How to write a generic method for lambda?

THE Waterfall :

I have the following interface:

public interface Mapper {
    public <T> T map(T element);
}

and when I do Mapper mapper = (int n) -> n*2; I get the problem:

Illegal lambda expression: Method map of type Mapper is generic

What am I missing here? How is it possible to create a generic method to use in lambda expression?

Naman :

You should change your definition to

public interface Mapper<T> { // type bound to the interface
    T map(T element);
}

and then use it as :

Mapper<Integer> mapper = element -> element * 2; // notice Integer and not 'int' for the type

which can also be written as :

Mapper<Integer> mapper = (Integer element) -> element * 2;