Java Generics: Unknown wildcards and T

First, T may well be replaced by E or Z or TYPE . It is just the name given to the generic type parameter.

Most of the time, the following letters chosen by convention:

• K to " K andy", example: Map<K,V>
• V to " V alue", example: Map<K,V>
• E to " E lement", example: List<E>
• T to " T ype", example: Collections#addAll

Note that for T it should be seen as class rather than typo , by definition all of them represent a type anyway.

About wildcards , it can not be used in a statement. This can only be used in generic context (between <> ) when it does not know the type.

Examples:

public <?> ? foo(? bar) //não irá compilar.

public <T> T foo(T bar) //ele irá compilar.

public foo(List<?> bar) //ele irá compilar.

By convention, T is used to represent a type parameter , or type parameter , used in the declaration of a generic class.

As already mentioned in Jean's response, there is a convention for more common cases, although you can use any letter in practice.

A classic example is the declaration of class List , which represents a list of elements. So we have:

public interface List<E> extends Collection<E> { ... }

At the time of using the list, we can then define the type E of the list, like this:

List<String> nomes = new ArrayList<>();

And now the compiler will warn if we try to put anything other than String in the list.

Only sometimes we want to explicitly leave a list without a defined type. This could only be done with List , but in this case the compiler will think you forgot to declare the type.

For this we use the wildcard ? , as in the example:

int contar(List<?> qualquerTipo) {
    return qualquerTipo.size();
}

Note that I do not need to know the type to count the elements of the list, so it is a perfectly valid use.

There are other applications for this, but it is generally a type of generic meta-programming used in utilitarian frameworks or routines where the actual type does not matter.

This should not be used frequently in system implementations we do on a day-to-day basis, as it is much more insecure.