Why do I get "object is not an instance of declaring class" when invoking a method using reflection?

Question

public class Shared {

    public static void main(String arg[]) throws SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException {
        Shared s1 = new Shared();

        Object obj[] = new Object[2];
        obj[0] = "object1";
        obj[1] = "object2";
        s1.testParam(null, obj);

        Class param[] = new Class[2];
        param[0] = String.class;
        param[1] = Object[].class; //// how to define the second parameter as array
        Method testParamMethod = s1.getClass().getDeclaredMethod("testParam", param);
        testParamMethod.invoke("", obj); ///// here getting error
    }

    public void testParam(String query,Object ... params){
        System.out.println("in the testparam method");
    }

}

Here is the output:

in the testparam method
Exception in thread "main" java.lang.IllegalArgumentException: object is not an instance of declaring class
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
    at java.lang.reflect.Method.invoke(Unknown Source)
    at pkg.Shared.main(Shared.java:20)

Stuart Cook · Accepted Answer · 2011-08-26 09:57:24Z

72

When you invoke a method via reflection, you need to pass the object you are calling the method on as the first parameter to Method#invoke.

// equivalent to s1.testParam("", obj)
testParamMethod.invoke(s1, "", obj);

answered Aug 26, 2011 at 9:57

Stuart Cook

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Thilo · Accepted Answer · 2011-08-26 09:57:48Z

22

 testParamMethod.invoke("", obj); ///// here getting error

The first parameter to invoke must be the object to invoke it on:

 testParamMethod.invoke(s1, "", obj);

answered Aug 26, 2011 at 9:57

Thilo

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