Java 8 Optional cannot be applied to interface

I would write that without an explicit cast:

Optional.ofNullable(string)
        .map(s -> {
             MyInterface m = new First();
             return m;  
        })
        .orElse(new Second())
        .number();

The problem is that Java infers the mapped type to be First, and Second is not an instance of First. You need to explicitly give Java a bit of a nudge to know the right type:

private static void main(String... args)
{
    final String string = "";
    final Number number = Optional.ofNullable(string)
        .<MyInterface>map(str -> new First())  // Explicit type specified 
        .orElse(new Second())
        .number();
}

This is a generic limitation of type inference along method chains. It's not limited to Optional.

There has been some suggestion to have type inference work along method chains. See this question: Generic type inference not working with method chaining?

Maybe in a future version of Java the compiler will be clever enough to figure this out. Who knows.

I have discovered out that the method Optional::map returns U which doesn't allow apply returned First to another type such Second is. An explicit casting to its interface or requiring it within the map method is a way to go:

final Number number = Optional.ofNullable("")
        .<MyInterface>map(string -> new First())
        .orElse(new Second())
        .number(); 

__

^{Edit: I have found this out after posting the question. However, I am keeping both since I haven't found a similar solution anywhere else yet.}