Purpose of Functional Interfaces in Java8

Question

I\'ve come across many questions in regards of Java8 in-built Functional Interfaces, including this, this, this and this. But all ask about \"why only one m

Answer 1

Although you ask "Is that it?", it's very nice that we don't have to write a new interface ever time we want type for a lambda.

Ask yourself, if you're reading an API, which is easier for a programmer to use:

public void processUsers(UserProcessor userProcessor);

... or ...

public void processUsers(Consumer userProcessor);

With the former, I have to go and take a look at UserProcessor to find out what one is, and how I could create one; I don't even know it could be implemented as a lambda until I go and find out. With the latter, I know immediately that I can type u -> System.out.println(u) and I'll be processing users by writing them to stdout.

Also the author of the library didn't need to bloat their library with Yet Another Type.

In addition, if I coerce a lambda to a Functional Type, I can use that type's composition methods, for example:

 candidates.filter( personPredicates.IS_GRADUATE.negate());

That gives you Predicate methods and(), or(), negate(); Function methods compose(), andThen() -- which your custom type would not have unless you implemented them.