When to use wildcards in Java Generics?

[siddharth das]

Hi,

public <T extends Animal> void takeThing(ArrayList<T> list)

Does the same thing as this:

public void takeThing(ArrayList<? extends Animal> list)

So here is my question: if they are exactly same, why don't we write

public <? extends Animal> void takeThing(ArrayList<?> list)

or

public void takeThing(ArrayList<T extends Animal> list

Also, when would it be useful to use a ? instead of a T in a method declaration ( as above ) with Generics, or for a Class declaration? What are the benefits?


Thanks!

[Jesper de Jong]

This:

siddharth das wrote:public <? extends Animal> void takeThing(ArrayList<?> list)

and this:

siddharth das wrote:public void takeThing(ArrayList<T extends Animal> list)

both do not compile.

The first one doesn't because it's simply a syntax error - after the 'public' you can specify one or more type arguments between < and >, and a wildcard is not a type argument. A wildcard is not some kind of anonymous type argument.

The second one doesn't because you didn't specify that T is a type argument, so the compiler will expect that there is a type named 'T', which there isn't (unless you have a class T { ... } or interface T { ... } in scope).

A bit more about the second: Note that the convention is that type arguments are named with a single, upper-case letter, but this is just a common convention - the name of a type argument is just a name, and can be anything. If you don't specify the type argument between < and > after 'public', then the compiler cannot know if T is a type parameter or a concrete type. An example to make it more clear:

// Here, Stuff is a type argument
public <Stuff> void takeThing(List<Stuff> list)

// Here, Stuff is just a normal type
public void takeThing(List<Stuff> list)

[siddharth das]

then where to use T and where to use ?

[Jesper de Jong]

It depends on what the method does. In some cases the method needs to know the type, so then you have to use a type parameter, and in other cases it doesn't need to know the type so you can use a wildcard.

You can find everything you ever wanted to know about Java generics in Angelika Langer's Java Generics FAQ.