What is a capture conversion in Java and can anyone give me examples?

Question

I\'ve noticed JLS talks of 5.1.10 Capture Conversion, but I fail to understand what they are.

Can anyone explain them to me/give examples?

Answer 1

A parameterized type involving wildcard type arguments is really a union type. For example

List = Union{ List | S <: Number }

In 2 cases, instead of using List, Java uses the captured version List, where S is a just-created type variable with upper bound Number.

(1) http://java.sun.com/docs/books/jls/third_edition/html/expressions.html

To narrow the type of an expression. If an expression's type is List, we know for sure that the runtime type of the object is actually a List for some concrete type S (S <: Number>). So compiler uses List instead, to perform more accurate type analysis.

Capture conversion is applied to each expression individually; this leads to some dumb results:

 void test1(List a){}
 void test2(List a, List b){}

List x = ...;
test1(x);    // ok
test2(x, x); // error

(2) http://java.sun.com/docs/books/jls/third_edition/html/typesValues.html#4.10.2

In subtype checking A :< B where A involves wildcard arguments. For example,

List  :< B
<=>
Union{ List | S <: Number}  :< B
<=>
List :< B, for all S <: Number

So in effect, we are checking the captured version of type A