[newbe] casting at run time

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polilop - 11 Jan 2007 22:18 GMT

i have next set of code
interface A{}
class B implements A{something}
clas C implements A{something}

class D
{

public doSomethingWithTypeA(A aClass)

/***So what i do is***/
if(B instanceof A)
cast A to B
if(C instanceof A)
cast A to C
}

main
B b=new B();
D d=new D();
d.doSomethingWithTypeA(b);

is it possible to get the class type from A, and then cast to eather B or C
without checking instanceof (in runtime)?
Thanks

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Tom Hawtin - 11 Jan 2007 22:54 GMT

> interface A{}
> class B implements A{something}
[quoted text clipped - 13 lines]
> is it possible to get the class type from A, and then cast to eather B or C
> without checking instanceof (in runtime)?

It's much better to design your types to use polymorphism, rather than
attempting to switch on type.

interface A {
void doSomething();
}
class B implements A {
public void doSomething() { ...}
}
class C implements A {
public void doSomething() { ...}
}

static void doSomethingWithTypeA(A a) {
a.doSomething();
}

If you must check and cast, then the syntax is:

static void doSomethingWithTypeA(A a) {
if (a instanceof B) {
B b = (B)a;
...
} else if (a instanceof C) {
C c = (C)a;
...
} else {
throw new Error("Bad design causes errors...");
}
}

Tom Hawtin

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Lew - 12 Jan 2007 00:50 GMT

> class D
> {
[quoted text clipped - 12 lines]
> D d=new D();
> d.doSomethingWithTypeA(b);

Your code as posted will not compile.

Create a real example. Try it before you post it.

You cannot invoke the "instanceof" operator with a type as the left operand.

The variable name "aClass" is misleading since the variable is not of type
Class but of type A. It is bad practice to embed type information in a
variable name.

- Lew

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polilop - 12 Jan 2007 07:13 GMT

Sorry for me being hasty, here is the real code

public interface J2EEVo {
public abstract boolean isSuccess();
public abstract void setSuccess(boolean success);
}
public class UsersVo implements J2EEVo
{
private String USERNAME;
private String FAMILYNAME;
private boolean success;

.. a class constructor and set get methods
}
public class ReportVo implements J2EEVo
{
private String HEADER;
private String FOOTER;
private boolean success;

.. add class constructor and set get methods
}

public class ResultsetToVo {

public J2EEVo fillVo(J2EEVo uvo,ResultSet rs) throws SQLException
{
if(uvo instanceof UsersVo) {
uvo = (UsersVo)uvo;
}
else if(uvo instanceof ReportVo )
{
uvo=(ReportVo)uvo;
}
}

To answer Tom Hawtin: I am aware of instanceof, but is there another way to
cast the uvo at run time, so that i don't have to add a new if (uvo
insanceof someVo), every time a write a new J2EEVo that needs to use
ResultsetToVo. Something like (just a thought):

uvo =(getUvoClassInstance(uvo))uvo
Thanks

>> class D
>> {
[quoted text clipped - 25 lines]
>
> - Lew

Reply to this Message

Lew - 13 Jan 2007 18:48 GMT

Please do not top post.

> Sorry for me being hasty, here is the real code
> public class UsersVo implements J2EEVo
[quoted text clipped - 13 lines]
> .. add class constructor and set get methods
> }

By convention, non-constant variable (and method) names should begin with a
lower-case letter and use camel case. All-upper-case names are reserved for
static final variables used as class constants.

> public class ResultsetToVo {
>
[quoted text clipped - 8 lines]
> }
> }

The downcasts accomplish exactly nothing. The declared type of uvo will not
change, and it already knows its own runtime type.

> To answer Tom Hawtin: I am aware of instanceof, but is there another way to
> cast the uvo at run time, so that i don't have to add a new if (uvo
> insanceof someVo), every time a write a new J2EEVo that needs to use
> ResultsetToVo. Something like (just a thought):

You might need to rethink the design of fillVo(). Declaring it to take J2EEVo
arguments implies that it is only interested in the interface behaviors. The
downcasts tell us that that is a lie, the method really does care about the
implementation type (aside from the fact that you throw away the result of the
downcast in your code). You shouldn't take both points of view in the same code.

You might consider making fillVo() part of the interface, naturally without
the "uvo" argument. Each overriding class will implement fillVo() knowing full
well that the implementing type is itself. This is "polypmorphism", which is a
key concept to good (object-oriented) design.

A hint that this applies is your explicit use of the uvo argument in
fill...(), which would be the implicit "this" argument in an instance method.

Study the idea. You will end up with something similar to:

public interface J2EEVo {
public boolean isSuccess();
public void setSuccess(boolean success);
public void fill( ResultSet rs );
}

- Lew

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polilop - 15 Jan 2007 10:53 GMT

> Please do not top post.
>
[quoted text clipped - 66 lines]
>
> - Lew

sorry

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polilop - 12 Jan 2007 11:38 GMT

I thought about it, and rewrote my code, so now simpler and works. thanks
for help

>> class D
>> {
[quoted text clipped - 25 lines]
>
> - Lew

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[newbe] casting at run time

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