Java Generics: Unknown wildcards and T

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I was able to verify that in Java we can use both a Unknown wildcard ("?"), and "T" (what is this called?) to create generic mechanisms (classes, methods, interfaces...). But it was not clear to me what are the main differences between these artifices and when I should use one or the other. Could someone help, please?

java generic

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4

By convention, T is used to represent a type Parameter, or type parameter, used in declaring a generic class.

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

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

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

When using the list, we can then define the type E of the list, thus:

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

And now the compiler will let you know if we try to put anything other than a String on the list.

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

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

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

Note that I don’t need to know the type to count the list elements, so it’s a perfectly valid use.

There are other applications for this, but it is usually a type of generic meta-programming used in frameworks or utility routines where the actual type doesn’t matter.

This should not be used often in the day-to-day implementations of systems we do, as it is far more insecure.

3

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

Most often, the following letters chosen by convention :

Note that for T should be seen as class instead of the typo, since by definition, they all represent a type any way.

About wildcards, it cannot be used in a statement. This can only be used in generic context (between <>) when you don’t 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.

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