Sep 12, 2022 2 min read

Java Interview Questions (Series) - Functional Interface

The devil is always in the details! 😈

Brief

If you are to join a new project that's using at least Java 1.8 ( if not, you should consider looking for another project), you should be able to explain and what is a FunctionalInterface and what it is made of.

Implementation

Let's get into it! 🚀

A FunctionalInterface in Java is an interface having only one abstract method.

That's it!

@FunctionalInterface
public interface Flyable {
    void fly();
}

Flyable.java

❗️ Keep in mind that the @FunctionalInterface annotation is not mandatory, but it's a good practice to add it for it's validation purpose.

If you have an interface annotated with @FunctionalInterface and you try to add a second abstract method, you will get a nice compile-time error telling you that:

Multiple non-overriding abstract methods found in: YourFunctionalInterface.java.

🤨 Wait a minute, the compiler is not complaining that there are multiple abstract methods in your interface, but that they are non-overriding.

❓ Hmm, does it mean that we can have multiple abstract but overriding methods in our functional interface?

The answer is yes! ✅

Long story short

Every Java interface contains abstract methods with the same signature from Object class for equals, toString and hashCode, methods.

public interface YourInterface {
   String toString(); // this method is already there but you can still explicitly declare it
}

For this reason, they are not taken into account when validating a FunctionalInterface.

🧩 If this part was a bit confusing, it's totally fine, let's summarize it:

A FunctionalInterface can contain:

🔸 one abstract non-overriding method

🔸 any number of abstract overriding methods // those from Object

🔸 any number of default methods

🔸 any number of static methods.

Full example

@FunctionalInterface
public interface Flyable {

    void fly();

    String toString();   // overriding method from Object class

    default void defaultMethod() {
        System.out.print("Log from default method!");
    }

    static void staticMethod() {
        System.out.print("Log from static method!");
    }

}

Flyable.java