Difference between generic method and wildcard

[anthony chan]

Hi,

Is anyone able to explain what is the difference between generic method and wildcard like

public <T> void testing(Collection<T> c){}
public void testing(Collection<?> c){}

One difference I know is once you use wildcard you cannot add any element into the collection c but generic method is able to do that. Are there any other differences?

Best regards
anthony chan

[Seetharaman Venkatasamy]

Have a look at this

[anthony chan]

Thanks,

I have read it, although I am not quite understand about it previously. Can I interpret that generic method is used to express dependencies among the type of one or more arguments to a method and/or return type.

anthony

[Seetharaman Venkatasamy]

<edit>edited the wrong message</edit>

public <T> void testing(Collection<T> c){}
public void testing(Collection<?> c){}

in the above example, there is not much difference, but I prefer second one.

Generic methods allow type parameters to be used to express dependencies among the types of one or more arguments to a method and/or its return type. If there isn't such a dependency, a generic method should not be used

consider the below code:

class Collections {
    public static <T> void copy(List<T> dest, List<? extends T> src) {
    ...
}

it is more expressive that src may contains subclass of T and you are going to copy the element from src to dest which is a T. here <T> expression depends on <? extends T> . so in this situation you should use generic method instead of wilcard. else I prefere wildcard.

[Jesper de Jong]

Seetharaman Venkatasamy wrote:

public <T> void testing(Collection<T> c){}
public void testing(Collection<?> c){}

in the above example, there is not much difference, but I prefer second one.

Why do you prefer the second one?

You should avoid using "?" as a type argument, because it is less type-safe than using type arguments, as in the first line.

[Rob Spoor]

It depends. If you don't need T inside the method, then the second form is just fine. It allows you to read the contents without being able to modify the collection. And since T is not needed inside the method, the type obviously does not matter, so using Object for the reference type of the individual elements will be OK.

[Seetharaman Venkatasamy]

Jesper de Jong wrote:
Why do you prefer the second one?

even if you use T inside the method, I suppose you can not do useful operations on T . still we can mange with object reference in the case of 2nd.Please, correct me if I am wrong

[Stephan van Hulst]

Here's a recent discussion that's similar: http://www.coderanch.com/t/540581/java-programmer-SCJP/certification/Generics-method-parameters-wildcards

[Rob Spoor]

Seetharaman Venkatasamy wrote:

Jesper de Jong wrote:
Why do you prefer the second one?

even if you use T inside the method, I suppose you can not do useful operations on T . still we can mange with object reference in the case of 2nd.Please, correct me if I am wrong

You're not completely right. No, you cannot use T to create instances or arrays. But you can use it as the generic type for other objects. By using a method generic type T you can create an ArrayList<T> inside the method. By using just <?> you cannot. And that's what I meant by needing T inside the method.

[Seetharaman Venkatasamy]

Hmm, thanks Rob