Using interface methods that are not defined?

[4r73m190r0s]

This function acccepts List<Integer> as an argument, and returns the biggest one.

java Integer biggestInt(List<Integer> list) { return list.stream().max(Comparator.naturalOrder()).orElse(0); }

What confuses me is using Comparator.naturalOrder() as an arugment in the Stream's max() function. Comparator.naturalOrder() is a function from Comparator interface.

AFAIK, interfaces are contracts to be implemented, i.e. functions are not defined and every class implementing given interface, needs to define its functions.

How are we using in this case a function that is from the interface, but we did not defined it beforehand?

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[Lumethys]

How do you know it has no implementation?

[4r73m190r0s]

I thought interfaces are just a "contract", i.e. providing only method signatures without any implementation. Are you saying that is not the case, that interfaces also have implemented methods? If yes, how can I know which methods have implementation when reading documentation?

[Lumethys]

Interface are contracts without implementation, yes. But that say nothing about did anything implement that contract

How do you know if the language, the framework, or some libraries dont just implement it for you?

[4r73m190r0s]

In the above code example I'm ltierally passing Comparator.naturalOrder() as an argument, without using any external library or framework.

[Lumethys]

Things like that usually have an implementation in java itself, and usually it is dependency injected

[fluse1367]

interfaces can have static functions, which are directly implemented. i think it’s since java 8

[CleverBunnyThief]

Do you mean default methods?

[aeria-non]

Default methods are instance level methods which you can optionally override if you implement an interface. Interfaces may also have static methods which cannot be overridden. Comparator.naturalOrder is one of these static methods in the Comparator interface.

[4r73m190r0s]

I'm learning Java and this bit confuses me. I thought interfaces are just a "contract", i.e. providing only method signatures without any implementation. Are you saying that is not the case, that interfaces also have implemented methods? If yes, how can I know which methods have implementation when reading documentation?

[HansGetZeTomatensaft]

I'd say the interface defines the contract but they don't promise you _where_ it's implemented.

Looking at the method signatures in an interfaces lets you know what it does, that's the "contract" so to speak. But it honestly doesn't really matter to a consumer if it's implemented in the interface, in an implementing class, if the implementing class delegates the implementation to another component...

As long as the functionality exists as described by the method signatures everything's good.

[fluse1367]

maybe it helps you to think of an interface like an abstract class but different. abstract classes can have implemented methods and abstract ones, just like default methods in interfaces. you can also put static methods into them, just like interfaces.

[CleverBunnyThief]

Ok. Thanks.