Utility for extracting type parameters

[Maurizio Cimadamore]

On 30/09/11 09:22, Florian Weimer wrote:
> I've got a type T (in the most general sense; that is, it can be a
> generic type with or with type parameters applied) and a generic type X
> such that T is assignable to X.  If no raw types are involved and I'm
> not mistaken, the type parameters of X which make this assignment valid
> are uniquely determined.  Is there a helper function to compute them?
>
> At first glance, this does not seem to be too difficult to compute, but
> generics are tricky, so I wonder if there's existing code.
>
Is X a completely unknown type or do you know its class name?

i.e.

X = List<...>

In that case you can use the code in Types.asSuper [1]; see line 1397:

*) call Types.asSuper passing in the type of T (as first parameter) and 
the symbol of X (as second parameter) - assuming that T is a subtype of 
X, this method will return the supertype of T whose class name is the 
same as X - call it X'.

*) now that you have X', you can easily determine its type-parameters, 
which will be the ones that make the assignment legal (when applied to X).

Hope this helps.

[1] - 
http://hg.openjdk.java.net/jdk8/tl/langtools/file/4e754e4b0a52/src/share/classes/com/sun/tools/javac/code/Types.java

Maurizio