java - How to solve IllegalArgumentException? -

i have application uses classloader , reflection load , invoke methods. @ runtime receive exception : java.lang.illegalargumentexception : object not instance of declaring class. here code:

file operatorfile = new file(operatorspath); url operatorfilepath = operatorfile.tourl();           url[] operatorfilepaths = new url[]{operatorfilepath}; classloader operatorsloader = new urlclassloader(operatorfilepaths);   class operatorinterface = operatorsloader.loadclass("operator");   filenamefilter filterdotclass = new filenamefilter()  {      public boolean accept(file dir, string name)      {          return name.endswith(".class");      }  }; file[] operatorfiles = new file(operatorspath).listfiles(filterdotclass);  class[] operatorclass = new class[operatorfiles.length-1];  int index = 0; method methodsinoperator; object instance; string operatorsign; map<string , object> operatormap = new hashmap<string , object>();  (file oprfile : operatorfiles) {     string filename = oprfile.getname();          //string filename = "operators."+oprfile.getname();          filename = filename.substring(0, filename.length() - 6);          class currentclass = operatorsloader.loadclass(filename);           if (!filename.equalsignorecase("operator") && operatorinterface.isassignablefrom(currentclass))          {             operatorclass[index] = currentclass;             instance = currentclass.newinstance();             methodsinoperator = currentclass.getmethod("getsign", null);              operatorsign = (string)methodsinoperator.invoke(instance, null);             operatormap.put(operatorsign,instance);             ++index;          }  }  while(expression != null) {     string[] elementinexpression = expression.split(",");      if(elementinexpression.length != 3)     {         system.out.println("expression should have 2 operand , 1 operator ");         throw new validationexception("validation on number of element failed1");     }      validation.validateoperand(elementinexpression[0],elementinexpression[1]);     validation.validateoperator(elementinexpression[2]);      double firstnumber = double.parsedouble(elementinexpression[0]);     double secondnumber = double.parsedouble(elementinexpression[1]);      double output = 0;     object obj;      for(class operatorcls : operatorclass)     {         obj = (object)operatormap.get(elementinexpression[2]);         methodsinoperator = operatorcls.getmethod("calculate", new class[] { double.class, double.class } );          output =(double)methodsinoperator.invoke(obj, firstnumber, secondnumber);     }     processingresult.add(output);     expression = mathexpreader.readline();  } readerwriter.writefile(path,processingresult);  

can me?