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Design patterns for Agent / Actor based concurrent design.

- by nso1

Recently i have been getting into alternative languages that support an actor/agent/shared nothing architecture - ie. scala, clojure etc (clojure also supports shared state). So far most of the documentation that I have read focus around the intro level. What I am looking for is more advanced documentation along the gang of four but instead shared nothing based. Why ? It helps to grok the change in design thinking. Simple examples are easy, but in a real world java application (single threaded) you can have object graphs with 1000's of members with complex relationships. But with agent based concurrency development it introduces a whole new set of ideas to comprehend when designing large systems. ie. Agent granularity - how much state should one agent manage - implications on performance etc or are their good patterns for mapping shared state object graphs to agent based system. tips on mapping domain models to design. Discussions not on the technology but more on how to BEST use the technology in design (real world "complex" examples would be great).

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Get class of caller's method (via inspect) in Python (alt: super() emulator)

- by Slava Vishnyakov

Is it possible to get reference to class B in this example? class A(object): pass class B(A): def test(self): test2() class C(B): pass import inspect def test2(): frame = inspect.currentframe().f_back cls = frame.[?something here?] # cls here should == B (class) c = C() c.test() Basically, C is child of B, B is child of A. Then we create c of type C. Then the call to c.test() actually calls B.test() (via inheritance), which calls to test2(). test2() can get the parent frame frame; code reference to method via frame.f_code; self via frame.f_locals['self']; but type(frame.f_locals['self']) is C (of course), but not B, where method is defined. Any way to get B?

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Trying to understand the usage of class_eval

- by eMxyzptlk

Hello everyone, I'm using the rails-settings gem, and I'm trying to understand how you add functions to ActiveRecord classes (I'm building my own library for card games), and I noticed that this gem uses one of the Meta-programming techniques to add the function to the ActiveRecord::Base class (I'm far from Meta-programming master in ruby, but I'm trying to learn it) module RailsSettings class Railtie < Rails::Railtie initializer 'rails_settings.initialize', :after => :after_initialize do Railtie.extend_active_record end end class Railtie def self.extend_active_record ActiveRecord::Base.class_eval do def self.has_settings class_eval do def settings RailsSettings::ScopedSettings.for_thing(self) end scope :with_settings, :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}')", :select => "DISTINCT #{self.table_name}.*" scope :with_settings_for, lambda { |var| { :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}') AND settings.var = '#{var}'" } } scope :without_settings, :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}')", :conditions => 'settings.id IS NULL' scope :without_settings_for, lambda { |var| { :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}') AND settings.var = '#{var}'", :conditions => 'settings.id IS NULL' } } end end end end end end What I don't understand is why he uses class_eval on ActiveRecord::Base, wasn't it easier if he just open the ActiveRecord::Base class and define the functions? Specially that there's nothing dynamic in the block (What I mean by dynamic is when you do class_eval or instance_eval on a string containing variables) something like this: module ActiveRecord class Base def self.has_settings class_eval do def settings RailsSettings::ScopedSettings.for_thing(self) end scope :with_settings, :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}')", :select => "DISTINCT #{self.table_name}.*" scope :with_settings_for, lambda { |var| { :joins => "JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}') AND settings.var = '#{var}'" } } scope :without_settings, :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}')", :conditions => 'settings.id IS NULL' scope :without_settings_for, lambda { |var| { :joins => "LEFT JOIN settings ON (settings.thing_id = #{self.table_name}.#{self.primary_key} AND settings.thing_type = '#{self.base_class.name}') AND settings.var = '#{var}'", :conditions => 'settings.id IS NULL' } } end end end end I understand the second class_eval (before the def settings) is to define functions on the fly on every class that 'has_settings' right ? Same question here, I think he could use "def self.settings" instead of "class_eval.... def settings", no ?

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Ruby: Calling class method from instance

- by Peter

In Ruby, how do you call a class method from one of that class's instances? Say I have class Truck def self.default_make # Class method. "mac" end def initialize # Instance method. Truck.default_make # gets the default via the class's method. # But: I wish to avoid mentioning Truck. Seems I'm repeating myself. end end the line Truck.default_make retrieves the default. But is there a way of saying this without mentioning Truck? It seems like there should be.

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spring-nullpointerexception- cant access autowired annotated service (or dao) in a no-annotations class

- by user286806

I have this problem that I cannot fix. From my @Controller, i can easily access my autowired @Service class and play with it no problem. But when I do that from a separate class without annotations, it gives me a NullPointerException. My Controller (works)- @Controller public class UserController { @Autowired UserService userService;... My separate Java class (not working)- public final class UsersManagementUtil { @Autowired UserService userService; or @Autowired UserDao userDao; userService or userDao are always null! Was just trying if any one of them works. My component scan setting has the root level package set for scanning so that should be OK. my servlet context - <?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx" xsi:schemaLocation=" http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.0.xsd">   <context:property-placeholder location="classpath:jdbc.properties" /> <context:component-scan base-package="x" /> <tx:annotation-driven transaction-manager="hibernateTransactionManager" />  <bean id="messageSource" class="o.s.c.s.ReloadableResourceBundleMessageSource"> <property name="basename" value="/WEB-INF/messages" /> </bean> <bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource"> <property name="driverClassName" value="${database.driver}" /> <property name="url" value="${database.url}" /> <property name="username" value="${database.user}" /> <property name="password" value="${database.password}" /> </bean>  <bean id="viewResolver" class="o.s.w.s.v.InternalResourceViewResolver"> <property name="prefix"> <value>/</value> </property> <property name="suffix"> <value>.jsp</value> </property> <property name="order"> <value>0</value> </property> </bean>  <bean id="sessionFactory" class="o.s.o.h3.a.AnnotationSessionFactoryBean"> <property name="dataSource" ref="dataSource" /> <property name="annotatedClasses"> <list> <value>orion.core.models.Question</value> <value>orion.core.models.User</value> <value>orion.core.models.Space</value> <value>orion.core.models.UserSkill</value> <value>orion.core.models.Question</value> <value>orion.core.models.Rating</value> </list> </property> <property name="hibernateProperties"> <props> <prop key="hibernate.dialect">${hibernate.dialect}</prop> <prop key="hibernate.show_sql">${hibernate.show_sql}</prop> <prop key="hibernate.hbm2ddl.auto">${hibernate.hbm2ddl.auto}</prop> </props> </property> </bean> <bean id="hibernateTransactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager"> <property name="sessionFactory" ref="sessionFactory" /> </bean> Any clue?

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PHP: friend classes and ungreedy caller function/class

- by avetis.kazarian

Is there any way to get the caller function with something else than debug_backtrace()? I'm looking for a less greedy way to simulate scopes like friend or internal. Let's say I have a class A and a class B. Until now, I've been using debug_backtrace(), which is too greedy (IMHO). I thought of something like this: <?php class A { public function __construct(B $callerObj) {} } class B { public function someMethod() { $obj = new A($this); } } ?> It might be OK if you want to limit it to one specific class, but let's say I have 300 classes, and I want to limit it to 25 of them? One way could be using an interface to aggregate: public function __construct(CallerInterface $callerObj) But it's still an ugly code. Moreover, you can't use that trick with static classes. Have any better idea?

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Ruby: Locate class definition at run time?

- by Thor Thurn

I'm having an odd probably with rails right now... a class is being defined somewhere, and I can't find it. Grepping for "class ClassName" hasn't managed to locate it, but it's definitely there when I load up the rails console. It's just a vanilla class inheriting from Object with nothing else defined... quite boring. So, what I'd like is a way to figure out where this class constant was originally defined from the rails console. Something to print out the value of '__ FILE __' when this class was declared, in other words. I feel like some type of metaprogramming should make this possible.

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How do I attach a link (to a View) to an image in ASP.NET MVC?

- by Ryan Pitts

Ok, so here is my situation. I am creating a web application using ASP.NET MVC 2 using the C# language. I have programmed in HTML, CSS, and PHP for several years and I am very new to ASP.NET. The part that I am having trouble with is the image gallery. The setup: I have a link on the navigation bar that goes to a "Galleries" page. This page will show a list of galleries. Each gallery has a title, an image, and a description. All of this information is pulled from an XML file. I'm using the XML file like a database. I wanted to use this method so that i could easily update the list of galleries and have the updated XML file automatically be reflected by the website. Now, the galleries should link to an "Images" page. This page will display a list of images within the gallery based on what gallery was selected. This page will also pull from an XML file. The problem: I cannot seem to attach a dynamic link to the image? I am also stuck and not sure how to get the correct View to display. I know I need to do something with the controllers and models, right? I have some code if needed? I would greatly appreciate any help or direction for this! Thanks!

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C++: select argmax over vector of classes w.r.t. arbitrary expression

- by karpathy

Hello, I have trouble describing my problem so I'll give an example: I have a class description that has a couple of variables in it, for example: class A{ float a, b, c, d; } Now, I maintain a vector<A> that contains many of these classes. What I need to do very very often is to find the object inside this vector that satisfies that one of it's parameters is maximal w.r.t to the others. i.e code looks something like: int maxi=-1; float maxa=-1000; for(int i=0;i<vec.size();i++){ res= vec[i].a; if(res > maxa) { maxa= res; maxi=i; } } return vec[maxi]; However, sometimes I need to find class with maximal a, sometimes with maximal b, sometimes the class with maximal 0.8*a + 0.2*b, sometimes I want a maximal a*VAR + b, where VAR is some variable that is assigned in front, etc. In other words, I need to evaluate an expression for every class, and take the max. I find myself copy-pasting this everywhere, and only changing the single line that defines res. What makes it even more complicated is that even the name of the vector changes. Sometimes it's vec, sometimes it can be something else. I have many vectors that contain A's. This could be changed if this makes the problem too hard. Is there some nice way to avoid this insanity in C++? What's the neatest way to handle this? Thank you!

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Java hashcode based on identity

- by hjfreyer

The default behavior of Object.hashCode() is to return essentially the "address" of the object so that a.hashCode() == b.hashCode() if and only if a == b. How can I get this behavior in a user-defined class if a superclass already defines hashCode()? For instance: class A { public int hashCode() { return 0; } } class B extends A { public int hashCode() { // Now I want to return a unique hashcode for each object. // In pythonic terms, it'd look something like: return Object.hashCode(this); } } Ideas?

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SQL - Updating records based on most recent date

- by Remnant

I am having difficulty updating records within a database based on the most recent date and am looking for some guidance. By the way, I am new to SQL. As background, I have a windows forms application with SQL Express and am using ADO.NET to interact with the database. The application is designed to enable the user to track employee attendance on various courses that must be attended on a periodic basis (e.g. every 6 months, every year etc.). For example, they can pull back data to see the last time employees attended a given course and also update attendance dates if an employee has recently completed a course. I have three data tables: EmployeeDetailsTable - simple list of employees names, email address etc., each with unique ID CourseDetailsTable - simple list of courses, each with unique ID (e.g. 1, 2, 3 etc.) AttendanceRecordsTable - has 3 columns { EmployeeID, CourseID, AttendanceDate, Comments } For any given course, an employee will have an attendance history i.e. if the course needs to be attended each year then they will have one record for as many years as they have been at the company. What I want to be able to do is to update the 'Comments' field for a given employee and given course based on the most recent attendance date. What is the 'correct' SQL syntax for this? I have tried many things (like below) but cannot get it to work: UPDATE AttendanceRecordsTable SET Comments = @Comments WHERE AttendanceRecordsTable.EmployeeID = (SELECT EmployeeDetailsTable.EmployeeID FROM EmployeeDetailsTable WHERE (EmployeeDetailsTable.LastName =@ParameterLastName AND EmployeeDetailsTable.FirstName =@ParameterFirstName) AND AttendanceRecordsTable.CourseID = (SELECT CourseDetailsTable.CourseID FROM CourseDetailsTable WHERE CourseDetailsTable.CourseName =@CourseName)) GROUP BY MAX(AttendanceRecordsTable.LastDate) After much googling, I discovered that MAX is an aggregate function and so I need to use GROUP BY. I have also tried using the HAVING keyword but without success. Can anybody point me in the right direction? What is the 'conventional' syntax to update a database record based on the most recent date?

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How can I cluster short messages [Tweets] based on topic ? [Topic Based Clustering]

- by Jagira

Hello, I am planning an application which will make clusters of short messages/tweets based on topics. The number of topics will be limited like Sports [ NBA, NFL, Cricket, Soccer ], Entertainment [ movies, music ] and so on... I can think of two approaches to this Ask for users to tag questions like Stackoverflow does. Users can select tags from a predefined list of tags. Then on server side I will cluster them on based of tags. Pros:- Simple design. Less complexity in code. Cons:- Choices for users will be restricted. Clusters will not be dynamic. If a new event occurs, the predefined tags will miss it. Take the message, delete the stopwords [ predefined in a dictionary ] and apply some clustering algorithm to make a cluster and depending on its popularity, display the cluster. The cluster will be maintained according to its sustained popularity. New messages will be skimmed and assigned to corresponding clusters. Pros:- Dynamic clustering based on the popularity of the event/accident. Cons:- Increased complexity. More server resources required. I would like to know whether there are any other approaches to this problem. Or are there any ways of improving the above mentioned methods? Also suggest some good clustering algorithms.I think "K-Nearest Clustering" algorithm is apt for this situation.

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JBoss classloading when 2 WARs have the same class

- by Pacman

I have a web applications A.war which has two servlets AServlet and BServlet. Both instantiate a helper class com.mycompany.Foo.class (this is my own class, not a third party library). Now I want to split the two servlets into two separate WARs: A.war will have only AServlet and a new B.war will have BServlet. AServlet will invoke BServlet via HTTP GET. Both WARs will have com.mycompany.Foo.class. I want to deploy both WARs on the same JBoss instance. The question is, will there be any classloading issues due to the same class being present in both WARs, and the WARs being deployed on the same JBoss instance?

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Add CSS Class to link based on Character Count with jquery

- by Wes

I have links that load dynamically onto a background to make them look like buttons. Some of the links take up two lines and some of the links take up 1 line. Eithe way they need to be vertically centered. My plan to to append a class based on the number of characters and then adjust the padding from there. So if the link's text is 25 characters (including spaces) or less - append class 'small' if the link's text is greater than 25 characters - append class 'large' How would I go about doing this with jQuery?

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launch android activity from non-activity class

- by Alberto Barrera

im New on Android. I know theres a lot of similar Questions but anyone is helping. Im using a 3rd party app that just launch a class that extends their own class. So from that class i would like to launch an activity. public class SkyTest extends VtiUserExit { @Override public VtiUserExitResult execute() throws VtiExitException { // TODO Auto-generated method stub logInfo("TEST"); return null; } } How do i launch an activity named MainActivity from here. i tryed this: Context context = null; Intent intent = new Intent(context, MainActivity.class); context.startActivity(intent); but its not working, i know i cant use the null context, but how do i create a context o how it works? Thanks

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Get class of caller's method (via inspect) in Python; or: super(Class,self).method() replacement wit

- by Slava Vishnyakov

Is it possible to get reference to class B in this example? class A(object): pass class B(A): def test(self): test2() class C(B): pass import inspect def test2(): frame = inspect.currentframe().f_back cls = frame.[?something here?] # cls here should == B (class) c = C() c.test() Basically, C is child of B, B is child of A. Then we create c of type C. Then the call to c.test() actually calls B.test() (via inheritance), which calls to test2(). test2() can get the parent frame frame; code reference to method via frame.f_code; self via frame.f_locals['self']; but type(frame.f_locals['self']) is C (of course), but not B, where method is defined. Any way to get B?

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Pass logger instance to class

- by mridang

Hi Guys, I'm using a open-source Python library in my project. This library logs a lot of information using the logging class. ...but I can't see the output or log it to file. I know that i would have to create a logger instance and add a file-handler or a console-handler to it but how can i pass this logger instance to the class? Here's the init snippet of the class that I'm going to be using. class Periscope: ''' Main Periscope class''' def __init__(self): self.config = ConfigParser.SafeConfigParser({"lang": "en"}) if is_local: self.config_file = os.path.join(bd.xdg_config_home, "periscope", "config") if not os.path.exists(self.config_file): folder = os.path.dirname(self.config_file) if not os.path.exists(folder): logging.info("Creating folder %s" %folder) os.mkdir(folder) logging.info("Creating config file") configfile = open(self.config_file, "w") self.config.write(configfile) configfile.close() else: #Load it self.config.read(self.config_file) self.pluginNames = self.listExistingPlugins() self._preferedLanguages = None Any help? Thanks guys.

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Field Members vs Method Variables?

- by Braveyard

Recently I've been thinking about performance difference between class field members and method variables. What exactly I mean is in the example below : Lets say we have a DataContext object for Linq2SQL class DataLayer { ProductDataContext context = new ProductDataContext(); public IQueryable<Product> GetData() { return context.Where(t=>t.ProductId == 2); } } In the example above, context will be stored in heap and the GetData method variables will be removed from Stack after Method is executed. So lets examine the following example to make a distinction : class DataLayer { public IQueryable<Product> GetData() { ProductDataContext context = new ProductDataContext(); return context.Where(t=>t.ProductId == 2); } } (*1) So okay first thing we know is if we define ProductDataContext instance as a field, we can reach it everywhere in the class which means we don't have to create same object instance all the time. But lets say we are talking about Asp.NET and once the users press submit button the post data is sent to the server and the events are executed and the posted data stored in a database via the method above so it is probable that the same user can send different data after one another.If I know correctly after the page is executed, the finalizers come into play and clear things from memory (from heap) and that means we lose our instance variables from memory as well and after another post, DataContext should be created once again for the new page cycle. So it seems the only benefit of declaring it publicly to the whole class is the just number one text above. Or is there something other? Thanks in advance... (If I told something incorrect please fix me.. )

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Silencing GCC warnings when using an "Uncopyable" class

- by Kazade

I have several classes that I don't want to be copyable, some of these classes have pointer data members. To make these classes uncopyable I privately inherit the following class template: template <class T> class Uncopyable { protected: Uncopyable() {} virtual ~Uncopyable() {} private: Uncopyable(const Uncopyable &); T & operator=(const T&); }; Which I used like so: class Entity : private Uncopyable<Entity> { } This works fine, however when I compile with -Weffc++ I still get the following warning: class Entity has pointer data members but does not override Entity(const Entity&) or operator=(const Entity&) Why is it still giving me this warning?

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What are Class methods in Python for?

- by Dave Webb

I'm teaching myself Python and my most recent lesson was that Python is not Java, and so I've just spent a while turning all my Class methods into functions. I now realise that I don't need to use Class methods for what I would done with static methods in Java, but now I'm not sure when I would use them. All the advice I can find about Python Class methods is along the lines of newbies like me should steer clear of them, and the standard documentation is at its most opaque when discussing them. Does anyone have a good example of using a Class method in Python or at least can someone tell me when Class methods can be sensibly used?

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Prevent mapping all public members of a class in Fluent NHibernate

- by alimbada

I have a class generated from a WSDL that has a bunch of public properties and a public event. I'm extending this class with my own and adding some properties to it. All of my own properties are declared virtual, but the base class properties are not virtual. I'm using Fluent NHibernate's ClassMap to map only the properties from my extended class. How do I prevent (Fluent)NHibernate from trying to persist all the base class's public members? At the moment, I get the following exception when creating the ISessionFactory: NHibernate.InvalidProxyTypeException: The following types may not be used as proxies: Type: method get_<BaseClassProperty should be 'public/protected virtual' or 'protected internal virtual' Type: method set_<BaseClassProperty should be 'public/protected virtual' or 'protected internal virtual' ... Type: method add_<BaseClassEvent should be 'public/protected virtual' or 'protected internal virtual' Type: method remove_<BaseClassEvent should be 'public/protected virtual' or 'protected internal virtual'

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Template class implicit copy constructor issues

- by Nate

Stepping through my program in gdb, line 108 returns right back to the calling function, and doesn't call the copy constructor in class A, like (I thought) it should: template <class S> class A{ //etc... A( const A & old ){ //do stuff... } //etc... }; template <class T> class B{ //etc... A<T> ReturnsAnA(){ A<T> result; // do some stuff with result return result; //line 108 } //etc... }; Any hints? I've banged my head against the wall about this for 4 hours now, and can't seem to come up with what's happening here.

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define method for instance of class

- by aharon

Let there be class Example defined as: class Example def initialize(test='hey') self.class.send(:define_method, :say_hello, lambda { test }) end end On calling Example.new; Example.new I get a warning: method redefined; discarding old say_hello. This, I conclude, must be because it defines a method in the actual class (which makes sense, from the syntax). And that, of course, would prove disastrous should there be multiple instances of Example with different values in their methods. Is there a way to create methods just for the instance of a class from inside that instance? Thanks so much.

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Using the StackTrace Class in a production environment to get calling method info

- by andy

hey guys We have a "log" class which uses Relection.MethodBase to send current class info to the log. The reflection.MethodBase stuff happens in the class itself. However, I'd like to move that stuff to a single external "log" singleton type class. In this scenario the external log class needs to get the CALLING info, not the current method info. I'm using stacktrace to do this, which isn't in the Reflection namespace. Can I guarantee that "that" specific information (calling method) will be there in a production environment? var stackTrace = new StackTrace(); return LogManager.GetLogger(stackTrace.GetFrame(1).GetMethod().DeclaringType); cheers!

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NHibernate: discriminator without common base class?

- by joniba

Is it possible to map two classes to the same property without them sharing a common base class? For example, a situation like this: class Rule { public virtual int SequenceNumber { get; set; } public virtual ICondition Condition { get; set; } } interface ICondition { } class ExpressionCondition : ICondition { public virtual string Expression { get; set; } } class ThresholdCondition : ICondition { public virtual int Threshold { get; set; } } I also cannot add some empty abstract class that both conditions inherit from because the two ICondition implementations exist in different domains that are not allowed to reference each other. (Please no responses telling me that this situation should not occur in the first place - I'm aware of it and it doesn't help me.)

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