Classes - The Java Language Specification - page 182

These type variables are known as the type parameters of the class. The type parameter
section follows the class name and is delimited by angle brackets.

TypeParameters:
< TypeParameterList >
TypeParameterList:
TypeParameterList , TypeParameter
TypeParameter

In a class's type parameter section, a type variable T directly depends on a type variable S
if S is the bound of T , while T depends on S if either T directly depends on S or T directly
depends on a type variable U that depends on S (using this definition recursively).

It is a compile-time error if a type variable in a class's type parameter section depends on
itself.

The scope and shadowing of a class's type parameter is specified in § 6.3 and § 6.4 .

Example 8.1.2-1. Mutually Recursive Type Variable Bounds

Click here to view code image

interface ConvertibleTo<T> {
T convert();

}
class ReprChange<T extends ConvertibleTo<S>,
S extends ConvertibleTo<T>> {

T t;
void set(S s) { t = s.convert(); }
S get()

{ return t.convert(); }

}

A generic class declaration defines a set of parameterized types (§ 4.5 ) , one for each pos-
sible invocation of the type parameter section by type arguments. All of these parameter-
ized types share the same class at run time.

For instance, executing the code:
Click here to view code image

Vector<String> x = new Vector<String>();
Vector<Integer> y = new Vector<Integer>();
boolean b = x.getClass() == y.getClass();

will result in the variable b holding the value true .