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• Why should I declare a class as an abstract class?

  - by Pied Piper

  I know the syntax, rules applied to abstract class and I want know usage of an abstract class Abstract class can not be instantiated directly but can be extended by other class What is the advantage of doing so? How it is different from an Interface? I know that one class can implement multiple interfaces but can only extend one abstract class. Is that only difference between an interface and an abstract class? I am aware about usage of an Interface. I have learned that from Event delegation model of AWT in Java. In which situations I should declare class as an abstract class? What is benefits of that?

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• Python: query a class's parent-class after multiple derivations ("super()" does not work)

  - by henry

  Hi, I have built a class-system that uses multiple derivations of a baseclass (object-class1-class2-class3): class class1(object): def __init__(self): print "class1.__init__()" object.__init__(self) class class2(class1): def __init__(self): print "class2.__init__()" class1.__init__(self) class class3(class2): def __init__(self): print "class3.__init__()" class2.__init__(self) x = class3() It works as expected and prints: class3.__init__() class2.__init__() class1.__init__() Now I would like to replace the 3 lines object.__init__(self) ... class1.__init__(self) ... class2.__init__(self) with something like this: currentParentClass().__init__() ... currentParentClass().__init__() ... currentParentClass().__init__() So basically, i want to create a class-system where i don't have to type "classXYZ.doSomething()". As mentioned above, I want to get the "current class's parent-class". Replacing the three lines with: super(type(self), self).__init__() does NOT work (it always returns the parent-class of the current instance - class2) and will result in an endless loop printing: class3.__init__() class2.__init__() class2.__init__() class2.__init__() class2.__init__() ... So is there a function that can give me the current class's parent-class? Thank you for your help! Henry -------------------- Edit: @Lennart ok maybe i got you wrong but at the moment i think i didn't describe the problem clearly enough.So this example might explain it better: lets create another child-class class class4(class3): pass now what happens if we derive an instance from class4? y = class4() i think it clearly executes: super(class3, self).__init__() which we can translate to this: class2.__init__(y) this is definitly not the goal(that would be class3.__init__(y)) Now making lots of parent-class-function-calls - i do not want to re-implement all of my functions with different base-class-names in my super()-calls.

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• How to use derived class shared variables in shared methods of base class

  - by KoolKabin

  Hi guys, I am trying to add shared members in derived classes and use that values in base classes... I have base class DBLayer public shared function GetDetail(byval UIN as integer) dim StrSql = string.format("select * from {0} where uin = {1}", tablename, uin) .... end function end class my derived class class User inherits dblayer public shared tabledname as string = "users" end class class item inherits dblayer public shared tabledname as string = "item" end class class category inherits dblayer public shared tabledname as string = "category" end class currently there is error using the tablename variable of derived class in base class but i want to use it... coz i dun know other techniques... if other solutions are better then u can post it or u can say how can i make it work? confused...

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• Advantages of Singleton Class over Static Class?

  Point 1)Singleton We can get the object of singleton and then pass to other methods.Static Class We can not pass static class to other methods as we pass objectsPoint 2) Singleton In future, it is easy to change the logic of of creating objects to some pooling mechanism. Static Class Very difficult to implement some pooling logic in case of static class. We would need to make that class as non-static and then make all the methods non-static methods, So entire your code needs to be changed.Point3:) Singleton Can Singletone class be inherited to subclass? Singleton class does not say any restriction of Inheritence. So we should be able to do this as long as subclass is also inheritence.There's nothing fundamentally wrong with subclassing a class that is intended to be a singleton. There are many reasons you might want to do it. and there are many ways to accomplish it. It depends on language you use.Static Class We can not inherit Static class to another Static class in C#. Think about it this way: you access static members via type name, like this: MyStaticType.MyStaticMember(); Were you to inherit from that class, you would have to access it via the new type name: MyNewType.MyStaticMember(); Thus, the new item bears no relationships to the original when used in code. There would be no way to take advantage of any inheritance relationship for things like polymorphism. span.fullpost {display:none;}

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• Advantages of Singleton Class over Static Class?

  Point 1) Singleton We can get the object of singleton and then pass to other methods. Static Class We can not pass static class to other methods as we pass objects Point 2) Singleton In future, it is easy to change the logic of of creating objects to some pooling mechanism. Static Class Very difficult to implement some pooling logic in case of static class. We would need to make that class as non-static and then make all the methods non-static methods, So entire your code needs to be changed. Point3:) Singleton Can Singletone class be inherited to subclass? Singleton class does not say any restriction of Inheritence. So we should be able to do this as long as subclass is also inheritence.There's nothing fundamentally wrong with subclassing a class that is intended to be a singleton. There are many reasons you might want to do it. and there are many ways to accomplish it. It depends on language you use. Static Class We can not inherit Static class to another Static class in C#. Think about it this way: you access static members via type name, like this: MyStaticType.MyStaticMember(); Were you to inherit from that class, you would have to access it via the new type name: MyNewType.MyStaticMember(); Thus, the new item bears no relationships to the original when used in code. There would be no way to take advantage of any inheritance relationship for things like polymorphism. span.fullpost {display:none;}

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• c++ defining a static member of a template class with type inner class pointer

  - by Jack

  I have a template class like here (in a header) with a inner class and a static member of type pointer to inner class template <class t> class outer { class inner { int a; }; static inner *m; }; template <class t> outer <t>::inner *outer <t>::m; when i want to define that static member i says "error: expected constructor, destructor, or type conversion before '*' token" on the last line (mingw32-g++ 3.4.5)

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• Getting all inner classes by reflection

  - by Roman

  I have the following problem. I have this pretty class and now I want to get all the classes that extend that class (inner classes ) and fill 'classList' with it. ( in an automatic way of course ) public abstract class CompoundReference { private static List<Class<? extends CompoundReference>> classList = new ArrayList<Class<? extends CompoundReference>>(); @CompoundKey(gsType = User.class, dbType = UserDetailsMappings.class) public static class CUser extends CompoundReference { } @CompoundKey(gsType = Catalog.class, dbType = CatalogDetailsMappings.class) public static class CCatalog extends CompoundReference { } @CompoundKey(gsType = Product.class, dbType = ProductDetailsMappings.class) public static class CProduct extends CompoundReference { } @CompoundKey(gsType = Category.class) public static class CCategory extends CompoundReference { } @CompoundKey(gsType = Poll.class, dbType = PollDetailsMappings.class) public static class CPoll extends CompoundReference { } // much mroe inner classes Some manual solution would be just to main such a static block , that is something that I dont want to do. static { classList.addAll(Arrays.asList(CUser.class, CCatalog.class, CProduct.class, CCategory.class, CPoll.class, CComment.class, CWebPage.class, CReview.class, CPost.class, CMessage.class, CStory.class,CPicture.class)); }

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• Class accessing inner class privates?

  - by aloh

  Class Outer { ... private class Node { private T data; ... private T getData() { return data; } } } What's the purpose of using set and get methods if the outer class can access inner class private members? What's the purpose of making inner classes private? Package access?

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• Manager/Container class vs static class methods

  - by Ben

  Suppose I a have a Widget class that is part of a framework used independently by many applications. I create Widget instances in many situations and their lifetimes vary. In addition to Widget's instance specified methods, I would like to be able to perform the follow class wide operations: Find a single Widget instance based on a unique id Iterate over the list of all Widgets Remove a widget from the set of all widgets In order support these operations, I have been considering two approaches: Container class - Create some container or manager class, WidgetContainer, which holds a list of all Widget instances, support iteration and provides methods for Widget addition, removal and lookup. For example in C#: public class WidgetContainer : IEnumerable<Widget { public void AddWidget(Widget); public Widget GetWidget(WidgetId id); public void RemoveWidget(WidgetId id); } Static class methods - Add static class methods to Widget. For example: public class Widget { public Widget(WidgetId id); public static Widget GetWidget(WidgetId id); public static void RemoveWidget(WidgetId id); public static IEnumerable<Widget AllWidgets(); } Using a container class has the added problem of how to access the container class. Make it a singleton?..yuck! Create some World object that provides access to all such container classes? I have seen many frameworks that use the container class approach, so what is the general consensus?

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• Rails class << self

  - by xpepermint

  Hey. I would like to understand what "class << self" stands for in the next example. module Utility class Options #:nodoc: class << self def parse(args) end end end end Thx!

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• class inheretence of a attribute which is itself a class

  - by alex

  i have a class which inherets a attribute from a super-class. this attribute is a class itself. class classA(superClass): def func(self,x): if self.attributeB is None: do somthing and in the other class i have class superClass: self.attributB = classB() i get the error AttributeError: class classA has no attribute 'attributeB' when i access the attribute like i showed but if on command line i can see it works, x = classA() x.attributeB is None True so the test works. whats going on in the above code?

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• query for inner join of table 4.

  - by amol kadam

  hi.... I'm facing the problem of inner join of table 4 following is query given plz see & give me solution select INSURED.FNAME + ' ' + INSURED.LNAME AS MNAME ,INSURED.MEMBCODE as MEMBERCODE ,INSURED.POLICYNO AS POLICYNO ,INSURED.POLICYFRMDATE AS POLICYFROMDATE ,INSURED.POLICYTODATE AS POLICYTODATE , MEMBERSHIP.MRKEXTNAME AS MARKETINGEXECUTIVE ,MEMBERSHIP.EMPLOYEECOUNT AS EMPLOYEECOUNT ,INSURED.CLAIMID AS CLAIMID ,POLICY.POLICYTYPE ,POLICY.COVAMTHOSPITAL as SUMINSURED ,ORGANIZATION.ORGANIZATIONNAME from ((INSURED inner join MEMBERSHIP on MEMBERSHIP.MEMBERSHIPID=INSURED.MEMBERSHIPID) inner join POLICY on MEMBERSHIP.POLICYNAME=POLICY.POLICYNAME) inner join ORGANIZATION on ORGANIZATION.ORGANIZATIONID=MEMBERSHIP.ORGANIZATIONID WHERE INSUREDID=427

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• Using a database class in my user class

  - by Josh

  In my project I have a database class that I use to handle all the MySQL stuff. It connects to a database, runs queries, catches errors and closes the connection. Now I need to create a members area on my site, and I was going to build a users class that would handle registration, logging in, password/username changes/resets and logging out. In this users class I need to use MySQL for obvious reasons... which is what my database class was made for. But I'm confused as to how I would use my database class in my users class. Would I want to create a new database object for my user class and then have it close whenever a method in that class is finished? Or do I somehow make a 'global' database class that can be used throughout my entire script (if this is the case I need help with that, no idea what to do there.) Thanks for any feedback you can give me.

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• Using a datanase class in my user class?

  - by Josh

  In my project I have a database class that I use to handle all the MySQL stuff. It connects to a database, runs queries, catches errors and closes the connection. Now I need to create a members area on my site, and I was going to build a users class that would handle registration, logging in, password/username changes/resets and logging out. In this users class I need to use MySQL for obvious reasons... which is what my database class was made for. But I'm confused as to how I would use my database class in my users class. Would I want to create a new database object for my user class and then have it close whenever a method in that class is finished? Or do I somehow make a 'global' database class that can be used throughout my entire script (if this is the case I need help with that, no idea what to do there.) Thanks for any feedback you can give me.

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• Can i access outer class objects in inner class

  - by Shantanu Gupta

  I have three classes like this. class A { public class innerB { //Do something } public class innerC { //trying to access objB here directly or indirectly over here. //I dont have to create an object of innerB, but to access the object created by A //i.e. innerB objInnerB = objB; //not like this innerB objInnerB= new innerB(); } public innerB objB{get;set;} } I want to access the object of class B in Class C that is created by class A. Is it possible somehow to make changes on object of Class A in Class C. Can i get Class A's object by creating event or anyhow.

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• Linking a template class using another template class (error LNK2001)

  - by Luís Guilherme

  I implemented the "Strategy" design pattern using an Abstract template class, and two subclasses. Goes like this: template <class T> class Neighbourhood { public: virtual void alter(std::vector<T>& array, int i1, int i2) = 0; }; and template <class T> class Swap : public Neighbourhood<T> { public: virtual void alter(std::vector<T>& array, int i1, int i2); }; There's another subclass, just like this one, and alter is implemented in the cpp file. Ok, fine! Now I declare another method, in another class (including neighbourhood header file, of course), like this: void lSearch(/*parameters*/, Neighbourhood<LotSolutionInformation> nhood); It compiles fine and cleanly. When starting to link, I get the following error: 1>SolverFV.obj : error LNK2001: unresolved external symbol "public: virtual void __thiscall lsc::Neighbourhood<class LotSolutionInformation>::alter(class std::vector<class LotSolutionInformation,class std::allocator<class LotSolutionInformation> > &,int,int)" (?alter@?$Neighbourhood@VLotSolutionInformation@@@lsc@@UAEXAAV?$vector@VLotSolutionInformation@@V?$allocator@VLotSolutionInformation@@@std@@@std@@HH@Z)

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• Inner Join with more than a field

  - by Leandro

  Precise to do a select with inner join that has relationship in more than a field among the tables Exemple: DataSet dt = new Select().From(SubConta.Schema) .InnerJoin(PlanoContabilSubConta.EmpSubContaColumn, SubConta.CodEmpColumn) .InnerJoin(PlanoContabilSubConta.FilSubContaColumn, SubConta.CodFilColumn) .InnerJoin(PlanoContabilSubConta.SubContaColumn, SubConta.TradutorColumn) .Where(PlanoContabilSubConta.Columns.EmpContabil).IsEqualTo(cEmp) .And(PlanoContabilSubConta.Columns.FilContabil).IsEqualTo(cFil) .And(PlanoContabilSubConta.Columns.Conta).IsEqualTo(cTrad) .ExecuteDataSet(); But the generated sql is wrong: exec sp_executesql N'/* GetDataSet() */ SELECT [dbo].[SubContas].[CodEmp], [dbo].[SubContas].[CodFil], [dbo].[SubContas].[Tradutor], [dbo].[SubContas].[Descricao], [dbo].[SubContas].[Inativa], [dbo].[SubContas].[DataImplantacao] FROM [dbo].[SubContas] INNER JOIN [dbo].[PlanoContabilSubContas] ON [dbo].[SubContas].[CodEmp] = [dbo].[PlanoContabilSubContas].[EmpSubConta] INNER JOIN [dbo].[PlanoContabilSubContas] ON [dbo].[SubContas].[CodFil] = [dbo].[PlanoContabilSubContas].[FilSubConta] INNER JOIN [dbo].[PlanoContabilSubContas] ON [dbo].[SubContas].[Tradutor] = [dbo].[PlanoContabilSubContas].[SubConta] WHERE EmpContabil = @EmpContabil0 AND FilContabil = @FilContabil1 AND Conta = @Conta2 ',N'@EmpContabil0 varchar(1),@FilContabil1 varchar(1),@Conta2 varchar(1)',@EmpContabil0='1',@FilContabil1='1',@Conta2='1' What should be made to generate this sql? exec sp_executesql N'/* GetDataSet() */ SELECT [dbo].[SubContas].[CodEmp], [dbo].[SubContas].[CodFil], [dbo].[SubContas].[Tradutor], [dbo].[SubContas].[Descricao], [dbo].[SubContas].[Inativa], [dbo].[SubContas].[DataImplantacao] FROM [dbo].[SubContas] INNER JOIN [dbo].[PlanoContabilSubContas] ON [dbo].[SubContas].[CodEmp] = [dbo].[PlanoContabilSubContas].[EmpSubConta] AND [dbo].[SubContas].[CodFil] = [dbo].[PlanoContabilSubContas].[FilSubConta] AND [dbo].[SubContas].[Tradutor] = [dbo].[PlanoContabilSubContas].[SubConta] WHERE EmpContabil = @EmpContabil0 AND FilContabil = @FilContabil1 AND Conta = @Conta2 ',N'@EmpContabil0 varchar(1),@FilContabil1 varchar(1),@Conta2 varchar(1)',@EmpContabil0='1',@FilContabil1='1',@Conta2='1'

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• Invoke a subclass method of an anonymous class

  - by arjacsoh

  I am trying right now to dig into anonymous classes and one question was just arised I 'd prefer not to refer to much details and to pose my question straightforward: How can I invoke the method sizzle() in the following anonymous class: public class Popcorn { public void pop() { System.out.println("popcorn"); } } class Food { Popcorn p = new Popcorn() { public void sizzle() { System.out.println("anonymous sizzling popcorn"); } public void pop() { System.out.println("anonymous popcorn"); } }; public void popIt() { p.pop(); // OK, Popcorn has a pop() method p.sizzle(); // Not Legal! Popcorn does not have sizzle() } } It is known and definite in polymorphism rules that a refernce of a superclass cannot invoke methods of subclass without downcasting (even if it refers to an object of the given subclass). However in the above case what is the "key" to invoke the sizzle() method?

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• Building a database class in PHP

  - by Sprottenwels

  I wonder if I should write a database class for my application, and if so, how to accomplish it? Over there on SO, a guy mentioned it should be written as an abstract class. However, I can't understand why this would be a benefit. Do I understand correctly, that if I would write an abstract class, every other class that methods will need a database connection, could simply extend this abstract class and have it's own database object? If so, how is this different from a "normal" class where I could instantiate an database object? Another method would be to completely forget about my own class and to instantiate a mysqli object on demand. What do you recommend?

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• Class design for calling "the same method" on different classes from one place

  - by betatester07

  Let me introduce my situation: I have Java EE application and in one package, I want to have classes which will act primarily as cache for some data from database, for example: class that will hold all articles for our website class that will hold all categories etc. Every class should have some update() method, which will update data for that class from database and also some other methods for data manipulation specific for that data type. Now, I would like to call update() method for all class instances (there will be exactly one class instance for every class) from one place. What is the best design?

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• Java: reusable encapsulation with interface, abstract class or inner classes?

  - by HH

  I try to encapsulate. Exeption from interface, static inner class working, non-static inner class not working, cannot understand terminology: nested classes, inner classes, nested interfaces, interface-abstract-class -- sounds too Repetitive! Exception 'illegal type' from interface apparently because values being constants(?!) static interface userInfo { File startingFile=new File("."); String startingPath="dummy"; try{ startingPath=startingFile.getCanonicalPath(); }catch(Exception e){e.printStackTrace();} } Working code but no succes with non-static inner class import java.io.*; import java.util.*; public class listTest{ public interface hello{String word="hello word from Interface!";} public static class hej{ hej(){} private String hejo="hello hallo from Static class with image"; public void printHallooo(){System.out.println(hejo);} } public class nonStatic{ nonStatic(){} //HOW TO USE IT? public void printNonStatic(){System.out.println("Inside static class with an image!");} } public static void main(String[] args){ //INTERFACE TEST System.out.println(hello.word); //INNNER CLASS STATIC TEST hej h=new hej(); h.printHallooo(); //INNER CLASS NON-STATIC TEST nonStatic ns=new nonStatic(); ns.printNonStatic(); //IS there a way to it without STATIC? } } Output The above code works but how non-staticly? Output: hello word from Interface! hello hallo from Static class with image! StaticPrint without an image of the class! Related Nesting classes inner classes? interfacses

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• How to easily substitute a Base class

  - by JTom

  Hi, I have the following hierarchy of classes class classOne { virtual void abstractMethod() = 0; }; class classTwo : public classOne { }; class classThree : public classTwo { }; All classOne, classTwo and classThree are abstract classes, and I have another class that is defining the pure virtual methods class classNonAbstract : public classThree { void abstractMethod(); // Couple of new methods void doIt(); void doItToo(); }; And right now I need it differently...I need it like class classNonAbstractOne : public classOne { void abstractMethod(); // Couple of new methods void doIt(); void doItToo(); }; class classNonAbstractTwo : public classTwo { void abstractMethod(); // Couple of new methods void doIt(); void doItToo(); }; and class classNonAbstractThree : public classThree { void abstractMethod(); // Couple of new methods void doIt(); void doItToo(); }; But all the nonAbstract classes have the same new methods, with the same code...and I would like to avoid copying all the methods and it's code to every nonAbstract class. How could I accomplish that? Hopefully it's understandable...

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• Using a class within a class?

  - by Josh

  I built myself a MySQL class which I use in all my projects. I'm about to start a project that is heavily based on user accounts and I plan on building my own class for this aswell. The thing is, a lot of the methods in the user class will be MySQL queries and methods from the MySQL class. For example, I have a method in my user class to update someone's password: class user() { function updatePassword($usrName, $newPass) { $con = mysql_connect('db_host', 'db_user', 'db_pass'); $sql = "UPDATE users SET password = '$newPass' WHERE username = '$userName'"; $res = mysql_query($sql, $con); if($res) return true; mysql_close($con); } } (I kind of rushed this so excuse any syntax errors :) ) As you can see that would use MySQL to update a users password, but without using my MySQL class, is this correct? I see no way in which I can use my MySQL class within my users class without it seeming dirty. Do I just use it the normal way like $DB = new DB();? That would mean including my mysql.class.php somewhere too... I'm quite confused about this, so any help would be appreciated, thanks.

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• Identifying that a variable is a new-style class in Python?

  - by Dave Johansen

  I'm using Python 2.x and I'm wondering if there's a way to tell if a variable is a new-style class? I know that if it's an old-style class that I can do the following to find out. import types class oldclass: pass def test(): o = oldclass() if type(o) is types.InstanceType: print 'Is old-style' else: print 'Is NOT old-style' But I haven't been able to find anything that works for new-style classes. I found this question, but the proposed solutions don't seem to work as expected, because simple values as are identified as classes. import inspect def newclass(object): pass def test(): n = newclass() if inspect.isclass(n): print 'Is class' else: print 'Is NOT class' if inspect.isclass(type(n)): print 'Is class' else: print 'Is NOT class' if inspect.isclass(type(1)): print 'Is class' else: print 'Is NOT class' if isinstance(n, object): print 'Is class' else: print 'Is NOT class' if isinstance(1, object): print 'Is class' else: print 'Is NOT class' So is there anyway to do something like this? Or is everything in Python just a class and there's no way to get around that?

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• How to get a Class literal from a generically specific Class

  - by h2g2java

  There are methods like these which require Class literals as argument. Collection<EmpInfo> emps = SomeSqlUtil.select( EmpInfo.class, "select * from emps"); or GWT.create(Razmataz.class); The problem presents itself when I need to supply generic specific classes like EmpInfo<String> Razmataz<Integer> The following would be wrong syntax Collection<EmpInfo<String>> emps = SomeSqlUtil.select( EmpInfo<String>.class, "select * from emps"); or GWT.create(Razmataz<Integer>.class); Because you cannot do syntax like Razmataz<Integer>.class So, how would I be able to squeeze a class literal out of EmpInfo<String> Razmataz<Integer> so that I could feed them as arguments to methods requiring Class literals? Further info Okay, I confess that I am asking this primarily for GWT. I have a pair of GWT RPC interface Razmataz. (FYI, GWT RPC interface has to be defined in server-client pairs). I plan to use the same interface pair for communicating whether it be String, Integer, Boolean, etc. GWT.create(Razmataz) for Razmataz<T> complains that, since I did not specify T, GWT compiler treated it as Object. Then GWT compiler would not accept Object class. It needs to be more specific than being an Object. So, it seems there is no way for me to tell GWT.create what T is because a Class literal is a runtime concept while generics is a compile time concept, Right?

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