CODEWITHSUNDEEP
javagenerics
Sankalp
Sankalp

Reputation: 2824

Clarification about wildcards (Generics) in Java

I have recently started reading Core Java. But I am having a hard time grasping the concept of wildcards.

Specifically, I'm confused about the difference between the following:

public class A<T extends {some_class}> {/*...*/}

and

public class A<? extends {some_class}> {/*...*/}

Can anyone help me understand the difference if there is at all?

Upvotes: 1

Views: 160

Answers (4)

Dave
Dave

Reputation: 5173

The difference is that you cannot use the ? elsewhere while you can use T. For example:

public class Foo<T extends Number> {
  T value; // you can declare fields of type T here

  int foo() {
    // Since you said T extends Number, you can call methods of Number on value
    return value.intValue(); 
  }
}

So why would you use ? at all? If you don't need the type. It wouldn't make sense to use it in a class definition any way that I can think of. But you could use it in a method like this:

int getListSize(List<?> list) {
    return list.size();
}

Any kind of method where you're more interested in the overall class and it has a method that doesn't involve the paramaterized type would work here. Class.getName() is another example.

Upvotes: 1

Suresh Atta
Suresh Atta

Reputation: 121998

? extends some class   or T extends some_class

means some_class itself or any of its children and anything that would work with instanceof some_class.

As per conventions T is meant to be a Type and ? is unknown type.

Upvotes: 0

Jono
Jono

Reputation: 171

In generic code, the question mark (?), called the wildcard, represents an unknown type. The wildcard can be used in a variety of situations: as the type of a parameter, field, or local variable; sometimes as a return type (though it is better programming practice to be more specific). The wildcard is never used as a type argument for a generic method invocation, a generic class instance creation, or a supertype.

The Type T is a defined type or known type

Hope this helps

Upvotes: 0

Bohemian
Bohemian

Reputation: 425033

They are the same, except with the wildcard you can't refer to the type in the code of your class. Using T names the type.

Upvotes: 0

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