public static double logPdf(double x,double lambda){
  if (x < 0)   return Double.NEGATIVE_INFINITY;
  return Math.log(lambda) - (lambda * x);
}

public static double logPdf ( double x , double lambda ) { if ( x < 0 ) return Double . NEGATIVE_INFINITY ; return Math . log ( lambda ) - ( lambda * x ) ; }
the natural log of the probability density function of the distribution ( mean = 1 / lambda )
