Using the Reflection API to dynamically modify methods

[Serge]

Hello,

I am wondering if I could use the Java reflection API to dynamically
modify the methods of a class. For example, I need a function that will
evaluate any f(x) at some x. Say I have something like:

static double eval_f(double val){
double result = (val)**2 + 2*(val) + 2;
return result;
}

The above eval_f() will evaluate f(x) = x^2 + 2x + 2 at any x. However, in
my program I need to be able to modify f(x) to whatever the user wants
(the user enters the function he wants and it is copied to a string; my
program simply finds roots of these functions using different numerical
methods). Thus, if the user wants f(x) = x^11 + 2 I want to be able to
dynamically modify the first line in the eval_f() function to:

double result = (val)**11 + 2;

It seems to me as if there might be a way to accomplish this using the
Reflection class although I am not a pro at using it. I would appreciate
if someone could tell me if it is indeed possible and how I can go about
doing this. Thanks so much.

Cheers,
Serge

 

[Andrew Thompson]

Serge wrote:
....

I am wondering if I could use the Java reflection API to dynamically
modify the methods of a class.
No.

..For example, I need a function that
will evaluate any f(x) at some x. Say I have something like:

static double eval_f(double val){
double result = (val)**2 + 2*(val) + 2;
return result;
}

The above eval_f() will evaluate f(x) = x^2 + 2x + 2 at any x.
However, in my program I need to be able to modify f(x) to whatever
the user wants (the user enters the function he wants and it is
copied to a string; my program simply finds roots of these functions
using different numerical methods).

First figure how to parse the
string to a function.
That is 95% of the problem.

By the time you have done that,
evaluating it will be easy.

 

[Adam Jenkins]

Hello,

I am wondering if I could use the Java reflection API to dynamically
modify the methods of a class. For example, I need a function that will
evaluate any f(x) at some x. Say I have something like:

static double eval_f(double val){
double result = (val)**2 + 2*(val) + 2;
return result;
}

The above eval_f() will evaluate f(x) = x^2 + 2x + 2 at any x. However, in
my program I need to be able to modify f(x) to whatever the user wants
(the user enters the function he wants and it is copied to a string; my
program simply finds roots of these functions using different numerical
methods). Thus, if the user wants f(x) = x^11 + 2 I want to be able to
dynamically modify the first line in the eval_f() function to:

double result = (val)**11 + 2;

What you want to do is beyond the scope of what reflection provides. If
you want to allow a user to enter functions at runtime, the simplest
solution would be to use an embeddable scripting language like Beanshell
or Rhino. Beanshell implementes an interpreted java-like language, and
Rhino is a Javascript interpreter written in Java. You can get
Beanshell at
http://www.beanshell.org
and Rhino at
http://www.mozilla.org/rhino
I think there's another one called Dynamic Java. All these make it easy
to access java classes from the scripting language as well should you
need to do so.

Adam

 

[Serge]

Thanks for the help. I have actually considered using a scripting language
before. Actually, I simply passed arguments to a Perl script
evaluated the function using eval() in Perl and passed the result back to
my program by means of a temp file. However, all that stuff made the
program rather slow. On another note, I can't really use things like Rhino
because I have to make all my programs work on an external computer on
which I don't have install rights (it has jsdk 1.4.2 though and all the
std unix stuff). So I am wondering if there is any way to implement what i
discussed without using external tools (outside the jsdk 1.4.2). Thanks in
advance for the help.

 

[Adam Jenkins]

Thanks for the help. I have actually considered using a scripting language
before. Actually, I simply passed arguments to a Perl script
evaluated the function using eval() in Perl and passed the result back to
my program by means of a temp file. However, all that stuff made the
program rather slow. On another note, I can't really use things like Rhino
because I have to make all my programs work on an external computer on
which I don't have install rights (it has jsdk 1.4.2 though and all the
std unix stuff). So I am wondering if there is any way to implement what i
discussed without using external tools (outside the jsdk 1.4.2). Thanks in
advance for the help.

I'm not sure I understand the problem. You obviously have some install
rights since you're able to run the Java classes that you're writing.
Beanshell and Rhino are just jar files containing some Java classes
which implement an interpreter, which you can use from your own code.
You can just put them wherever you're putting your own classes. You can
even unpack the Beanshell or Rhino jar and make a single jar containing
your classes and the Rhino or Beanshell classes.

The performance should be a bit better than passing scripts to perl,
since you don't have to start a separate process an communicate via temp
files; you just call the evaluate function of whatever scripting library
you choose. Also, Beanshell and Rhino both integrate very well with
Java; you can access Java classes from your scripts, you can call script
functions from Java, and the script functions can return Java objects.

Adam

 

[Serge]

Oh if its just a jar file then I can surely use it! Thanks. But what about
the precision though. When I used Perl, it gave me precision problems.
It would work fine, for low precision, but when I looped a lot of times,
the approximation I got for the root of f(x) would not change. I wonder if
it's possible to use BigDecimal and BigInteger classes with Beanshell or
Rhino? Thanks for all the help.

 

[Jon A. Cruz]

Oh if its just a jar file then I can surely use it! Thanks. But what about
the precision though. When I used Perl, it gave me precision problems.
It would work fine, for low precision, but when I looped a lot of times,
the approximation I got for the root of f(x) would not change. I wonder if
it's possible to use BigDecimal and BigInteger classes with Beanshell or
Rhino? Thanks for all the help.

Other alternatives include Jython and JACL.

http://www.jython.org/

http://tcljava.sourceforge.net/docs/website/index.html

I've used the latter, and it was quite trivial to integrat and use.

 

[Adam Jenkins]

Oh if its just a jar file then I can surely use it! Thanks. But what about
the precision though. When I used Perl, it gave me precision problems.
It would work fine, for low precision, but when I looped a lot of times,
the approximation I got for the root of f(x) would not change. I wonder if
it's possible to use BigDecimal and BigInteger classes with Beanshell or
Rhino? Thanks for all the help.

Yup, as I said, both Rhino and Beanshell make it trivial to use any Java
class from them. Their own number types are probably just based on Java
numerical types as well.

Adam