Wrapper for multiple instanceof

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getsanjay.sharma@gmail.com - 22 Aug 2007 02:01 GMT

Hello to all Java programmers out there.

Recently I have been trying to do a multiple instanceof and thought it
would be better to write a function than do those ugly || with
repeated instanceofs'.

public class MyInstance
{
public static boolean check(Object obj, String...classes) throws
Exception
{
boolean result = false;
for(String str : classes)
{
Class c = Class.forName(str);
result = result | (obj instanceof c);
if(result) break;
}
return(result);
}

public static void main(String args[]) throws Exception
{
String str = "hello";
String classes[] = {"String", "StringBuffer"};
System.out.println(MyInstance.check(str, classes));

}
}

I do very well realize that the instanceof operator requires a TYPE on
its RHS, but I can't seem to find out how to convert the class name
string to a type?

Is such kind of thing even possible here or doing multiple || with
instanceofs' is the only way to go?

Thanks and regards,
STS

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Joshua Cranmer - 22 Aug 2007 02:22 GMT

> Recently I have been trying to do a multiple instanceof and thought it
> would be better to write a function than do those ugly || with
> repeated instanceofs'.

Most instanceof's can be replaced with good OOP design.

> public class MyInstance
> {
[quoted text clipped - 9 lines]
> }
> return(result);

Err.. ugly and uncompilable? The proper or for a boolean variable is ||
(short circuit boolean or) as opposed to | (bitwise or). This isn't C(++).

correct code:

for (String str : classes) {
Class<?> c = Class.forName(str);
if (c.isInstance(obj))
return true;
}
return false;

Even better would be to redeclare check as:
public static boolean check(Object obj, Class... classes)

circumventing the reflection needed.

> }
>
[quoted text clipped - 3 lines]
> String classes[] = {"String", "StringBuffer"};
> System.out.println(MyInstance.check(str, classes));

Use varargs properly:
System.out.println(MyInstance.check(str,"String","StringBuffer"));

Or, using redefined check:
System.out.println(MyInstance.check(str,String.class,StringBuffer.class));

> }
> }
>
> I do very well realize that the instanceof operator requires a TYPE on
> its RHS, but I can't seem to find out how to convert the class name
> string to a type?

By the nature of the underlying bytecode, the RHS of the instanceof
operator has to be known at compile time, rendering the ability to do
runtime loading impossible without using reflection. You started using
the Class class but seemed to have dropped off before getting to the
important part (it does have a method named instanceOf, after all).

> Is such kind of thing even possible here or doing multiple || with
> instanceofs' is the only way to go?

See above. I recommend, however, that the code be redesigned to avoid
using instanceof's if possible.

Signature

Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

Reply to this Message

Patricia Shanahan - 22 Aug 2007 02:31 GMT

>> Recently I have been trying to do a multiple instanceof and thought it
>> would be better to write a function than do those ugly || with
[quoted text clipped - 18 lines]
> Err.. ugly and uncompilable? The proper or for a boolean variable is ||
> (short circuit boolean or) as opposed to | (bitwise or). This isn't C(++).

"|", in Java, is not just bitwise or. It is also a correct, but rarely
used, boolean operator, performing the same operation as "||" but always
evaluating the right hand side. See
http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.22.2

In this case, it makes no difference which of "|" or "||" is used. The
left hand side is always false so the right hand side has to be
evaluated even with "||".

However, it is a good idea to make a habit of using "||" unless there
is a reason to force evaluation of the right hand side when the left
hand side is true.

Patricia

Reply to this Message

Lew - 22 Aug 2007 02:47 GMT

getsanjay.sharma@gmail.com wrote:

>>> Recently I have been trying to do a multiple instanceof and thought it
>>> would be better to write a function than do those ugly || with
[quoted text clipped - 15 lines]
>>> }
>>> return(result);

This idiom is a red flag that polymorphism should be used instead.

Define an interface, I'll call it "DoesSomething", with a method, say,
"doSomething()":

public interface DoesSomething
{
public void doSomething();
}

Then define implementing classes specific to your situation:

public class HelloWorld implements DoesSomething
{
private final String something;
public HelloWorld( String s )
{
something = s;
}
public void doSomething()
{
System.out.println( something );
}
}

In some client class you would have code like this:

DoesSomething doer = Factory.getDoesSomething();
// this factory method will deliver some implementation
// of DoesSomething depending on environment or data

doer.doSomething();

Notice the client code doesn't "know" that doer is "really" a HelloWorld, or
some other implementing class. It just lets the object "know" what to do for
itself. No run-time type checking needed on your part.

This idiom is at the heart of object-oriented programming.

Signature

Lew

Reply to this Message

Roedy Green - 22 Aug 2007 04:50 GMT

>I do very well realize that the instanceof operator requires a TYPE on
>its RHS, but I can't seem to find out how to convert the class name
>string to a type?

See http://mindprod.com/jgloss/classforname.html

There are dynamic and static versions of instanceof.

Signature

Roedy Green Canadian Mind Products
The Java Glossary
http://mindprod.com

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Bent C Dalager - 22 Aug 2007 10:27 GMT

>I do very well realize that the instanceof operator requires a TYPE on
>its RHS, but I can't seem to find out how to convert the class name
>string to a type?

Class stringClass = String.class;

Then you can use Class.isAssignableFrom() to do your checking.

Note that as others have pointed out, explicitly checking what class
an object is, is a danger sign (or a bad smell if you like). Chances
are there is a more appropriate way of accomplishing your goal using
polymorphism. Then again, sometimes there is not (this can typically
be the case when you need to work with third-party classes).

Cheers
Bent D

Signature

Bent Dalager - bcd@pvv.org - http://www.pvv.org/~bcd
powered by emacs

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Twisted - 22 Aug 2007 14:48 GMT

On Aug 21, 9:01 pm, getsanjay.sha...@gmail.com wrote:

> public static boolean check(Object obj, String...classes) throws
> Exception
[quoted text clipped - 8 lines]
> return(result);
> }

Good Lord! When I saw the subject line I held my nose and clicked the
link to read the post, expecting something interesting but with a fair
bit of code smell.

This goes way beyond code smell -- I think maybe something crawled in
there and died!

Reply to this Message

getsanjay.sharma@gmail.com - 22 Aug 2007 16:07 GMT

Thanks a lot Joshua for putting things right, I knew there was some
bit missing, I guess it was the isInstance() method. And like Patricia
said, in this scenario the || or | didn't make a difference since I
was anyways breaking out of the loop if any of the class satisfied the
condition.

Thank you Patricia for clarifying in my absence, and oh, what we would
do without your JLS plugs. :-)

On Aug 22, 6:47 am, Lew <l...@lewscanon.com> wrote:

> getsanjay.sha...@gmail.com wrote:
> [...]
[quoted text clipped - 3 lines]
>
> This idiom is at the heart of object-oriented programming.

Thanks a lot Lew for letting me know of this. But this concept is
still a bit hazy to me. Can you provide a real world example on how
your paradigm can be put to use?

Thanks Roedy, like always, your link was really helpful. :)

And Bent, your method gave me an alternate solution to my problem.
Both isInstance() and isAssignableFrom() really seem to do the trick.
Thank you.

> On Aug 21, 9:01 pm, getsanjay.sha...@gmail.com wrote:
>
[quoted text clipped - 17 lines]
> This goes way beyond code smell -- I think maybe something crawled in
> there and died!

Its OK my friend, we all try to learn. Maybe by writing smelly code
but creative code today, I would live tomorrow to write better code...

Thanks and regards,
STS.

Reply to this Message

Lew - 22 Aug 2007 22:32 GMT

Lew wrote:

>> ... polymorphism ...
>> This idiom is at the heart of object-oriented programming.

> Thanks a lot Lew for letting me know of this. But this concept is
> still a bit hazy to me. Can you provide a real world example on how
> your paradigm can be put to use?

GIYF.

I already provided an example.

Signature

Lew

Reply to this Message

Lew - 22 Aug 2007 22:38 GMT

> Lew wrote:
>>> ... polymorphism ...
[quoted text clipped - 7 lines]
>
> I already provided an example.

Here's the first hit I got from searching
Java polymorphism

<http://www.javaworld.com/javaworld/javatips/jw-javatip30.html>

Signature

Lew

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Daniel Pitts - 22 Aug 2007 22:38 GMT

On Aug 22, 8:07 am, getsanjay.sha...@gmail.com wrote:

> Thanks a lot Joshua for putting things right, I knew there was some
> bit missing, I guess it was the isInstance() method. And like Patricia
[quoted text clipped - 52 lines]
> Thanks and regards,
> STS.

Start with your the bad instanceof check:

class Foo {}
class Bar {}
class Baz {}

class NeedsToKnow {
public void needsToKnow(Object o) {
if ((o instanceof Foo) || o instanceof Bar) {
System.out.println("Fubar!");
} else {
System.out.println("Not Fubar!");
}
}
}

-----------

Refactoring to polymorphism gives you something like:

interface Knowable {
void needsToKnow();
}

abstract class Fubar implements Knowable{
public void needsToKnow() {
System.out.println("Fubar!");
}
}
/* Note, that these are free to further change the rules for
needsToKnow if they need to know.
class Foo extends Fubar {}
class Bar extends Fubar {}
class Baz implements Knowable {
public void needsToKnow() {
System.out.println("Not Fubar!");
}
}

class NeedsToKnow {
/* Note that Object o becomes Knowable o */
public void needsToKnow(Knowable o) {
/* Note the delegation. Tell, don't ask */
o.needsToKnow();
}
}

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Wrapper for multiple instanceof

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