Issues with generic type detection of SAM types implemented using lambdas

[Oliver Gierke]

Hi all,

Lambda based implementations of SAM types currently don't support inspecting the type for generic type parameters. This can cause unexpected surprise as some high-level API taking a SAM type as parameter is usually an indicator to users, that they can be used with Lambdas. If the object passed in is then inspected for generic types somewhere down the call stack this causes issues. Handing in a dedicated implementation of the SAM type is a workaround bit I think that's highly confusing and can be a source of errors hard to understand and debug.

I've added an example below.

Cheers,
Ollie

public class LambdaTypeDetectionSample {

	public static void main(String[] args) {

		Function<Integer, String> lambdaFunction = i -> i.toString();
		Function<Integer, String> oldschoolFunction = new Function<Integer, String>() {

			public String apply(Integer t) {
				return t.toString();
			}
		};

		printTypeArguments(oldschoolFunction);

		// Yields:
		// java.util.function.Function<java.lang.Integer, java.lang.String> is a class sun.reflect.generics.reflectiveObjects.ParameterizedTypeImpl
		// class java.lang.Integer
		// class java.lang.String

		printTypeArguments(lambdaFunction);

		// Yields:
		// interface java.util.function.Function is a class java.lang.Class
	}

	private static void printTypeArguments(Function<?, ?> function) {

		Type type = function.getClass().getGenericInterfaces()[0];

		System.out.println(type + " is a " + type.getClass());

		if (type instanceof ParameterizedType) {

			ParameterizedType functionInterface = (ParameterizedType) type;
			Arrays.stream(functionInterface.getActualTypeArguments()).forEach(System.out::println);
		}
	}
}
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