Base class/Derived class

Hi all,
I have
class Base {
int a;
class Derived{
int b;
now if i do
Base X = new Base();
Derived D = new Derived();
It Doesn't allow me to do D=X
But it does allow me to do X=D.
whats the reason behind it. And what actually happens when i do X=D , does the copy constructor gets called ?
Thanx

Manthana wrote:
Hi, u said
the value of X becomes the reference to the instance of Derived that you created when you called "new Derived()".
So now i can access attributes of derived class from the reference X right X.b
But i cant do X.b
Why?Please spell out words like "you". It makes things easier to read. :)
With Derived extending Base, you could say:
X = D;
X.a = 5;Because the compiler and runtime see X as a reference to a Base object, you can only access variables/methods defined in Base. Since X is declared as being a reference to Base, you always know that 'a' is defined. 'b' wouldn't be defined for X if you said:
X = new Base();
X.b = 52;So, in order to access the b, you'd have to access it with something declared as a reference to Derived, such as 'D':
D.b = 10;You could say:
X = D; // Line 1
Derived d2 = (Derived)X;
d2.b = 10;But, then you'll get a ClassCastException if X is only referring to a Base object [e.g., if Line 1 was "X = new Base();"], not to a Derived object.

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Is nested class = derived class?

When I am playing around with nestedclasses, I found an interesting thing:
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cyclic inheritance involving NestedInterfaceDemo
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Hello caspar again, so I retry helping you in another thread, perhaps I overreacted or misinterpreted something in the other thread. You know, my english is not the best. Perhaps I did not get you right.
ANd if I change the Outer class to something else,
say Outer.Inner inner = new BlaBla.Inner(),
the compiler will say it cant find package BlaBla.
so this kinda tells me the nestedclass has a package
of a name of its outerclass. so the inner class fully
qualified name would be Outer.Inner
Assume we have the following:
package mypackage;
class Outer {
class Inner {
}The full qualified class name of Inner is mypackage.Outer.Inner.
The java compiler cannot tell only from syntax if an element in such expression is a package or class or an inner class. It says it cant find package BlaBla in your example, because it assumed it would be a package. A better error message of the compiler would have been: "cannot find package or class BlaBla".
Beg the compiler to be more clear ;-)
This can be a little confusion with inner classes, but there is a naming convention that helps us to distinguish: class names start with upper letters, packages with lower. But still the compiler cannot know.
I hope this could help at least a little.
From this, it should imply that the compiler
can resolve the i1 interface, but does the
"cyclic inheritance" mean that i1 interface also
inherits from NestedInterfaceDemo??No, i don't know, why Java does not allow this.
if does not inherit from NestedInterfaceDemo.
Your construction could work in runtime if Java would allow.
Does anybody know any reason except for bad style?
regards

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virtual void markRead() const;

The Sun C++ FAQ discusses the warning message:
http://developers.sun.com/prodtech/cc/documentation/ss11/mr/READMEs/c++_faq.html#Coding1
Notice that warnings are not necessarily errors. But I applaud your desire to fix the code so that it generates no warnings. I wish more of our customers could be persuaded to do the same. :-)
C++ 5.3 issues this warning, by the way. Example:
struct B { virtual int foo(int); };
struct D : B { virtual int foo(double); }; // line 2
D d;
line 2: Warning: D::foo hides the virtual function B::foo(int).
If for your particular code you do not see a warning with C++ 5.3, it would be due to a bug in C++ 5.3 that was later fixed.

How to call derived class to base class

Hello everybody,
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by
(kamal)

Sorry, I've got major difficulties understanding your query:
I create a GUi application in java
ication in java swing. ok
Now i want to navigate between
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bcoz i imported the class from package to
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Does the Java Language Specification explicitly allow setting of fields of a derived class from within the base class' constructor via reflection? The following test case runs green, but I would really like to know if this Java code is compatible among different VM implementations.
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public class DerivedClassReflectionWorksInBaseClassConstructorTest extends TestCase {
abstract static class A {
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Framework:
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}Plug-In:
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Base class vs derived class

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Flex/AS3 Best way to construct a derived class instance from an existing base class instance?

What is the best way to handle the instantiation of a derived class from an existing base class.
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[RemoteClass (alias="com.myco...')]
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In my Cairgorm command which retrieves this object from ther server I want to do this:
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Thanks for the assistance. Let me try to clarify.
MY UI requires a number of composite classes.    The individual components of the composite classes are being transfered to/from the server at different times depending upone which component has changed state.    The construction of the composite classes from the base class happens in my clients business logic.
Composition happens in a derived class; but server syncronization happens using the base class.    When I recieve the object from Blazeds through the remote object event, it is in the form of the base class. I then need to instantiate the derived class and copy the elements of the base class into it (for later composite construction).   And likewise when sending the base class back to the server, I need to upcast the derived class to its base class.   But in this case just a mere upcast does not work. I actually need to create a new base class and copy the attrbutes into it. I believe this is limitation of how remoting works on Flex/AS3.
My question is, what is the best way to turn my base class into it's derived class so further composite construction can take place.   The way I am currently doing it is to create a load method on the base class, that takes the base class as on argument. The load function, copies all of the instance attribute references from the base class to the target class.
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Then, after I recieve the base class from the server.   I create a new derived class and pass the base class into the load function like this:
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Thanks for your help.

Why cannnot derived class pointer point to a base class object?

Hi,
Pleaseeee... explain me why cannot the derived class pointer point to a base class object.
I know that Base class pointer can point to a derived class object.
Thanks & Regards,
Vig....

Example:
class Base
{ public: void foo(); } * pBase;
class Derived : public Base
{ public: void bar(); } * pDerived;
Now, what would happen if you assign pDerived = new Base() and then call pDerived->bar()?By forbidding such an assignment, compiler prevents you from writing error-prone code.

Casting base class object to derived class object

interface myinterface
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class Base implements myinterface
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class Derived extends Base
public class File1
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     public static void main(String args[])
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Giving the exception ClassCastException......
Can't we convert from base class object to derived class.
Can any one help me please...
Thnaks in Advance
Bharath kumar.

When posting code, please use tags
The object returned by File1.fun() is of type Base. You cannot cast an object to something it isn't - in this case, Base isn't Dervied. If you added some member variables to Derived, and the compiler allowed your cast from Base to Derived to succeed, where would these member variables come from?
Also, you don't need the cast to myinterface in File1.fun()
Also, normal Java coding conventions recommend naming classes and interfaces (in this case, myinterface) with leading capital letters, and camel-case caps throughout (e.g. MyInterface)

Can a derived class catch base class object

can a derived class object catch a base class object
e-g if i have a class
class Test {
// code
}and
class Test2 extends Test {
}and lets say there is a method that return an object of Test (base class), so is it possible to write
Test2 t=getTestObj(); // i-e subclass holding base class objectif its not.. how can we do so? so that derived class should hold base class object....

sorry..
here is how it goes
i have a function a class
public Test getObj() {
// code
}The constraint is that i cant change the implementation of this above function.
Now when i call this function, it returns me a Test object. But i cant do
Test t=getObj();because the Test class is written purely in java whereas i want to use write a new Test class that implements the same functionality [using the above given function] in JNI.
so what i did, i created a new package [nativ] and created a class "Test" in that. But since i have to use the function that returns me Test and not "nativ.Test", so what i did, i inherited nativ.Test from "Test" so that the object might resolve the reference.
So this is it.. that i cant modify the function and i need some way to do it.. plz reply

Managing constants in base / derived classes

Hi there,
The problem:
I'm facing what is certainly a simple problem but fail to come up with an acceptable solution. What I'd like to do is managing constants in a base class and derived class... which for some reasons are not parts of the same package. Also, these constants may have different values in the derived base.
Down to earth description:
Class base uses a set of constants A=1, B=2, C=3.
Class derived should be provided with similarly named constants but they may have different values A=10, B=20, C=30. I could declared these constants in the classes themselves but somehow I think this is ugly.
A "solution" that does not work:
Assume for a moment that I'm just a very naive programmer (which actually I am... but anyway...). I would them come up with something like:
package base;
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package derived;
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Bonus question:
I'm not a pro in OOP (seriously guys... C rocks! Oops, wrong forum :-) but it seems to be a case of "code inheritance" vs "behaviour inheritance" (I just made those expressions up!). What I mean is: if what is inherited is the actual "code" of the methods (as if a huge copy paste took place) then the naive solution would work. But in the real world, a "behaviour inheritance" is at work in the sense that what is inherited is the "behaviour" (or, more bluntly, simply the code as compiled in the base class). Hmm... Am I making any sense here? Is that distinction theorized in some way? I tried to search a bit by myself but did not know where to start.
Thanks,

I mucked around with this exact problem a while back.
The (simplified) scenario is AbstractCourse is extended by CookingCourse, WritingCourse, and BusinessCourse. Every course has a basePrice and a materialsCost... both of which vary from course to course... The same price calculation applies to all types of course, just the amounts will vary.
totalPrice = basePrice + materialsCost The traditional non-oo solution is simple enough... you would just create a "table" of all six contstants, and a function with a switch statement...
but How to do this "The OO Way"?
What I came up with is:
class BaseCourse {
abstract double getBasePrice();
abstract double getMaterialsCost();
public double getPrice() {
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private static final double BASE_PRICE = 300.00;
private static final double MATERIALS_COST = 112.60;
public double getBasePrice() { return CookingCourse.BASE_PRICE }
public double getMaterialsCost() { return CookingCourse.MATERIALS_COST }
public class WritingCourse {
private static final double BASE_PRICE = 200.00;
private static final double MATERIALS_COST = 50.00;
public double getBasePrice() { return WritingCourse.BASE_PRICE }
public double getMaterialsCost() { return WritingCourse.MATERIALS_COST }
public class BusinessCourse {
private static final double BASE_PRICE = 600.00;
private static final double MATERIALS_COST = 18.12;
public double getBasePrice() { return BusinessCourse.BASE_PRICE }
public double getMaterialsCost() { return BusinessCourse.MATERIALS_COST }
}The base class specifies that each subclass must be able to tell it's base price, and it's materials cost... then we implement the calculation on those values just once, in the base class.
For all I know there are much more succinct, flexible, efficient, and basically much more smarter ways of doing this... this is just a way that worked for me... and it's simple enough so even I can follow it.
Java can be a very very very very verbose language :-( ... But hey it still shits on C ;-)
Cheers. Keith.
Edited by: corlettk on 23/05/2008 11:17 - typos

Problems using different tables for base class and derived class

I have a class named SuperProject and another class Project derived from
it. If I let SchemaTool generate the tables without specifying a "table"
extension, I get a single TABLE with all the columns from both classes and
everything works fine. But if I specify a "table" for the derived class,
SchemaTool generates the derived class with just one column (corresponds
to the attribute in derived class). Also it causes problems in using the
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JDO file:
<jdo>
<package name="jdo">
<class name="Project" identity-type="application"
persistence-capable-superclass="SuperProject">
<extension vendor-name="kodo" key="table" value="PROJECT"/>
</class>
<class name="SuperProject" identity-type="application"
objectid-class="ProjectId">
<field name="id" primary-key="true"/>
</class>
</package>
</jdo>
java classes:
public class Project extends SuperProject
String projectSpecific
public class SuperProject
BigDecimal id;
String name;
tables generated by SchemaTool:
TABLE SUPERPROJECTSX (IDX, JDOCLASSX, JDOLOCKX, NAMEX);
TABLE PROJECT(PROJECTSPECIFICX)
Thanks,
Justine Thomas

Justine,
This will be resolved in 2.3.4, to be released later this evening.
-Patrick
In article <aofo2q$mih$[email protected]>, Justine Thomas wrote:
I have a class named SuperProject and another class Project derived from
it. If I let SchemaTool generate the tables without specifying a "table"
extension, I get a single TABLE with all the columns from both classes and
everything works fine. But if I specify a "table" for the derived class,
SchemaTool generates the derived class with just one column (corresponds
to the attribute in derived class). Also it causes problems in using the
Project class in collection attributes.
JDO file:
<jdo>
<package name="jdo">
<class name="Project" identity-type="application"
persistence-capable-superclass="SuperProject">
<extension vendor-name="kodo" key="table" value="PROJECT"/>
</class>
<class name="SuperProject" identity-type="application"
objectid-class="ProjectId">
<field name="id" primary-key="true"/>
</class>
</package>
</jdo>
java classes:
public class Project extends SuperProject
String projectSpecific
public class SuperProject
BigDecimal id;
String name;
tables generated by SchemaTool:
TABLE SUPERPROJECTSX (IDX, JDOCLASSX, JDOLOCKX, NAMEX);
TABLE PROJECT(PROJECTSPECIFICX)
Thanks,
Justine Thomas
Patrick Linskey [email protected]
SolarMetric Inc. http://www.solarmetric.com

My derived class suddenly doesn't recognize the parent class

I had my assignment coded, debugged, working and ready to go. So I bring my flash drive upstairs to play around with it on my other computer. When I compile the base class it compiles fine, but now when I try to compile the derived class, and the test class with the main method, I'm coming up with errors.
They have something to do with the base class not being recognized, because I can't instantiate any objects of the class now, and i get an error when attempting to compile the derived class to the effect of "cannot find symbol; symbol: class BaseClass". i get this error throughout the derived class, from the initial declaration of the class (public class DerivedClass extends BaseClass), to any references made toward methods of the parent class.
My base class compiles with no problem, but it's like nothing else is able to "use" it. I'm not sure if it has something to do with the PC im using now, and I can't confirm this at the moment because the PC I used to write the code is down (waiting on an upgrade).
ehhh.....help.
Message was edited by:
asphaltninja
null

It's not the fault of the computer hardware.
It's not the fault of the operating system.
It's not the fault of the JDK.
So don't go rebooting or reinstalling anything. The problem is that you didn't set up things on the new computer in the same way they were set up on the old computer.
In particular the compiling problem is that the compiled version of BaseClass isn't in the classpath when you try to compile DerivedClass. I can't tell why from your description.

How to load function from derived class from dll

Dear all,
how to access extra function from derived class.
for Example
//==========================MyIShape.h
class CMyIShape
public:
CMyIShape(){};
virtual ~CMyIShape(){};
virtual void Fn_DrawMe(){};
// =========== this is ShapRectangle.dll
//==========================ShapRectangle .h
#include "MyIShape.h"
class DLL_API ShapRectangle :public CMyIShape
public:
ShapRectangle(){};
virtual ~ShapRectangle(){};
virtual void Fn_DrawMe(){/*something here */};
virtual void Fn_ChangeMe(){/*something here */};
__declspec (dllexport) CMyIShape* CreateShape()
// call the constructor of the actual implementation
CMyIShape * m_Obj = new ShapRectangle();
// return the created function
return m_Obj;
// =========== this is ShapCircle .dll
//==========================ShapCircle .h
#include "MyIShape.h"
class DLL_API ShapCircle :public CMyIShape
public:
ShapCircle(){};
virtual ~ShapCircle(){};
virtual void Fn_DrawMe(){/*something here */};
virtual void Fn_GetRadious(){/*something here */};
__declspec (dllexport) CMyIShape* CreateShape()
// call the constructor of the actual implementation
CMyIShape * m_Obj = new ShapCircle();
// return the created function
return m_Obj;
in exe there is no include header of of ShapCircle and ShapRectangle
and from the exe i use LoadLibrary and GetProcAddress .
typedef CMyIShape* (*CREATE_OBJECT) ();
CMyIShape*xCls ;
//===================== from ShapeCircle.Dll
pReg= (CREATE_OBJECT)GetProcAddress (hInst ,"CreateShape");
xCls = pReg();
now xCls give all access of base class. but how to get pointer of funciton Fn_GetRadious() or how to get access.
thanks in advance.

could you please tell me in detail. why so. or any reference for it. i love to read.
i don't know this is bad way.. but how? i would like to know.
I indicated in the second sentence. Classes can be implemented differently by different compilers. For example, the alignment of member variables may differ. Also there is the pitfall that a class may be allocated within the DLL but deallocated in the client
code. But the allocation/deallocation algorithms may differ across different compilers, and certainly between DEBUG and RELEASE mode. This means that you must ensure that if the DLL is compiled in Visual Studio 2010 / Debug mode, that the client code is also
compiled in Visual Studio 2010 / Debug mode. Otherwise your program will be subject to mysterious crashes.
is there any other way to archive same goal?
Of course. DLL functionality should be exposed as a set of functions that accept and return POD data types. "POD" means "plain-ole-data" such as long, wchar_t*, bool, etc. Don't pass pointers to classes.
Obviously classes can be implemented within the DLL but they should be kept completely contained within the DLL. You might, for example, expose a function to allocate a class internally to the DLL and another function that can be called by the client code
to free the class. And of course you can define other functions that can be used by the client code to indirectly call the class's methods.
and why i need to give header file of ShapCircle and shapRectangle class, even i am not using in exe too. i though it is enough to give only MyIShape.h so with this any one can make new object.
Indeed you don't have to, if you only want to call the public properties and methods that are defined within MyIShape.h.

Implements interface method at the derived class

Hi all.
I have a class (Derived) that extends another class (Base).
In the base class (Base) there is a method f() with its implementation.
In the interface (C) there is also f() method exactlly like in the base class (Base).
The derived class (Derived) is implements the interface (C).
My question is:
Do i have to implement the method f() in the derived class (Derived) ?

My guess is that you probably have to, even if it's just to call the parent's method.
This all sounds pretty sketchy. Why don't you just make the BASE class implement your interface?

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