# Functional interfaces

**URL:** <https://forum.codewithmosh.com/t/functional-interfaces/9038>\
**Category:** Java\
**Created:** [December 6, 2021, 5:24pm UTC](https://forum.codewithmosh.com/t/functional-interfaces/9038 "2021-12-06T17:24:17Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![vanessa](https://avatars.discourse-cdn.com/v4/letter/v/51bf81/32.png) [@vanessa](https://forum.codewithmosh.com/u/vanessa)\
**Post date:** [December 6, 2021, 5:24pm UTC](https://forum.codewithmosh.com/t/functional-interfaces/9038/1 "2021-12-06T17:24:17Z")

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Hey everyone, I am currently working my way through the lambda expression and functional interfaces in part 3 of the Java course.

After some googling I realised that List.of() is a factory method but I am still unsure what this means. Does it instantiate a List object? If so how is this possible since List is an interface and not a class. My understanding was that interfaces could not be instantiated. And if it is not instantiating a List object, what class does this object belong to?

My second question also relates to this.  
Consumer print = item → System.out.println(item);

Would someone be so kind as to explain what is being instantiated here? I am unsure which class this object belongs to since Consumer is an interface. I feel like I am missing something very important here. I am not sure whether there is a specific section that I should re-watch perhaps. Any clarification would be greatly appreciated 🙂

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**Author:** ![jmrunkle](https://sea2.discourse-cdn.com/flex020/user_avatar/forum.codewithmosh.com/jmrunkle/32/3765_2.png) [@jmrunkle](https://forum.codewithmosh.com/u/jmrunkle)\
**Post date:** [August 18, 2022, 5:06am UTC](https://forum.codewithmosh.com/t/functional-interfaces/9038/2 "2022-08-18T05:06:54Z")

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> [@vanessa](#):
>
> After some googling I realised that List.of() is a factory method but I am still unsure what this means. Does it instantiate a List object? If so how is this possible since List is an interface and not a class. My understanding was that interfaces could not be instantiated. And if it is not instantiating a List object, what class does this object belong to?

Yes, it instantiates a new `List` object. Specifically, it instantiates an unmodifiable list (meaning the contents of the list itself cannot be changed even if the elements themselves are mutable). This is not just trying to call `new List(...)`, but it is creating an instance of some unspecified class that implements the `List` interface. The unspecified class is considered an implementation detail.

> [@vanessa](#):
>
> My second question also relates to this.  
> Consumer print = item → System.out.println(item);
> 
> Would someone be so kind as to explain what is being instantiated here? I am unsure which class this object belongs to since Consumer is an interface. I feel like I am missing something very important here. I am not sure whether there is a specific section that I should re-watch perhaps.

The `item -> System.out.println(item)` bit there is a lambda expression. The code is creating a function and storing it in a variable `print` which has the type `Consumer`. Technically `Consumer` is a generic interface so we should specify a type argument, but if not specified it is basically `Consumer<Object>`.

You should probably re-watch the whole “Lambda Expressions and Functional Interfaces” section of Part 3 (~45 minutes long), but in particular you may be interested in [https://codewithmosh.com/courses/711980/lectures/12836332](https://codewithmosh.com/courses/711980/lectures/12836332) and [https://codewithmosh.com/courses/711980/lectures/12836341](https://codewithmosh.com/courses/711980/lectures/12836341).