What are Functional interfaces? Why they were added? Is FI annotation mandatory?

What is a Functional Interface?

An interface that has exactly one abstract method. It can have any number of default and static methods.

Why were they added?

• To enable Lambda Expressions.
• To support Functional Programming patterns in Java.
• To use methods as arguments (behavioral parameterization).

Is @FunctionalInterface mandatory?

• No, it is not mandatory. Any interface with one abstract method is technically a functional interface.
• However, using the annotation is highly recommended because it triggers a compiler error if you accidentally add a second abstract method.