How to define a Generic class that can have type T or type List

Question

My Java Generics is rusty returning to Java after a long time away.

I am trying to create a generic base class to wrap Jacksons ObjectMapper

abstract class MyBaseClass<T> {
  private Class<T> clazz;
  public void read() {
       objectMapper.readValue(json, clazz)
  }

  abstract Class<T> getGenericTypeClass();
}

The issue I am having is that sometimes T is an instance of a class, and sometimes it needs to be List<T>. My code works for the former, but I can't get it to compile when I have the list scenario. So this

class Sub extends MyBaseClass<Thing> {
    @Override
    getGenericTypeClass() {
       return Thing.class;
    }
}

works fine, but when I need List<Thing> I'm a bit lost.

If you need `List`, you might need to consider type tokens (from http://stackoverflow.com/a/75345/2413303 ) - I tinkered with them [here](http://stackoverflow.com/a/30096160/2413303) but I don't know if it's of any help. — EpicPandaForce, Dec 05 '16 at 15:33
There is a `Thing.class` but there is not a `List.class` only a `List.class`, so the type information of `` would be lost. This external library you're using might have some other api to deal with cases like that. It really depends on the library in this case. — Jorn Vernee, Dec 05 '16 at 15:33
But you can also look at [this article](https://bthurley.wordpress.com/2012/07/18/accessing-generic-type-info-at-runtime/) and Spring's `GenericTypeResolver` — EpicPandaForce, Dec 05 '16 at 15:40
You need a `TypeReference` for parameterized types with Jackson. But the type has to be known at compile time, ie. not the type variable `T`. — Sotirios Delimanolis, Dec 05 '16 at 15:42
@hvgotcodes Sotirios is right. Jackson's `TypeReference` does have the capability to store the type parameter. You could change the `Class` to `TypeReference`. You would have to pass in the concrete type in a `TypeReference` when instantiating your class. — Jorn Vernee, Dec 05 '16 at 15:47

Bohemian · Accepted Answer · 2016-12-05T17:29:40.993

You can't. And you shouldn't. A Thing and a List<Thing> are totally different. They are analogous to a Car and a Carpark.

I suggest always returning List<Thing>, wrapping a single Thing in a singleton List if required.

Here's one (easy) way;

public void read() {
   objectMapper.readValue("[" + json.replaceAll("^\\[|]$", "") + "]", clazz);
}

Where clazz is a TypeReference for List<Thing>.

The string manipulation guarantees the JSON will be wrapped in "[...]", adding wrapping square brackets if there are none.

How to define a Generic class that can have type T or type List

1 Answers1