Creating a mock library

I want to create a Mock Library class that implements InvocationHandler interface from Java Reflection.

This is the template I have created:

import java.lang.reflect.*;
import java.util.*;

class MyMock implements InvocationHandler {
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            // todo
        }
        
        public MyMock when(String method, Object[] args) {
            // todo
        }
        
        public void thenReturn(Object val) {
            // todo
        }
}

The when and thenReturn methods are chained methods.

Then when method registers the given mock parameters.

thenReturn method registers the expected return values for the given mock parameters.

Also, I want to throw java.lang.IllegalArgumentException if the proxied interface calls methods or uses parameters that are not registered.

This is a sample interface:

interface CalcInterface {
    int add(int a, int b);
    String add(String a, String b);
    String getValue();
}

Here we have two overloaded add methods.

This is a program to test the mock class I wanted to implement.

class TestApplication {     
        public static void main(String[] args) {
            MyMock m = new MyMock();
            CalcInterface ref = (CalcInterface) Proxy.newProxyInstance(MyMock.class.getClassLoader(), new Class[]{CalcInterface.class}, m);
            
            m.when("add", new Object[]{1,2}).thenReturn(3);
            m.when("add", new Object[]{"x","y"}).thenReturn("xy");
            
            System.out.println(ref.add(1,2)); // prints 3
            System.out.println(ref.add("x","y")); // prints "xy"
        }
}

This is the code which I have implemented so far to check the methods in CalcInterface:

class MyMock implements InvocationHandler {
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            int n = args.length;
            if(n == 2 && method.getName().equals("add")) {
                Object o1 = args[0], o2 = args[1];
                if((o1 instanceof String) && (o2 instanceof String)) {
                    String s1 = (String) o1, s2 = (String) o2;
                    return s1+ s2;
                } else if((o1 instanceof Integer) && (o2 instanceof Integer)) {
                    int s1 = (Integer) o1, s2 = (Integer) o2;
                    return s1+ s2;
                }
            }
            throw new IllegalArgumentException();
        }
        
        public MyMock when(String method, Object[] args) {
            return this;
        }
        
        public void thenReturn(Object val) {
        
        }
}

Here I am checking only for methods with the name add and having 2 arguments, with their type as String or Integer.

But I wanted to create this MyMock class in a general fashion, supporting different interfaces not just CalcInterface, and also supporting different methods not just the add method I implemented here.