functional interface vs interface

Java 8 FunctionalInterface and Interface

stackoverflow.com › questions › 66203627 › java-8-functionalinterface-and-interface

FunctionalInterface is an interface with :

only one abstract method.
zero or several default methods (with body).

Thus, the compiler will be able to find the method corresponding to the lambda you will pass.

Eg;

@FunctionalInterface
public interface Supplier<T> {
    T get();
    default String greet() {
        return "Hello, World ";
    }
    default String greet2() {
        return "Hello, World 2";
    }
}

Thus java will easily find the right (abstract) method because is is unique.

        Supplier<Car> fordCarProvider = () -> new Car("Ford", 1500, 4, 43000);
        Supplier<Car> bmwCarProvider = () -> new Car("BMW", 1500, 4, 83000);

        System.out.println(fordCarProvider.get());
        System.out.println(bmwCarProvider.get());

But in case you have several abstract method, it is no more a FunctionalInterface. Because ambiguity might occur.

// No more @FunctionalInterface
public interface Supplier<T> {
    T get();
    String greet();
    default String greet2() {
        return "Hello, World 2";
    }
}

Answer from g.momo on Stack Overflow

Medium

medium.com › @qingedaig › java-functional-interface-vs-normal-interface-76889460bfd4

Java functional interface vs normal interface | by Alice Dai | Medium

November 11, 2024 - Normal Interface: Can have multiple abstract methods, allowing you to define a variety of behaviors that implementing classes must handle. Functional Interface: Can have multiple default and static methods, as they don’t affect its status as a functional interface (since only the single abstract method matters for lambda compatibility).

DEV Community

dev.to › johnscode › interface-vs-functional-interface-523m

Interface v.s. Functional Interface - DEV Community

June 29, 2024 - An interface may inherit from multiple interfaces. It may contain abstract methods, constants, static methods and default methods. ... Functional interface can also be called a single abstraction interface.

Discussions

What's the difference between using function types and functional interfaces as listeners/callbacks?

The difference is mostly semantic: what does it "mean" to the person using it? The main, practical, difference is that a functional interface is just a regular object interface that comes with its own factory function- you can make instances by "calling" this auto-generated factory function and passing a lambda. A function type is, well, a function- not an object. The difference is negligible for the downstream implementer of the type. The difference is mostly in how the author intends to use a provided instance of the type. If you do a functional interface, you'll use it with a dot-notation method call, and if you do a function type, you'll just invoke it directly. So, again, it's all about semantics. Do you want to communicate that the thing in question is "an object that does a thing" or do you want to communicate that the thing in question is "an operation"? There is one more difference that's sometimes worth thinking about, which is that since a functional interface is just an interface, it means that a user must pass in something that was explicitly created for the purpose of being used in that way. A function type is obviously satisfied by any function/method that happens to have the same signature. I'm not sure that's ever really a problem, but maybe if you have a bunch of function types and some of them share the same signature, you might pass the wrong thing to the wrong API- maybe forcing an explicit named interface would be the safer option. More on reddit.com

r/Kotlin

10

4

September 30, 2022

Guide to Functional Interfaces in Java

When I don't remember which to use I always refer to this list . I've always felt this functional interface business is just bad design when compared with C# simple Action/Function. Another victim of type erasure. More on reddit.com

r/programming

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18

March 7, 2017

(ELI5) What exactly is the use of "functional Interface"?

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r/developersIndia

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October 23, 2024

What is the purpose of an Interface?

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r/learnjava

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February 26, 2023

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