C++/CLI :
public interface class ITest{
public:
virtual void doSomething (){
}
}
public ref Base {
...........
...........
}
generic <typename T> where T : ITest
public ref Derived : Base{
public:
virtual void doNothing (){
}
}
public ref AnotherClass {
public:
generic<class T> where T : Base
static int justDoThis(){
//Problem!!
}
}
C# :
In C# there are two classes A and B. A inherits from the ITest and B inherits from Derived where A is used as the typename. Also, B has a private variable of type A. So, from main function AnotherClass.justDoThis<B>() is called where B is passed as the generic type.
"//Problem!!" Part :
Now I have to create a new instance of B in this section and also access the A which is private variable in B.
So if I take your paragraph of description of the C# code:
class A : ITest {}
class B : Derived<A>
{
private A someVariableOfTypeA;
}
class Program
{
void Main(string[] args)
{
AnotherClass.justDoThis<B>();
}
}
And the problem is that you want to do this:
public ref AnotherClass {
public:
generic<class T> where T : Base
static int justDoThis()
{
// Problem!!
Something^ actuallyB = gcnew Something();
A^ a = actuallyB->someVariableOfTypeA;
}
}
Issue #1: You can allow creation of new objects of the generic type by specifying gcnew as another generic constraint. (In C#, this would be new.) This will require that the generic type have a default (i.e., parameterless) constructor, which you can access with the normal gcnew.
generic<class T> where T : Base, gcnew
static int justDoThis()
{
T^ t = gcnew T();
}
Issue #2: You cannot access private variables within an object. That's what private means. If you want to give justDoThis access to the A object, then add an appropriate public method or property to Base. The method or property would return type ITest. You could also put that method/property on a new interface (perhaps named IHaveAnITestAccessorMethod), and add that as another generic constraint, and B satisfies all the constraints.
Note that it won't do any good to make the variable public on type B: justDoThis doesn't know about B, it only knows about T, which is a Base with a no parameter constructor.
Disclaimers:
I didn't check my syntax with a compiler.
Yes, you can do anything with reflection, but that's a bad design. Don't do that, fix your code the right way.
Related
Why I cannot do that? I get compilation error.
public interface A {
void update(Object a);
}
public class B implements A{
void update(Long a) {
}
}
That is Java 8
I do not see here violating any OO principle.
That's really make my time difficult to implement a generic API...
(I try to get Generics out of the play because the generic API gets counter-intuitive)
You get a compilation error because void update(Long a) does not implement void update(Object a).
You can implement it as follows:
public class B implements A {
void update(Object o) {
if (!(o instanceof Long)) {
// possibly throw an exception
}
Long a = (Long) o;
...
}
}
A class that implements an interface must implement all the methods declared in the interface. The methods must have the exact same signature (name + parameters) as declared in the interface. The class does not need to implement (declare) the variables of an interface. Only the methods.
Source:
http://tutorials.jenkov.com/java/interfaces.html#:~:text=A%20class%20that%20implements%20an,Only%20the%20methods.
I'm new to OSGI framework and I'm trying to access the 'Derived' Class variable 'publicVariable' from another class 'Derived2' like "Derived.publicVariable" but publicVariable is always shows null. I really appreciate if someone can help me out with this.
Thanks
Manifest file - Derived2
Require-Bundle:com.xxxxxx.Derived1
Java code
abstract class Base {
protected Vector <String> supportedCommands = new Vector <String> ();
protected abstract void initialiseCommands();
}
class Derived extends Base {
private static Derived derivedPlugin = null;
public Derived()
{
derivedPlugin = this;
}
public static Derived getPlugin()
{
return derivedPlugin;
}
public String publicVariable = null;
protected void initialiseCommands()
{
publicVariable = "someData";
System.out.println("Derived" + publicVariable);
}
}
class Derived2 extends Base {
protected void initialiseCommands()
{
supportedCommands.add(Derived.getPlugin().publicVariable);
System.out.println("IMRSAUtilitiesPlugin" +supportedCommands);
}
Also referred below link, which is a similar issue but i'm not using any static variable, it is just a public variable.
how use Singleton object in different class loader....?
The code in the question will not compile. You are trying to access an instance field (publicVariable in class Derived) in a static way, i.e. Derived.publicVariable.
OSGi does not change the semantics of the Java language, and if you cannot even compile your code then OSGi will certainly not be able to run it.
I'm developing an application which is using a library and I would like to wrap this library so that it does not goes to deep into my application code. Thanks to that I could change the library I'm using just by re-implementing my wrapper classes.
Suppose that I have a library LibA. It gives me 2 objects to work with, LibAObj1 and LibAObj2. LibAObj2 has a method using LibAObj1.
Here can be a simple definition of their declaration
class LibAObj1 {};
class LibAObj2
{
void action(LibAObj1 &obj);
};
Now I would like to define an interface that my application can use to wrap those objects in my application code
For instance:
class ItfLibAObj1 {};
class ItfLibAObj2
{
public:
void action(ItfLibAObj1 &obj) = 0;
};
The problem comes whenever I want to implement my interface ItfLibAObj2.
class ImplLibAObj2 : public ItfLibAObj2
{
public:
void action(ItfLibAObj1 &itfObj)
{
<how to get my LibAObj1 from itfObj>?
obj.action(LibAObj1);
}
private:
LibObj2 obj;
}
The question is actually in the pseudo code. How to get my LibAObj1 contained in my ItfLibAObj1 reference? I could add a getter function in LibAObj1 interface to return a void pointer that I would cast but I don't find that elegant in C++.
Is there any kind of design pattern I could use to solve my problem? Or do I just have a design issue?
Note that I'm not wishing to select which library to use at run time.
Thanks a lot for your help.
Kind regards
You problem perfectly explains why in Proxy Pattern, both the Real and Proxy must implement the same interface:
And your code should look like this:
// interfaces
class ItfLibAObj1 {};
class ItfLibAObj2
{
public:
void action(ItfLibAObj1 &obj) = 0;
};
// real
class RealLibAObj1 : public ItfLibAObj1 {};
class RealLibAObj2 : public ItfLibAObj2
{
void action(ItfLibAObj1 &obj)
{
...
}
};
// proxy
class ProxyLibAObj1 : public ItfLibAObj1
{
private:
RealLibAObj1 real;
};
class ProxyLibAObj2 : public ItfLibAObj2
{
private:
RealLibAObj2 real;
void action(ItfLibAObj1 &obj)
{
// do something
real.action(obj); // delegate to the real
// do something
}
};
However, if the whole purpose of your "wrapping" is adding a new layer between your core/real and the outside (client), please consider the Facade Pattern which provides a simpler interface to the client, instead of merely mimic the classes/methods of the core.
I'm in a method of a derived class, loosely as follows:
Class Base
{
private:
int variableIWantToAccess;
}
Class Derived : public Base
{
public someMethod() {
variableIWantToAccess++; <<-----ERROR
}
How do I access the variable that's declared in the base class? I'm unable to access it because it is private.
You should declare it as protected instead of private.
Protected members of a class are accessible for the class descendants only.
Leave the field private and create a pair of protected getter / setter methods instead (for the same reasons you wouldn't expose a public field).
Class Base
{
private:
int variableIWantToAccess;
protected:
int GetVariable() { return variableIWantToAccess; }
void SetVariable(int var) { variableIWantToAccess = var; }
}
I am trying to find a way to have a factory class / method that would take in an object or some kind of identifier (string or type) then based off the input parameter determine which implementation of the interface to create and return.
how do I setup my factory method and register the dependency for the interface? following is what I have roughly.
public class ISampleFactory
{
public ISample GetSample(Type type)
{
// do something here to return an implementation of ISample
}
}
public class SampleA : ISample
{
public void DoSomething();
}
public class SampleB : ISample
{
public void DoSomething();
}
public interface ISample
{
void DoSomethin();
}
Have a look at ninject Contextual Bindings Documentation:
You can either use Named Bindings:
this.Bind<ISample>().To<SampleA>().Named("A");
this.Bind<ISample>().To<SampleB>().Named("B");
or a conditional binding with any of the already available extensions or write your own:
this.Bind<ISample>().To<SampleA>().When(...);
this.Bind<ISample>().To<SampleB>().When(...);
see https://github.com/ninject/ninject/wiki/Contextual-Binding