JAVA and generic types issue

Question

I'm facing an issue with generic types:

public static class Field<T> {

    private Class<? extends T> clazz;

    public Field(Class<? extends T> clazz) {
        this.clazz = clazz;
    }

}

public static void main(String[] args) {

    // 1. (warning) Iterable is a raw type. References to generic type Iterable<T> should be parameterized.
    new Field<Iterable>(List.class);

    // 2. (error) The constructor Main.Field<Iterable<?>>(Class<List>) is undefined.
    new Field<Iterable<?>>(List.class);

    // 3. (error) *Simply unpossible*
    new Field<Iterable<?>>(List<?>.class);

    // 4. (warning) Type safety: Unchecked cast from Class<List> to Class<? extends Iterable<?>>.
    new Field<Iterable<?>>((Class<? extends Iterable<?>>) List.class);

}

What's the best solution between the 1. and the 4. (or any other one by the way)?

What do you want to achieve? It's hard to answer your question without knowing that... — Anders R. Bystrup
– Anders R. Bystrup, Commented Feb 6, 2013 at 10:07
Class can be a nasty beast. 5th option: new Field<Iterable<Object>>(List.class) ('warning: unchecked assignment'). — akaIDIOT
– akaIDIOT, Commented Feb 6, 2013 at 10:11
@AndersR.Bystrup It's an example that reflects an issue I'm facing with the custom objects of the project I'm working on. So I'm afraid I can't be more specific, sorry... — sp00m
– sp00m, Commented Feb 6, 2013 at 10:33
@akaIDIOT With your solution, I'm not getting a warning but an error: The constructor Main.Field<Iterable<Object>>(Class<List>) is undefined. — sp00m
– sp00m, Commented Feb 6, 2013 at 10:34

Mikhail Vladimirov · Accepted Answer · 2013-02-06 10:43:11Z

5

public class Field <T> {
    private Class <? extends T> clazz;

    public <TT extends T> Field (Class <TT> clazz) {
        this.clazz = clazz;
    }

    public static void main (String [] args) {
        new Field <Iterable <?>> (List.class);
    }
}

edited Feb 6, 2013 at 10:43

answered Feb 6, 2013 at 10:17

Mikhail Vladimirov

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3 Comments

SpaceTrucker Over a year ago

I haven't seen type parameters on a constructor before, but nice solution

sp00m Over a year ago

This is really awesome, I had never noticed that you could infer generic types to constructors. That said, I don't really understand why this solution doesn't cause the same error as in 2...

Ben Schulz Over a year ago

@sp00m Pretty sure it abuses a compiler bug ( see stackoverflow.com/q/11570215/774444 ).

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