Should I put for the generic type?

Question

Consider this snipped code:

public class MaxSizeHandler extends AbstractValueHandler<Collection> {      
}

and I use eclipse, and It warns me to add infer generic arguments type for Collection and the code changes like this:

public class MaxSizeHandler extends AbstractValueHandler<Collection<?>> {       
}

My question is what's the problem if I don't put it, or what's the advantage if I put it?

Eclipse is asking - "A collection of What?." – TheLostMind Apr 11 '14 at 14:45 — TheLostMind, Apr 11 '14 at 14:45

score 5 · Answer 1 · answered Apr 11 '14 at 14:49

5

Passing a raw Collection will imply that the Collection is not parametrized, hence you lose the ability to strongly type (i.e. at compile time) what goes in the Collection.

Passing a Collection<?> is not substantially different, as the wildcard will match anything extending Object.

Of course, it will remove the warning.

The best way would be to pass a Collection<MyObject> or a Collection<? extends MyObject>, etc.

answered Apr 11 '14 at 14:49

Mena

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Great answer. Minor compile time benefits, major benefit in making Eclipse shut up. =) – corsiKa Apr 11 '14 at 14:51
@corsiKa another way of making eclipse shut up is not using eclipse ;) – Rogue Apr 11 '14 at 15:04
The wildcard makes a big difference if you want to add items to the collection. – Sam Harwell Apr 11 '14 at 19:37

Raju Sharma · Answer 2 · 2014-04-11T15:20:42.727

1

you need to mention Collection type before itself in generic format like below :

public class MaxSizeHandler extends AbstractValueHandler<Collection<? extends T>> {      
}

T-> type of collections Otherwise java compiler will take as default type of collection.

edited Apr 11 '14 at 15:20

answered Apr 11 '14 at 14:49

Raju Sharma

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score 0 · Answer 3 · answered Apr 11 '14 at 14:48

0

Adding the correct type will allow the class to return the correct type of value, is there a specific type of Object your Collection will hold i.e Integer, then use AbstractValueHandler<Collection<Integer>>.

answered Apr 11 '14 at 14:48

jpdymond

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score 0 · Answer 4 · answered Apr 11 '14 at 15:01

Depending how you're using the MaxSizeHandler class it may make sense to make this class itself generic. For example, if you need to iterate over the collection:

public class MaxSizeHandler<T>
       extends AbstractValueHandler<Collection<? extends T>> {
  public void handle(Collection<? extends T> coll) {
    for(T item : coll) {
      // ...
    }
  }
}

or if you need to add new items to the collection:

public class MaxSizeHandler<T>
       extends AbstractValueHandler<Collection<? super T>> {
  public void handle(Collection<? super T> coll) {
    T item = createAnItem();
    coll.add(item);
  }
}

(These are just toy examples, as you haven't said what kind of methods AbstractValueHandler declares)

Should I put for the generic type?

4 Answers4