Exception when invoke reflection Class.getMethod

Question

I have below code which invokes a method using reflection. But I am getting,

java.lang.IllegalArgumentException: wrong number of arguments

exception at Method.invoke. What is the reason?

public class B {
    public static void main(String[] args) {
        A a = new A();
        Method m;
        try {
            m = a.getClass().getMethod("m3",Integer.class);
            m.invoke(a);
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

public class A {    
    public void m3(Integer x){
        System.out.println("ssss");
    }
}

Answer 1

The invoke(Object, Object...) -method takes as it's first parameter the object that the method should be invoked on (as you did correctly) and then as a variable-length parameter any parameters that should be passed to the method call.

In your case, you're forgetting about the methods Integer -parameter. The method you're trying to call is A.m3() , which doesn't exist in the class.

The correct call would be:

m.invoke(a, 12); // or any int/Integer as it's second parameter

Answer 2

Look at the original Method.invoke() method signature -

public Object invoke(Object obj,
            Object... args)
              throws IllegalAccessException,
                     IllegalArgumentException,
                     InvocationTargetException

And it has no overloaded version. So your method call - m.invoke() is not correct