Get generic of generic in Java

Question

I have a class. It will be simple container, which may contain some type T and type Optional<T> (guava Optional link) With Container.of(T) there is no problems. But, how can create Container.ofOptional(T)?
Here is my class:

public final class Container<T>{
  Class<?> firstClass;
  Class<T> seconClass;

  Container(Class<?> first, Class<T> second){
    this.firstClass = first;
    this.seconClass = second;
  }

  public static <T> Container<T> of(Class<?> first, Class<T> second){
    return new Container(first, second);
  }

  public static <T> Container<Optional<T>> ofOptional(Class<?> first, Class<T> second){
    return of(first, <what should I pass here>);
  }
}

I tried Optional.of(second) but it doesn't work.

So, in result, I want method Container.ofOptional(T) return Container<Optional<T>>

Optional.of(second) isn't viable since you claim to be returning a Container<Optional<T>>, and at best with that, you're returning an Optional<Class<T>>. I've been looking at this for a while now and I'm quite perplexed about its use case. Why are you just passing classes around instead of anything more concrete? — Makoto
– Makoto, Commented Dec 31, 2015 at 23:33
If you put return new Container(first, second); to ofOptional, Java compiles correctly. But, what is your purpose? What you need to do with ofOptional? — Valijon
– Valijon, Commented Dec 31, 2015 at 23:41
@Valijon, may be java compiles it correctly, but Intellij show me error about second argument Required Optional<T>, Found Optional — Aleksandr
– Aleksandr, Commented Jan 1, 2016 at 9:10

ruakh · Accepted Answer · 2016-01-01 00:26:37Z

3

Due to type erasure, this doesn't really make sense. There's only one Optional.class — Optional.of("foo").getClass() and Optional.of(3).getClass() both return the same instance — so although you can obtain an expression of type Class<Optional<T>> by lots of casting, its runtime-type will necessarily be Class<Optional> (or just null), so you can't do anything really useful with it.

So the simple answer to your question is that you can write

  public static <T> Container<Optional<T>> ofOptional(Class<?> first, Class<T> second){
    return of(first, (Class<Optional<T>>) (Class) Optional.class);
  }

but the real answer is that you shouldn't do this.

(You might want to post a new question about your real problem — the problem that Container is trying to solve — and perhaps someone can help you find a better approach.)

answered Jan 1, 2016 at 0:26

ruakh

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1 Comment

Aleksandr Over a year ago

Thanks you very much. I don't reed to post new question, because your answer solved my problem.

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