Function Error

I'm trying to import a function in Java, but am having problems with it. The java function is

public static double chi2cdf (double x)
{
  return chi.cdf(x);
}

I copied the sim.class file to the Postgres data directory and I added it to Postgres with the following
select sqlj.replace_jar('file:///D:/workspace/SimplePrediction/sim.jar','sim',true);

SELECT sqlj.set_classpath('public', 'sim');

CREATE OR REPLACE FUNCTION sim.chi2cdf(double precision)
                 RETURNS double precision
                 AS 'simpleMonteCarlo.sim.chi2cdf'
                 LANGUAGE java;

When I try to run it, I get the following error:
ERROR:  Unable to find static method simpleMonteCarlo.sim.chi2cdf with signature (D)D

I thought it was the signature of double precision, but then tried it with two other functions, one a float and one a
int.Does anyone have any ideas? 

Thank you,
Susan