Functional interface in Java SE 8+ = an interface with 1 abstract method (not derived from Object class)

Sources:

https://www.javappa.com/kurs-java/java8/interfejs-funkcyjny-co-to-jest

https://www.javappa.com/kurs-angularjs

https://www.javappa.com/kurs-spring/projekt-startowy-spring-boot

Functional interface in Java SE 8+ = an interface with 1 abstract method (not derived from Object class). 

Abstract method means with no body and not implemented.

We can also tell the compiler to treat it as a @FunctionalInterface with this annotation.

Default methods of such interfaces (with "default" keyword) are implemented with body here, and are not abstract since Java SE 8.