Java - Generics Method

Question

I've been reading on Generics lately, and I came across this method:

protected <V> RunnableScheduledFuture<V> decorateTask(Callable<V> callable, RunnableScheduledFuture<V> task) {
        return new ExceptionHandlingFutureTask<V>(callable, task);
    }

You see, I understand why there's a <V> after protected. What I don't understand is why there's a <V> again after RunnableScheduledFuture. I took this particular <V> out of the method, compiled it and there was no error. So why then did the writer decide to put it in there in the first place?

Andrzej Doyle · Accepted Answer · 2012-03-29 08:44:59Z

2

Generally speaking, you can always remove generics with "no error", because they're implemented in Java via erasure, so the actual compiled code simply refers to the raw classes anyway. (That said, you'd need to insert some casts to keep the compiler happy).

In light of the latter case, by removing the generic parameter from the future, you've turned it from "a future that will return a V" into "a future that will return something". Without the generic parameter, people will need to cast the result into the type they want, and the compiler won't be able to check the correctness of this for them.

It's just the same as being able to use a raw ArrayList instead of ArrayList<Integer>; both of them "work", but the latter is cleaner, easier to understand, type-checked by the compiler and doesn't require manual casting.

answered Mar 29, 2012 at 8:44

Andrzej Doyle

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Akinwale Agbaje Over a year ago

Aaaaaaah, I see! Thanks a lot. Makes a lot of sense now :)

manub · Accepted Answer · 2012-03-29 08:43:03Z

1

Because you're specifying that you're returning a RunnableScheduledFuture<V>, in which V is the same type of the Callable you put in your input.

If you remove <V>, you will get no error if you call the method in this way

RunnableScheduledFuture<String> rsf = decorateTask(Callable<Integer> callable, RunnableScheduledFuture<Integer> task);

But, if you don't remove the <V>, this will give you a compiler error.

answered Mar 29, 2012 at 8:43

manub

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Luca · Accepted Answer · 2012-03-29 08:43:55Z

0

Because he wants avoid cast warning when

ExceptionHandlingFutureTask<V> var = decorateTask(...);

answered Mar 29, 2012 at 8:43

Luca

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Buhake Sindi · Accepted Answer · 2012-03-29 08:46:12Z

0

Because RunnableScheduledFuture requires a parameterized type. Even though you can remove parameterized type from your code (and thus becoming a raw-type), the class definition is parameterized.

Removing the parameterized type, will require you to typecast to the appropriate type later.

answered Mar 29, 2012 at 8:46

Buhake Sindi

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