declarations with <> operator with classes like S , T , K, V, M, B

[ulvi ugur]

Hi all,

Can anybody tell me what the below conventions are called and with which version of Java the are introduced :

     public interface Converter<M, B> {}
     public final class BeanProperty<S, V> extends PropertyHelper<S, V> {}

or better with a question mark (?)

    private Property<S, ?> baseProperty;

Thanks in advance,

Ulvi

[Bear Bibeault]

Generics, Java 1.5

[ulvi ugur]

Hi Bear,

I am actually using generics for collections to declare the content of it but what do these single letters mean ? Are they real classes or is it just a dummy identifier to be replaced with a real class ?

Cheers,

Ulvi

[Rob Spoor]

It's the generic type, and it indeed gets replaced with something "real" like Integer, String or whatever you want when you use the class.

[Shanky Sohar]

these are called generics and introduced in java 1.5........
hava a look in chapter 7 of kathy book......scjp1.6...if you want to learn it.

[Seetharaman Venkatasamy]

Generally
<T,V> or <T> is called as type parameter.
<?> is called as unbounded wildcard notation
bounded wildcard :
<? extends Class> is called as upper bounded wildcard notation
<? super Class> is called as lower bounded wildcard notation

hope it clear the generic terminology.

[Jesper de Jong]

The single letters are the type parameters of the generic class that they are specified in. Note that they don't have to be single letters; using single, upper-case letters for type parameters is just convention.

[Campbell Ritchie]

Look in the Java™ Tutorials: here, and here.

[ulvi ugur]

Thanks guys, very helpful !

[Campbell Ritchie]

You're welcome

[Campbell Ritchie]

Seetharaman Venkatasamy wrote:Generally
<T,V> or <T> is called as type parameter.

Isn't that (<T>) called a formal type parameter, and when you write List<String>, then "String" is the actual type parameter? I think that's correct, but I'm not sure.

[Jesper de Jong]

For the exact definition:

Section 4.4 of the Java Language Specification describes type variables. You can use type variables in generic class, interface, method and constructor declarations as parameters in those declarations - so there they are called type parameters.

When you use a generic class or interface with a specific type, in other words when you fill in specific, existing types for the type arguments, you get a parameterized type. For example, List<String> is a parameterized type - you've filled in String for the type parameter T of the generic type List<T>. The Java Language Specification calls the type that you fill the parameter with the actual type argument.

The words "parameter" and "argument" mean the same (so "type parameter" and "type argument" also mean the same).

[Campbell Ritchie]

Thank you, Jesper. I suspect I got my nomenclature from Effetive Java by Bloch, and Bloch uses a different nomenclature from the Java Language Specification.