Definition of type variable and parameter

Question

I am reading up on generics in Java using the Java Language Specification, Third edition. In section \"4.6 Erasure\" type erasure is defined. On the erasure of a ty

Answer 1

TL;DR — Context is everything!

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Informally, "type variable" and "type parameter" are used interchangeably as synonyms for each other [even by Gilad Bracha — the creator of Java's Generics implementation].

Formally, however, the JLS explicitly describes them as two different — but closely-related — abstractions.

In the past, I have had similar questions myself about the less-than-crystal-clear usage of the terms "type parameter" and "type variable" in a lot of the literature on generics.

From my interpretation of the more-recent Java 8 JLS, a TypeParameter is what appears in a parameterized class' or method's type parameter section [the <> part of a declaration].

The JLS says, "A type variable is introduced by the declaration of a type parameter...". I interpret that to mean that, in order to use a TypeVariable, you must first fill out the method's [or the class'] type parameter section by following the rules specified for declaring a TypeParameter...

TypeParameter:
    {TypeParameterModifier} Identifier [TypeBound]

TypeParameterModifier:
    Annotation

TypeBound:
    extends TypeVariable
    extends ClassOrInterfaceType {AdditionalBound}

AdditionalBound:
    & InterfaceType

Reading the above syntactic production for TypeParameter and this one for TypeVariable...

TypeVariable:
    {Annotation} Identifier 

...I interpret those two productions to mean that if an identifier T is being used in a context where T has [or can have] a TypeBound after it, then T is a TypeParameter. Alternatively, if an identifier T is being used in a context where TypeBounds are not allowed, then T is a TypeVariable.

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I think of a TypeParameter as being analogous to a formal parameter of a method declaration.

When the JLS says, "A type variable is an unqualified identifier used as a type in class, interface, method, and constructor bodies", I interpret that to mean that declaring a TypeParameter in the type parameter section, is also sort of analogous to declaring a class that can be subsequently used as a reference type of, say, an instance variable — in a sense.

By that I mean, in order for the following to be legal...

class Foo { 

   private Bar bar;
}

class Boff extends Foo { }

..then you must first introduce the declaration of the Bar type before it can be used in the body of Foo. Analogously, the type parameter <T extends Foo> must first be declared in order for the following to be legal...

class Baz<T extends Foo> { /* The TypeParameter that "introduces" T comes first */

    private T quux; /* now T is  a TypeVariable in this context */

    /* <U extends Number> is the TypeParameter that "introduces" the TypeVariable, U */

    public <U extends Number> List<? super U> m( Class<U> clazz ) throws Exception { /* <U extends Number> is the TypeParameter */

        U u = clazz.newInstance( ); /* U is a TypeVariable in this context */

        /*...*/

        List<? super U> list = new LinkedList<>(); /* U is a TypeVariable in this context; just like it is in this method's return type */          

        /*...*/

        list.add( u );

        /*...*/

        return list;

    }

} 

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And if I'm allowed to be even more specific...

Baz<Boff> buzz = new Baz<>(); 

...the Boff inside the <> diamond, is neither a type variable nor a type parameter. It is a type argument .

Answer 2

If no bound is given for a type variable, Object is assumed.

Found in your link. That means that given FirstClass<T extends String> and SecondClass<V> you get that:

1. Class: FirstClass Type Parameter: T extends String. Type Variable: T. Type Bound: String.
2. Class: SecondClass Type Parameter: V Type Variable: V. Type Bound: defaults to Object.

Edit: By Type Parameter, Type Variable and Type Bound I do not mean the grammar rule, but the concept. Therefore, extends is just the keyword.

About the leftmost bound you can find the answer in the same link, two sentences after the first quote:

The order of types in a bound is only significant in that the erasure of a type variable is determined by the first type in its bound, and that a class type or type variable may only appear in the first position.