What causes "object is not an instance of declaring class"? [duplicate]

Question

Possible Duplicate:
Why do I get “object is not an instance of declaring class” when invoking a method using reflection?

When I run the code below, why does it throw this error?

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 com.Test.main(Test.java:10)

Here's my main class:

    package com;
    public class TestMain {
        private String strName = "abcdefg...";

        @SuppressWarnings("unused")
        private void display(){
            System.out.println(strName);
        }
    }

And my test class:

    package com;
    import java.lang.reflect.Method;
    public class Test {
        public static void main(String[] args) {
            Class<TestMain> tm = null; 
            try{
                tm= TestMain.class;
                Method m1 =tm.getDeclaredMethod("display"); 
                m1.setAccessible(true);
                m1.invoke(tm);
            }catch(Exception e){
                e.printStackTrace();
            }
        }
    }

This is my modified code, thank you:

package com;
    import java.lang.reflect.Method;
    public class Test {
        public static void main(String[] args) {
            TestMain tm =new TestMain(); 
            try{
                Method m1 = tm.getClass().getDeclaredMethod("display"); 
                m1.setAccessible(true);
                m1.invoke(tm);
            }catch(Exception e){
                e.printStackTrace();
            }
        }
    }

Sean Patrick Floyd · Accepted Answer · 2011-09-13 09:35:15Z

23

You need an instance, not the class:

TestMain object = // get TestMain object here
m1.invoke(object);

Or if you mean a static method, supply null as first parameter:

m1.invoke(null);

answered Sep 13, 2011 at 9:35

Sean Patrick Floyd

301k72 gold badges481 silver badges598 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Mat · Accepted Answer · 2011-09-13 09:32:27Z

1

m1 needs to be invoked on an instance of the TestMain class, not on the class itself (i.e. on an object created by new TestMain()).

answered Sep 13, 2011 at 9:32

Mat

208k41 gold badges407 silver badges423 bronze badges

Collectives™ on Stack Overflow

What causes "object is not an instance of declaring class"? [duplicate]

2 Answers 2

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Linked

Related