java Generic wildcard "?"

Question

I referred to documentation on Java Generics, and trying to use the wildcard "?" in a simple program:

class Unknown <?> {

}

public class UnknownTypes {

    public static void main(String s[] ) {

    }
}

Wildcard "?" refers to an Unknown type, so in class Unknown, I used type-parameter wildcard itself; however when I compile I get compilation error. If I had used like this it would have worked.

class Unknown <T> {

}

If wildcard "?" refers to unknown type, why can't we use "?" as type parameter.

The following is the compile error that I get.

UnknownTypes.java:1: error: <identifier> expected
class Unknown <?> {
           ^
UnknownTypes.java:1: error: '{' expected
class Unknown <?> {
            ^
UnknownTypes.java:10: error: reached end of file while parsing
}

Is wildcard "?" used in conjunction with something else?

Unknown in the sense that it will never be known. The wildcard doesn't make sense in a class declaration because when you instantiate the class, you'll parameterize it with a type. It's more useful for things like `List> list = nonGenericGetList();` (you'll do this a lot when working with old libraries). — David Ehrmann, Jul 14 '14 at 15:53
To illustrate David's statement, imagine how you will use the parameter had you are allowed to use ?. For instance, a class member declaration such as `private ? a` would be legal, which is neither allowed nor make sense. — Evan Sebastian, Jul 14 '14 at 15:58
see exactly same question http://stackoverflow.com/questions/9914302/why-wild-cards-cant-be-used-in-generic-class-method-declaration — Vipin, Jul 14 '14 at 16:47

Bohemian · Answer 1 · 2014-07-14T16:07:44.643

You can't name a generic parameter as ?, because ? is not a valid identifier - a valid name of a variable. You have to give a generic parameter a valid java name so you can refer to it in the implementation.

You can only specify ? as a generic bound:

List<?> list; // a variable holding a reference to a list of unknown type

You can't use ? when creating an instance:

new ArrayList<?>(); // can't do this

or as a parameter name of a class :

class MyClass<?> { // can't do this

score 3 · Answer 2 · answered Jul 14 '14 at 15:55

To define a generic class with a type parameter, you cannot use a wildcard (in the generic class it is a type)

class Unknown <TYPE> {
  TYPE foo; // <-- specifies the type of foo.
}

When using it, you can use the wildcard

Unknown<?> some = new Unknown<String>(); // <-- some is any kind of Unknown.

score 2 · Answer 3 · answered Jul 14 '14 at 15:55

2

You only use the wildcard when referring to an instance of a class. Not in the class declaration.

class Unknown<T>{}

Unknown<?> instance= new Unknown<Integer>();


public void canHandleAnyUnknown(Unknown<?> wild){
}

answered Jul 14 '14 at 15:55

John B

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score 0 · Answer 4 · answered Jul 14 '14 at 16:00

0

Wildcard means 'anything' not unknown. You can use only Alphabets as type parameter.

answered Jul 14 '14 at 16:00

whoami

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java Generic wildcard "?"

4 Answers4