How is a generic type instantiated?

[ThomsonTang]

By providing a type argument per type parameter.

The type argument list is a comma separated list that is delimited by angle brackets and follows the type name. The result is a so-called parameterized type.

example of a generic type:

class Pair<X, Y> {
    private X first;
    private Y second;
}

example of a concrete parameterized type:

public void printPair(Pair<String, Long> pair) {}   

and:

Pair<String, Long> pair = new Pair<String, Long>("maxsize", 1024L);

wildcard instantiations

In addition to concrete instantiation there so-called wildcard instantiations. They do not have concrete types as arguments, but so-called wildcards. A wildcard is a syntactic construct with a ? that denotes not just one type, but a family of types.

Example of a wildcard parameterized type:

public void printPair(Pair<?, ?> pair) {
....
}
Pair<?, ?> pair = new Pair<String, Long>("maximum", 1024L);
printPair(pair);

raw types

It is permitted to leave out the type arguments altogether and not specify type arguments at all. A generic type without type arguments is called raw type and is only allowed for reasons of compatibility with non-generic Java code.

Use of raw types is discouraged.