Get fields of generic type object in Java

Question

I have a class called "Person":

public Class Person {
    String name;
    String address;
}

A public method is used to return a Person object by using RestTemplate as below. My question is that how I print the name and the address of the Person object if I use generic type T for Person? I know I can do "instanceOf", but if "log" needs to handle many different types, then I will have to do "instanceOf" for each of them. Anyone has a better approach? Thanks.

public Person getPerson(SomeObject request) throws Exception {

    String url =......;
    return post(url, request, Pereson.class);
}

// This method can be used by response types other than Person
private <T> T post(String url, 
                   Object request, 
                   Class<T> responseType) throws Exception {

    T response = 
          restTemplate.postForObject(url, request, responseType);

    log(response, responseType);
    return response;
  }

private <T> void log(T response, 
                     Class<T> responseType) throws Exception {

    // how do I print the name and the address of the 
    // Person response object here?
 }

Answer 1

The easiest thing to do in this context is to delegate to the instance. Since you have no way to determine what that instance will be - it may be anything, really - the most effective way to log out critical information about the response would be to just invoke toString on response .

This means you need a suitable toString method attached to Person , which displays the name and address.

Anything else - dealing with instanceof or any kind of dodgy casting - is too risky and brittle when you need to introduce new classes which have to be logged.

Answer 2

There are a couple reasonable approached.

• Have Person implement a Printable interface and declare T extends Person . (Using Object::toString makes this simpler, but may be the wrong thing to do.)
• Add an adaptor (probably functional) interface, Printer (or even Printer<T> ), to stand in for Person . Write a set of overloaded static (or not) functions to wrap the likes of Person into Printer s (or even write inline as a lambda).