Introduce a compiler option to store generic type information about a lambda expression using the Signature Attribute

Alex Buckley alex.buckley at
Wed Jan 7 20:18:43 UTC 2015

On 1/7/2015 7:22 AM, Timo Walther wrote:
> the Java Reflection API allows to access the generic signature of
> methods through the java.lang.reflect.Method#getGenericReturnType()
> method. However, this is not yet supported for Lambda expressions.
> Given the following classes:
> class Tuple2<F0,F1> {
>    F0 field0;
>    F1 field1;
>    public Tuple2(F0 f0, F1 f1) {
>      this.field0 = f0;
>      this.field1 = f1;
>    }
> }
> interface Map<IN, OUT> {
>    OUT map(IN in);
> }
> Currently, there is no solution that allows to get further type
> information about expressions like:
> Map<String, Tuple2<String, Integer>> map = (str) -> new Tuple2<>(str, 1);
> System.out.println(getReturnType(map)) // can only print Tuple2 =>
> information about the Tuple2's fields are always lost

map is a variable, so it doesn't have a return type.

If map is a class variable or an instance variable, then 
Field#getGenericType should help.

Expressions (including lambda expressions) aren't generally exposed 
through Core Reflection. If an expression (such as a lambda expression) 
happens to be implemented as a method, then relying on the [generic] 
signature of that method is dangerous since it may change from release 
to release.


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