CODEWITHSUNDEEP
javagenerics
shar
shar

Reputation: 2078

Is the unbounded wildcard type used only to create references?

I have seen a lot of cases where in, the ? type is used only to create references. And we are not able to add any Objects other than null. So, is the use of '?', only to create references?

Also, what is the need / use of giving the option to add a null? They could simply have implemented with no add() method at all, right? Any reason for giving the option of adding null into List < ? > ?

Upvotes: 0

Views: 150

Answers (2)

Grim
Grim

Reputation: 1638

List is a generic interface, and the same interface regardless of the generic type. As such, all implementors are required to redefine all methods in it. As far as being able to add null into an "unbounded" list, it's not as much a matter of methods as much as a requisiste of respecting the interface:

the unbound generic wildcard allows a reference to a List (or other generic class) to be passed around to methods without requiring a compile-time knowledge of the specific type it will contain. However, the contract of the generic must still be respected. This poses no big problem for read operations, since whatever is in the List, it will always (also) be a java.lang.Object. On the other hand, insert operations are almost impossible, since lacking informations about the accepted types, no type can be accepted - e.g. a List can only accept strings, but an Object could be anything else - a Number for example, and thus cannot be accepted safely.

Null works safely since (assuming the List accept null values), it will be the only value that's always valid to be put inside the list (a null can be cast to any valid type).

Edit: just since i noticed i didn't actually answer your question: yes, the <?> wildcard (and the extended forms as well <? extends X> and <? super X> are only valid when defining references, and cannot be used in generic instantiation.

Upvotes: 3

Bohemian
Bohemian

Reputation: 425003

Firstly, null is a member of all types. That's part of the language specification that pre-dates generics, so you can' do anything about that.

As for the use of ?, you can't * instantiate* a wildcard-typed generic class, that you can't do this:

List<? extends Number> list = new ArrayList<? extends Number>();  // error

because it does't make sense. The wildcard type is used to tell the compiler "I don't know what the type of the list will be, but it will be bounded by Number". However, when the code executes, a List with a type bounded by Number will be assigned to the variable.

Wildcards are most often seen as parameters:

public static void doSeomthing(List<? extends Number> list) {
    // use the list knowing only that the elements are a subclass of Number
    // but knowing which exact class they are
}

Upvotes: 0

Related Questions