About Question enthuware.ocpjp.ii.v11.2.1854 :

[Sergiy Romankov]

I am not sure. In some cases by writing Lamda I have to directly write type(s)
but here it is just
Doer d = (a, b)->b.substring(0, a);

why not

Doer<Integer, String> d = (a, b)->b.substring(0, a);

[liugengyu]

Because Interface Doer is not like Function<T,R> in JDK, which already has generics.
public interface Function<T, R> {

R apply(T t);
...
}