java.lang.IllegalArgumentException: object is not an instance of declaring class

This exception is thrown when a method is invoked on an object that is not an instance of the declaring class.

For example, suppose you have a class called `MyClass` with a method called `myMethod`:

public class MyClass {
    public void myMethod() {
        // do something
    }
}

If you try to call `myMethod` on an object that is not an instance of `MyClass`, you will get the `IllegalArgumentException`.

Object obj = new Object();
MyClass mc = new MyClass();
mc.myMethod(); // this is valid
obj.myMethod(); // throws IllegalArgumentException

In the above code, `obj` is an instance of `Object`, not `MyClass`, so calling `myMethod` on it will result in an `IllegalArgumentException`.