如何解决IllegalArgumentException？

Question

我有一个使用ClassLoader和Reflection来加载和调用方法的应用程序。在运行时，我收到此异常：java.lang.IllegalArgumentException : object is not an instance of declaring class。这是我的代码：

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>();

for (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 two operand and 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); 

任何人都可以帮助我吗？