource() { EmbeddedDatabaseBuilder embeddedDatabaseBuilder = new EmbeddedDatabaseBuilder(); embeddedDatabaseBuilder.setType(EmbeddedDatabaseType.HSQL); embeddedDatabaseBuilder.addScript("setup.sql"); return embeddedDatabaseBuilder.build(); } @Bean public Map<String, Object> jpaProperties() { Map<String, Object> props = new HashMap<>(); props.put("eclipselink.weaving", "false"); props.put("eclipselink.ddl-generation", "create-tables"); props.put("eclipselink.target-database", HSQLPlatform.class.getName()); return props; } @Bean public JpaVendorAdapter jpaVendorAdapter() { EclipseLinkJpaVendorAdapter eclipseLinkJpaVendorAdapter = new EclipseLinkJpaVendorAdapter(); eclipseLinkJpaVendorAdapter.setDatabasePlatform(HSQLPlatform.class.getName()); eclipseLinkJpaVendorAdapter.setGenerateDdl(true); eclipseLinkJpaVendorAdapter.setShowSql(true); return eclipseLinkJpaVendorAdapter; } @Bean public PlatformTransactionManager transactionManager() { JpaTransactionManager txManager = new JpaTransactionManager(); txManager.setEntityManagerFactory(entityManagerFactory().getObject()); return txManager; } @Bean public LocalContainerEntityManagerFactoryBean entityManagerFactory() { LocalContainerEntityManagerFactoryBean lef = new LocalContainerEntityManagerFactoryBean(); lef.setDataSource(dataSource()); lef.setJpaPropertyMap(jpaProperties()); lef.setJpaVendorAdapter(jpaVendorAdapter()); lef.setPackagesToScan("com.package.myapp.core.domain"); return lef; } } UserRepository (uses spring-data): public interface UserRepository extends JpaRepository<User, Long> { @Query("select u from User u where u.user_id = ?1") User findByUserId(String userId); } Can you see any problems with what I've done?

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  • Factory, Abstract Factory and Factory Method

    - by skydoor
    Hi, I am really confused about these three terms. My understanding is that: in the Factory pattern, there is no concrete factory. The factory builds the new objects according to the parameters. in Abstract Factory pattern, there are multiple concrete factories. The client has to create different concrete factories explicitly. Is that right? What are the other differences? Furthermore, what is the Factory Method pattern? Is it same as the Factory pattern?

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  • The Template Method Design Pattern using C# .Net

    - by nijhawan.saurabh
    First of all I'll just put this pattern in context and describe its intent as in the GOF book:   Template Method: Define the skeleton of an algorithm in an operation, deferring some steps to Subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the Algorithm's Structure.    Usage: When you are certain about the High Level steps involved in an Algorithm/Work flow you can use the Template Pattern which allows the Base Class to define the Sequence of the Steps but permits the Sub classes to alter the implementation of any/all steps.   Example in the .Net framework: The most common example is the Asp.Net Page Life Cycle. The Page Life Cycle has a few methods which are called in a sequence but we have the liberty to modify the functionality of any of the methods by overriding them.   Sample implementation of Template Method Pattern:   Let's see the class diagram first:            Normal 0 false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin-top:0in; mso-para-margin-right:0in; mso-para-margin-bottom:8.0pt; mso-para-margin-left:0in; line-height:107%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-font-kerning:1.0pt; mso-ligatures:standard;}   And here goes the code:EmailBase.cs     1 using System;     2 using System.Collections.Generic;     3 using System.Linq;     4 using System.Text;     5 using System.Threading.Tasks;     6      7 namespace TemplateMethod     8 {     9     public abstract class EmailBase    10     {    11     12         public bool SendEmail()    13         {    14             if (CheckEmailAddress() == true) // Method1 in the sequence    15             {    16                 if (ValidateMessage() == true) // Method2 in the sequence    17                 {    18                     if (SendMail() == true) // Method3 in the sequence    19                     {    20                         return true;    21                     }    22                     else    23                     {    24                         return false;    25                     }    26     27                 }    28                 else    29                 {    30                     return false;    31                 }    32     33             }    34             else    35             {    36                 return false;    37     38             }    39     40     41         }    42     43         protected abstract bool CheckEmailAddress();    44         protected abstract bool ValidateMessage();    45         protected abstract bool SendMail();    46     47     48     }    49 }    50    EmailYahoo.cs      1 using System;     2 using System.Collections.Generic;     3 using System.Linq;     4 using System.Text;     5 using System.Threading.Tasks;     6      7 namespace TemplateMethod     8 {     9     public class EmailYahoo:EmailBase    10     {    11     12         protected override bool CheckEmailAddress()    13         {    14             Console.WriteLine("Checking Email Address : YahooEmail");    15             return true;    16         }    17         protected override bool ValidateMessage()    18         {    19             Console.WriteLine("Validating Email Message : YahooEmail");    20             return true;    21         }    22     23     24         protected override bool SendMail()    25         {    26             Console.WriteLine("Semding Email : YahooEmail");    27             return true;    28         }    29     30     31     }    32 }    33   EmailGoogle.cs      1 using System;     2 using System.Collections.Generic;     3 using System.Linq;     4 using System.Text;     5 using System.Threading.Tasks;     6      7 namespace TemplateMethod     8 {     9     public class EmailGoogle:EmailBase    10     {    11     12         protected override bool CheckEmailAddress()    13         {    14             Console.WriteLine("Checking Email Address : GoogleEmail");    15             return true;    16         }    17         protected override bool ValidateMessage()    18         {    19             Console.WriteLine("Validating Email Message : GoogleEmail");    20             return true;    21         }    22     23     24         protected override bool SendMail()    25         {    26             Console.WriteLine("Semding Email : GoogleEmail");    27             return true;    28         }    29     30     31     }    32 }    33   Program.cs      1 using System;     2 using System.Collections.Generic;     3 using System.Linq;     4 using System.Text;     5 using System.Threading.Tasks;     6      7 namespace TemplateMethod     8 {     9     class Program    10     {    11         static void Main(string[] args)    12         {    13             Console.WriteLine("Please choose an Email Account to send an Email:");    14             Console.WriteLine("Choose 1 for Google");    15             Console.WriteLine("Choose 2 for Yahoo");    16             string choice = Console.ReadLine();    17     18             if (choice == "1")    19             {    20                 EmailBase email = new EmailGoogle(); // Rather than newing it up here, you may use a factory to do so.    21                 email.SendEmail();    22     23             }    24             if (choice == "2")    25             {    26                 EmailBase email = new EmailYahoo(); // Rather than newing it up here, you may use a factory to do so.    27                 email.SendEmail();    28             }    29         }    30     }    31 }    32    Final Words: It's very obvious that why the Template Method Pattern is a popular pattern, everything at last revolves around Algorithms and if you are clear with the steps involved it makes real sense to delegate the duty of implementing the step's functionality to the sub classes. Normal 0 false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin-top:0in; mso-para-margin-right:0in; mso-para-margin-bottom:8.0pt; mso-para-margin-left:0in; line-height:107%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi; mso-font-kerning:1.0pt; mso-ligatures:standard;}

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  • Switching from abstract class to interface

    - by nischayn22
    I have an abstract class which has all abstract methods except one which constructs objects of the subclasses. Now my mentor asked me to move this abstract class to an interface. Having an interface is no problem except with the method used to construct subclass objects. Where should this method go now? Also, I read somewhere that interfaces are more efficient than abstract classes. Is this true? Here's an example of my classes abstract class Animal { //many abstract methods getAnimalobject(some parameter) { return //appropriate subclass } } class Dog extends Animal {} class Elephant extends Animal {}

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  • Difference between Factory Method and Abstract Factory design patterns using C#.Net

    - by nijhawan.saurabh
    First of all I'll just put both these patterns in context and describe their intent as in the GOF book: Factory Method: Define an interface for creating an object, but let subclasses decide which class to instantiate. Factory Method lets a class defer instantiation to subclasses.   Abstract Factory: Provide an interface for creating families of related or dependent objects without specifying their concrete classes.   Points to note:   Abstract factory pattern adds a layer of abstraction to the factory method pattern. The type of factory is not known to the client at compile time, this information is passed to the client at runtime (How it is passed is again dependent on the system, you may store this information in configuration files and the client can read it on execution). While implementing Abstract factory pattern, the factory classes can have multiple factory methods. In Abstract factory, a factory is capable of creating more than one type of product (Simpilar products are grouped together in a factory)   Sample implementation of factory method pattern   Let's see the class diagram first:                   ProductFactory.cs // ----------------------------------------------------------------------- // <copyright file="ProductFactory.cs" company=""> // TODO: Update copyright text. // </copyright> // -----------------------------------------------------------------------   namespace FactoryMethod {     using System;     using System.Collections.Generic;     using System.Linq;     using System.Text;       /// <summary>     /// TODO: Update summary.     /// </summary>     public abstract class ProductFactory     {         /// <summary>         /// </summary>         /// <returns>         /// </returns>         public abstract Product CreateProductInstance();     } }     ProductAFactory.cs // ----------------------------------------------------------------------- // <copyright file="ProductAFactory.cs" company=""> // TODO: Update copyright text. // </copyright> // -----------------------------------------------------------------------   namespace FactoryMethod {     using System;     using System.Collections.Generic;     using System.Linq;     using System.Text;       /// <summary>     /// TODO: Update summary.     /// </summary>     public class ProductAFactory:ProductFactory     {         public override Product CreateProductInstance()         {             return new ProductA();         }     } }         // ----------------------------------------------------------------------- // <copyright file="ProductBFactory.cs" company=""> // TODO: Update copyright text. // </copyright> // -----------------------------------------------------------------------   namespace FactoryMethod {     using System;     using System.Collections.Generic;     using System.Linq;     using System.Text;       /// <summary>     /// TODO: Update summary.     /// </summary>     public class ProductBFactory:ProductFactory     {         public override Product CreateProductInstance()         {             return new ProductB();           }     } }     // ----------------------------------------------------------------------- // <copyright file="Product.cs" company=""> // TODO: Update copyright text. // </copyright> // -----------------------------------------------------------------------   namespace FactoryMethod {     using System;     using System.Collections.Generic;     using System.Linq;     using System.Text;       /// <summary>     /// TODO: Update summary.     /// </summary>     public abstract class Product     {         public abstract string Name { get; set; }     } }     // ----------------------------------------------------------------------- // <copyright file="ProductA.cs" company=""> // TODO: Update copyright text. // </copyright> // -----------------------------------------------------------------------   namespace FactoryMethod {     using System;     using System.Collections.Generic;     using System.Linq;     using System.Text;       /// <summary>     /// TODO: Update summary.     /// </summary>     public class ProductA:Product     {         public ProductA()         {               Name = "ProductA";         }           public override string Name { get; set; }     } }       // ----------------------------------------------------------------------- // <copyright file="ProductB.cs" company=""> // TODO: Update copyright text. // </copyright> // -----------------------------------------------------------------------   namespace FactoryMethod {     using System;     using System.Collections.Generic;     using System.Linq;     using System.Text;       /// <summary>     /// TODO: Update summary.     /// </summary>     public class ProductB:Product     {          public ProductB()         {               Name = "ProductA";         }         public override string Name { get; set; }     } }     Program.cs using System; using System.Collections.Generic; using System.Linq; using System.Text;   namespace FactoryMethod {     class Program     {         static void Main(string[] args)         {             ProductFactory pf = new ProductAFactory();               Product product = pf.CreateProductInstance();             Console.WriteLine(product.Name);         }     } }       Normal 0 false false false false EN-US X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:"Table Normal"; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:""; mso-padding-alt:0in 5.4pt 0in 5.4pt; mso-para-margin-top:0in; mso-para-margin-right:0in; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0in; line-height:115%; mso-pagination:widow-orphan; font-size:11.0pt; font-family:"Calibri","sans-serif"; mso-ascii-font-family:Calibri; mso-ascii-theme-font:minor-latin; mso-hansi-font-family:Calibri; mso-hansi-theme-font:minor-latin; mso-bidi-font-family:"Times New Roman"; mso-bidi-theme-font:minor-bidi;}

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  • Abstract Methods in "Product" - Factory Method C#

    - by Regina Foo
    I have a simple class library (COM+ service) written in C# to consume 5 web services: Add, Minus, Divide, Multiply and Compare. I've created the abstract product and abstract factory classes. The abstract product named WS's code: public abstract class WS { public abstract double Calculate(double a, double b); public abstract string Compare(double a, double b); } As you see, when one of the subclasses inherits WS, both methods must be overridden which might not be useful in some subclasses. E.g. Compare doesn't need Calculate() method. To instantiate a new CompareWS object, the client class will call the CreateWS() method which returns a WS object type. public class CompareWSFactory : WSFactory { public override WS CreateWS() { return new CompareWS(); } } But if Compare() is not defined as abstract in WS, the Compare() method cannot be invoked. This is only an example with two methods, but what if there are more methods? Is it stupid to define all the methods as abstract in the WS class? My question is: I want to define abstract methods that are common to all subclasses of WS whereas when the factory creates a WS object type, all the methods of the subclasses can be invoked (overridden methods of WS and also the methods in subclasses). How should I do this?

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  • Confused about this factory, as it doesn't look like an Abstract Factory nor Factory Method

    - by Pin
    I'm looking into Guice and I've been reading its documentation recently. Reading the motivation section I don't understand the factories part, why they name it that way. To me that factory is just a wrapper for the implementing class they want it to return after calling getInstance(). public class CreditCardProcessorFactory { private static CreditCardProcessor instance; public static void setInstance(CreditCardProcessor creditCardProcessor) { instance = creditCardProcessor; } public static CreditCardProcessor getInstance() { if (instance == null) { throw new IllegalStateException("CreditCardProcessorFactory not initialized. " + "Did you forget to call CreditCardProcessor.setInstance() ?"); } return instance; } } Why do they call it factory as well if it is neither an abstract factory nor a factory method? Or am I missing something? Thanks.

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  • Abstract class + Inheritance vs Interface

    - by RealityDysfunction
    Hello fellow programmers, I am reading a book on C# and the author is comparing Abstract classes and Interfaces. He claims that if you have the following "abstract class:" abstract class CloneableType { public abstract object Clone(); } Then you cannot do this: public class MiniVan : Car, CloneableType {} This, I understand. However he claims that because of this inability to do multiple inheritance that you should use an interface for CloneableType, like so: public interface ICloneable { object Clone(); } My question is, isn't this somewhat misleading, because you can create an abstract class which is "above" class Car with the method Clone, then have Car inherit that class and then Minivan will inherit Car with all these methods, CloneAble class - Car class - Minivan Class. What do you think? Thanks.

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  • C++: Create abstract class with abstract method and override the method in a subclass

    - by Martijn Courteaux
    Hi, How to create in C++ an abstract class with some abstract methods that I want to override in a subclass? How should the .h file look? Is there a .cpp, if so how should it look? In Java it would look like this: abstract class GameObject { public abstract void update(); public abstract void paint(Graphics g); } class Player extends GameObject { @Override public void update() { // ... } @Override public void paint(Graphics g) { // ... } } // In my game loop: for (int i = 0; i < objects.size(); i++) { objects.get(i).update(); } for (int i = 0; i < objects.size(); i++) { objects.get(i).paint(g); } Translating this code to C++ is enough for me.

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  • Abstract exception super type

    - by marcof
    If throwing System.Exception is considered so bad, why wasn't Exception made abstract in the first place? That way, it would not be possible to call: throw new Exception("Error occurred."); This would enforce using derived exceptions to provide more details about the error that occurred. For example, when I want to provide a custom exception hierarchy for a library, I usually declare an abstract base class for my exceptions: public abstract class CustomExceptionBase : Exception { /* some stuff here */ } And then some derived exception with a more specific purpose: public class DerivedCustomException : CustomExceptionBase { /* some more specific stuff here */ } Then when calling any library method, one could have this generic try/catch block to directly catch any error coming from the library: try { /* library calls here */ } catch (CustomExceptionBase ex) { /* exception handling */ } Is this a good practice? Would it be good if Exception was made abstract? EDIT : My point here is that even if an exception class is abstract, you can still catch it in a catch-all block. Making it abstract is only a way to forbid programmers to throw a "super-wide" exception. Usually, when you voluntarily throw an exception, you should know what type it is and why it happened. Thus enforcing to throw a more specific exception type.

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  • abstract class extends abstract class in php?

    - by user151841
    I am working on a simple abstract database class. In my usage of this class, I'll want to have some instance be a singleton. I was thinking of having a abstract class that is not a singleton, and then extend it into another abstract class that is a singleton. Is this possible? Recommended?

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  • factory girl - know when an attribute is overridden in factory

    - by carlos
    Is there any way to determine if attributes are overridden in a factory and adjust the behavior of the factory based on those overrides? The evaluator passed into the callbacks contains a __override_names__ method. Unfortunately, by the time it is passed to the callback, every attribute configured by the factory is included in the array returned. Moreover, the Evaluator is marked as api private.

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  • General Overview of Design Pattern Types

    Typically most software engineering design patterns fall into one of three categories in regards to types. Three types of software design patterns include: Creational Type Patterns Structural Type Patterns Behavioral Type Patterns The Creational Pattern type is geared toward defining the preferred methods for creating new instances of objects. An example of this type is the Singleton Pattern. The Singleton Pattern can be used if an application only needs one instance of a class. In addition, this singular instance also needs to be accessible across an application. The benefit of the Singleton Pattern is that you control both instantiation and access using this pattern. The Structural Pattern type is a way to describe the hierarchy of objects and classes so that they can be consolidated into a larger structure. An example of this type is the Façade Pattern.  The Façade Pattern is used to define a base interface so that all other interfaces inherit from the parent interface. This can be used to simplify a number of similar object interactions into one single standard interface. The Behavioral Pattern Type deals with communication between objects. An example of this type is the State Design Pattern. The State Design Pattern enables objects to alter functionality and processing based on the internal state of the object at a given time.

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  • Help to understand the abstract factory pattern

    - by Chobeat
    I'm learning the 23 design patterns of the GoF. I think I've found a way to understand and simplify how the Abstract Factory works but I would like to know if this is a correct assumption or if I am wrong. What I want to know is if we can see the result of the Abstract Factory method as a matrix of possible products where there's a Product for every "Concrete Factory" x "AbstractProduct" where the Concrete Factory is a single implementation among the implementations of an AbstractFactory and an AbstractProduct is an interface among the interfaces to create Products. Is this correct or am I missing something?

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  • Factory Pattern: Determining concrete factory class instantiation?

    - by Chris
    I'm trying to learn patterns and I'm stuck on determining how or where a Factory Pattern determines what class to instanciate. If I have a Application that calls the factory and sends it, say, an xml config file to determine what type of action to take, where does that logic for interpreting the config file happen? THE FACTORY using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace myNamespace { public abstract class SourceFactory { abstract public UploadSource getUploadSource(); } public class TextSourceFactory : SourceFactory { public override UploadSource getUploadSource() { return new TextUploadSource(); } } public class XmlSourceFacotry : SourceFactory { public override UploadSource getUploadSource() { return new XmlUploadSource(); } } public class SqlSourceFactory : SourceFactory { public override UploadSource getUploadSource() { return new SqlUploadSource(); } } } THE CLASSES using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace myNamespace { public abstract class UploadSource { abstract public void Execute(); } public class TextUploadSource : UploadSource { public override void Execute() { Console.WriteLine("You executed a text upload source"); } } public class XmlUploadSource : UploadSource { public override void Execute() { Console.WriteLine("You executed an XML upload source"); } } public class SqlUploadSource : UploadSource { public override void Execute() { Console.WriteLine("You executed a SQL upload source"); } } }

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  • Is there anything wrong with my Factory class?

    - by Alex
    class PieceFactory { @SuppressWarnings("rawtypes") public Piece createPiece(String pieceType) throws Throwable{ Class pieceClass = Class.forName(pieceType); Piece piece = (Piece) pieceClass.newInstance(); return piece; } } I'm not all used to handling exceptions yet therefore I'm just throwing them, but everywhere I use a method that uses this factory it tells me I have to throw exceptions like throwable. For example, in one of my classes I have a method that instantiates a lot of objects using the method that uses the factory. I can use the method in that class by just throwing the exception, however it won't work if I try to pass a reference to that class to another class and then use the method from there. Then it forces me to try catch the exception. I probably don't need a factory but it seemed interesting and I'd like to try to use patterns. The reason I created the factory was that I have 6 subclasses of Piece and I wan't to use a method to instantiate them by passing the type of subclass I want as an argument to the method.

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  • What are the differences between abstract classes, interfaces, and when to use them

    - by user66662
    Recently I have started to wrap my head around OOP, and I am now to the point where the more I read about the differences between Abstract classes and Interfaces the more confused I become. So far, neither can be instantiated. Interfaces are more or less structural blueprints that determine the skeleton and abstracts are different by being able to partially develop code. I would like to learn more about these through my specific situation. Here is a link to my first question if you would like a little more background information: What is a good design model for my new class? Here are two classes I created: class Ad { $title; $description $price; function get_data($website){ } function validate_price(){ } } class calendar_event { $title; $description $start_date; function get_data($website){ //guts } function validate_dates(){ //guts } } So, as you can see these classes are almost identical. Not shown here, but there are other functions, like get_zip(), save_to_database() that are common across my classes. I have also added other classes Cars and Pets which have all the common methods and of course properties specific to those objects (mileage, weight, for example). Now I have violated the DRY principle and I am managing and changing the same code across multiple files. I intend on having more classes like boats, horses, or whatever. So is this where I would use an interface or abstract class? From what I understand about abstract classes I would use a super class as a template with all of the common elements built into the abstract class, and then add only the items specifically needed in future classes. For example: abstract class content { $title; $description function get_data($website){ } function common_function2() { } function common_function3() { } } class calendar_event extends content { $start_date; function validate_dates(){ } } Or would I use an interface and, because these are so similar, create a structure that each of the subclasses are forced to use for integrity reasons, and leave it up to the end developer who fleshes out that class to be responsible for each of the details of even the common functions. my thinking there is that some 'common' functions may need to be tweaked in the future for the needs of their specific class. Despite all that above, if you believe I am misunderstanding the what and why of abstracts and interfaces altogether, by all means let a valid answer to be stop thinking in this direction and suggest the proper way to move forward! Thanks!

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  • UML Class Diagram: Abstract or Interface?

    - by J Smith
    I am modeling a class diagram and have spotted an opportunity to simplify it slightly. What I want to know is, would this it be better to implement an abstract class or an interface? The scenario is this, I have the classes: Artist Genre Album Song All of which share the methods getName, setName, and getCount (playcount that is). Would it be best to create an abstract 'Music' class with the aforementioned abstract methods, or should I create an interface, since the classes that implement the interface have to include all of the interface's methods (I think, correct me if I'm wrong). I hope I've given enough detail, please ask questions if I haven't. Thanks!

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  • Seam @Factory in abstract base class?

    - by Shadowman
    I've got a series of web actions I'm implementing in Seam to perform create, read, update, etc. operations. For my read/update/delete actions, I'd like to have individual action classes that all extend an abstract base class. I'd like to put the @Factory method in the abstract base class to retrieve the item that is to be acted upon. For example, I have this as the base class: public abstract class BaseAction { @In(required=false)@Out(required=false) private MyItem item=null; public MyItem getItem(){...} public void setItem(...){...} @Factory("item") public void initItem(){...} } My subclasses would extend BaseAction, so that I don't have to repeat the logic to load the item that is to be viewed, deleted, updated, etc. However, when I start my application, Seam throws errors saying I have declared multiple @Factory's for the same object. Is there any way around this? Is there any way to provide the @Factory in the base class without encoutnering these errors?

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  • Followup: Python 2.6, 3 abstract base class misunderstanding

    - by Aaron
    I asked a question at Python 2.6, 3 abstract base class misunderstanding. My problem was that python abstract base classes didn't work quite the way I expected them to. There was some discussion in the comments about why I would want to use ABCs at all, and Alex Martelli provided an excellent answer on why my use didn't work and how to accomplish what I wanted. Here I'd like to address why one might want to use ABCs, and show my test code implementation based on Alex's answer. tl;dr: Code after the 16th paragraph. In the discussion on the original post, statements were made along the lines that you don't need ABCs in Python, and that ABCs don't do anything and are therefore not real classes; they're merely interface definitions. An abstract base class is just a tool in your tool box. It's a design tool that's been around for many years, and a programming tool that is explicitly available in many programming languages. It can be implemented manually in languages that don't provide it. An ABC is always a real class, even when it doesn't do anything but define an interface, because specifying the interface is what an ABC does. If that was all an ABC could do, that would be enough reason to have it in your toolbox, but in Python and some other languages they can do more. The basic reason to use an ABC is when you have a number of classes that all do the same thing (have the same interface) but do it differently, and you want to guarantee that that complete interface is implemented in all objects. A user of your classes can rely on the interface being completely implemented in all classes. You can maintain this guarantee manually. Over time you may succeed. Or you might forget something. Before Python had ABCs you could guarantee it semi-manually, by throwing NotImplementedError in all the base class's interface methods; you must implement these methods in derived classes. This is only a partial solution, because you can still instantiate such a base class. A more complete solution is to use ABCs as provided in Python 2.6 and above. Template methods and other wrinkles and patterns are ideas whose implementation can be made easier with full-citizen ABCs. Another idea in the comments was that Python doesn't need ABCs (understood as a class that only defines an interface) because it has multiple inheritance. The implied reference there seems to be Java and its single inheritance. In Java you "get around" single inheritance by inheriting from one or more interfaces. Java uses the word "interface" in two ways. A "Java interface" is a class with method signatures but no implementations. The methods are the interface's "interface" in the more general, non-Java sense of the word. Yes, Python has multiple inheritance, so you don't need Java-like "interfaces" (ABCs) merely to provide sets of interface methods to a class. But that's not the only reason in software development to use ABCs. Most generally, you use an ABC to specify an interface (set of methods) that will likely be implemented differently in different derived classes, yet that all derived classes must have. Additionally, there may be no sensible default implementation for the base class to provide. Finally, even an ABC with almost no interface is still useful. We use something like it when we have multiple except clauses for a try. Many exceptions have exactly the same interface, with only two differences: the exception's string value, and the actual class of the exception. In many exception clauses we use nothing about the exception except its class to decide what to do; catching one type of exception we do one thing, and another except clause catching a different exception does another thing. According to the exception module's doc page, BaseException is not intended to be derived by any user defined exceptions. If ABCs had been a first class Python concept from the beginning, it's easy to imagine BaseException being specified as an ABC. But enough of that. Here's some 2.6 code that demonstrates how to use ABCs, and how to specify a list-like ABC. Examples are run in ipython, which I like much better than the python shell for day to day work; I only wish it was available for python3. Your basic 2.6 ABC: from abc import ABCMeta, abstractmethod class Super(): __metaclass__ = ABCMeta @abstractmethod def method1(self): pass Test it (in ipython, python shell would be similar): In [2]: a = Super() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Super with abstract methods method1 Notice the end of the last line, where the TypeError exception tells us that method1 has not been implemented ("abstract methods method1"). That was the method designated as @abstractmethod in the preceding code. Create a subclass that inherits Super, implement method1 in the subclass and you're done. My problem, which caused me to ask the original question, was how to specify an ABC that itself defines a list interface. My naive solution was to make an ABC as above, and in the inheritance parentheses say (list). My assumption was that the class would still be abstract (can't instantiate it), and would be a list. That was wrong; inheriting from list made the class concrete, despite the abstract bits in the class definition. Alex suggested inheriting from collections.MutableSequence, which is abstract (and so doesn't make the class concrete) and list-like. I used collections.Sequence, which is also abstract but has a shorter interface and so was quicker to implement. First, Super derived from Sequence, with nothing extra: from abc import abstractmethod from collections import Sequence class Super(Sequence): pass Test it: In [6]: a = Super() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Super with abstract methods __getitem__, __len__ We can't instantiate it. A list-like full-citizen ABC; yea! Again, notice in the last line that TypeError tells us why we can't instantiate it: __getitem__ and __len__ are abstract methods. They come from collections.Sequence. But, I want a bunch of subclasses that all act like immutable lists (which collections.Sequence essentially is), and that have their own implementations of my added interface methods. In particular, I don't want to implement my own list code, Python already did that for me. So first, let's implement the missing Sequence methods, in terms of Python's list type, so that all subclasses act as lists (Sequences). First let's see the signatures of the missing abstract methods: In [12]: help(Sequence.__getitem__) Help on method __getitem__ in module _abcoll: __getitem__(self, index) unbound _abcoll.Sequence method (END) In [14]: help(Sequence.__len__) Help on method __len__ in module _abcoll: __len__(self) unbound _abcoll.Sequence method (END) __getitem__ takes an index, and __len__ takes nothing. And the implementation (so far) is: from abc import abstractmethod from collections import Sequence class Super(Sequence): # Gives us a list member for ABC methods to use. def __init__(self): self._list = [] # Abstract method in Sequence, implemented in terms of list. def __getitem__(self, index): return self._list.__getitem__(index) # Abstract method in Sequence, implemented in terms of list. def __len__(self): return self._list.__len__() # Not required. Makes printing behave like a list. def __repr__(self): return self._list.__repr__() Test it: In [34]: a = Super() In [35]: a Out[35]: [] In [36]: print a [] In [37]: len(a) Out[37]: 0 In [38]: a[0] --------------------------------------------------------------------------- IndexError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() /home/aaron/projects/test/test.py in __getitem__(self, index) 10 # Abstract method in Sequence, implemented in terms of list. 11 def __getitem__(self, index): ---> 12 return self._list.__getitem__(index) 13 14 # Abstract method in Sequence, implemented in terms of list. IndexError: list index out of range Just like a list. It's not abstract (for the moment) because we implemented both of Sequence's abstract methods. Now I want to add my bit of interface, which will be abstract in Super and therefore required to implement in any subclasses. And we'll cut to the chase and add subclasses that inherit from our ABC Super. from abc import abstractmethod from collections import Sequence class Super(Sequence): # Gives us a list member for ABC methods to use. def __init__(self): self._list = [] # Abstract method in Sequence, implemented in terms of list. def __getitem__(self, index): return self._list.__getitem__(index) # Abstract method in Sequence, implemented in terms of list. def __len__(self): return self._list.__len__() # Not required. Makes printing behave like a list. def __repr__(self): return self._list.__repr__() @abstractmethod def method1(): pass class Sub0(Super): pass class Sub1(Super): def __init__(self): self._list = [1, 2, 3] def method1(self): return [x**2 for x in self._list] def method2(self): return [x/2.0 for x in self._list] class Sub2(Super): def __init__(self): self._list = [10, 20, 30, 40] def method1(self): return [x+2 for x in self._list] We've added a new abstract method to Super, method1. This makes Super abstract again. A new class Sub0 which inherits from Super but does not implement method1, so it's also an ABC. Two new classes Sub1 and Sub2, which both inherit from Super. They both implement method1 from Super, so they're not abstract. Both implementations of method1 are different. Sub1 and Sub2 also both initialize themselves differently; in real life they might initialize themselves wildly differently. So you have two subclasses which both "is a" Super (they both implement Super's required interface) although their implementations are different. Also remember that Super, although an ABC, provides four non-abstract methods. So Super provides two things to subclasses: an implementation of collections.Sequence, and an additional abstract interface (the one abstract method) that subclasses must implement. Also, class Sub1 implements an additional method, method2, which is not part of Super's interface. Sub1 "is a" Super, but it also has additional capabilities. Test it: In [52]: a = Super() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Super with abstract methods method1 In [53]: a = Sub0() --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: Can't instantiate abstract class Sub0 with abstract methods method1 In [54]: a = Sub1() In [55]: a Out[55]: [1, 2, 3] In [56]: b = Sub2() In [57]: b Out[57]: [10, 20, 30, 40] In [58]: print a, b [1, 2, 3] [10, 20, 30, 40] In [59]: a, b Out[59]: ([1, 2, 3], [10, 20, 30, 40]) In [60]: a.method1() Out[60]: [1, 4, 9] In [61]: b.method1() Out[61]: [12, 22, 32, 42] In [62]: a.method2() Out[62]: [0.5, 1.0, 1.5] [63]: a[:2] Out[63]: [1, 2] In [64]: a[0] = 5 --------------------------------------------------------------------------- TypeError Traceback (most recent call last) /home/aaron/projects/test/<ipython console> in <module>() TypeError: 'Sub1' object does not support item assignment Super and Sub0 are abstract and can't be instantiated (lines 52 and 53). Sub1 and Sub2 are concrete and have an immutable Sequence interface (54 through 59). Sub1 and Sub2 are instantiated differently, and their method1 implementations are different (60, 61). Sub1 includes an additional method2, beyond what's required by Super (62). Any concrete Super acts like a list/Sequence (63). A collections.Sequence is immutable (64). Finally, a wart: In [65]: a._list Out[65]: [1, 2, 3] In [66]: a._list = [] In [67]: a Out[67]: [] Super._list is spelled with a single underscore. Double underscore would have protected it from this last bit, but would have broken the implementation of methods in subclasses. Not sure why; I think because double underscore is private, and private means private. So ultimately this whole scheme relies on a gentleman's agreement not to reach in and muck with Super._list directly, as in line 65 above. Would love to know if there's a safer way to do that.

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  • What are the advantages of the delegate pattern over the observer pattern?

    - by JoJo
    In the delegate pattern, only one object can directly listen to another object's events. In the observer pattern, any number of objects can listen to a particular object's events. When designing a class that needs to notify other object(s) of events, why would you ever use the delegate pattern over the observer pattern? I see the observer pattern as more flexible. You may only have one observer now, but a future design may require multiple observers.

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  • Java - abstract class, equals(), and two subclasses

    - by msr
    Hello, I have an abstract class named Xpto and two subclasses that extend it named Person and Car. I have also a class named Test with main() and a method foo() that verifies if two persons or cars (or any object of a class that extends Xpto) are equals. Thus, I redefined equals() in both Person and Car classes. Two persons are equal when they have the same name and two cars are equal when they have the same registration. However, when I call foo() in the Test class I always get "false". I understand why: the equals() is not redefined in Xpto abstract class. So... how can I compare two persons or cars (or any object of a class that extends Xpto) in that foo() method? In summary, this is the code I have: public abstract class Xpto { } public class Person extends Xpto{ protected String name; public Person(String name){ this.name = name; } public boolean equals(Person p){ System.out.println("Person equals()?"); return this.name.compareTo(p.name) == 0 ? true : false; } } public class Car extends Xpto{ protected String registration; public Car(String registration){ this.registration = registration; } public boolean equals(Car car){ System.out.println("Car equals()?"); return this.registration.compareTo(car.registration) == 0 ? true : false; } } public class Teste { public static void foo(Xpto xpto1, Xpto xpto2){ if(xpto1.equals(xpto2)) System.out.println("xpto1.equals(xpto2) -> true"); else System.out.println("xpto1.equals(xpto2) -> false"); } public static void main(String argv[]){ Car c1 = new Car("ABC"); Car c2 = new Car("DEF"); Person p1 = new Person("Manel"); Person p2 = new Person("Manel"); foo(p1,p2); } }

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  • Factory Method Implementation

    - by cedar715
    I was going through the 'Factory method' pages in SO and had come across this link. And this comment. The example looked as a variant and thought to implement in its original way: to defer instantiation to subclasses... Here is my attempt. Does the following code implements the Factory pattern of the example specified in the link? Please validate and suggest if this has to undergo any re-factoring. public class ScheduleTypeFactoryImpl implements ScheduleTypeFactory { @Override public IScheduleItem createLinearScheduleItem() { return new LinearScheduleItem(); } @Override public IScheduleItem createVODScheduleItem() { return new VODScheduleItem(); } } public class UseScheduleTypeFactory { public enum ScheduleTypeEnum { CableOnDemandScheduleTypeID, BroadbandScheduleTypeID, LinearCableScheduleTypeID, MobileLinearScheduleTypeID } public static IScheduleItem getScheduleItem(ScheduleTypeEnum scheduleType) { IScheduleItem scheduleItem = null; ScheduleTypeFactory scheduleTypeFactory = new ScheduleTypeFactoryImpl(); switch (scheduleType) { case CableOnDemandScheduleTypeID: scheduleItem = scheduleTypeFactory.createVODScheduleItem(); break; case BroadbandScheduleTypeID: scheduleItem = scheduleTypeFactory.createVODScheduleItem(); break; case LinearCableScheduleTypeID: scheduleItem = scheduleTypeFactory.createLinearScheduleItem(); break; case MobileLinearScheduleTypeID: scheduleItem = scheduleTypeFactory.createLinearScheduleItem(); break; default: break; } return scheduleItem; } }

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  • Better solution then simple factory method when concrete implementations have different attributes

    - by danip
    abstract class Animal { function eat() {..} function sleep() {..} function isSmart() } class Dog extends Animal { public $blnCanBark; function isSmart() { return $this->blnCanBark; } } class Cat extends Animal { public $blnCanJumpHigh; function isSmart() { return $this->blnCanJumpHigh; } } .. and so on up to 10-20 animals. Now I created a factory using simple factory method and try to create instances like this: class AnimalFactory { public static function create($strName) { switch($strName) { case 'Dog': return new Dog(); case 'Cat': return new Cat(); default: break; } } } The problem is I can't set the specific attributes like blnCanBark, blnCanJumpHigh in an efficient way. I can send all of them as extra params to create but this will not scale to more then a few classes. Also I can't break the inheritance because a lot of the basic functionality is the same. Is there a better pattern to solve this?

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