myhook.js

class_name = "rena2019.dummyapp.LoginActivity"

if (Java.available) {
  setTimeout(function() { // avoid java.lang.ClassNotFoundException
	Java.perform(function() {        
		var my_class = Java.use(class_name);
                //replace the original implementation of the function `isEmailValid` with our custom function
                my_class.isEmailValid.implementation = function(x){
                  //print the original arguments
                  console.log( "original call: fun("+ x + ")");
                  //call the original implementation of `isEmailValid` with args
                  // var ret_value = this.isEmailValid("w@g.de");
                  //return ret_value;
                  return true;
                }
	});
  }, 0);
} else {
   send("Java not available in this process");
}