Why @Nonnull annotation checked at runtime?

Question

I have a function with following signature

public static String myFunction(@Nonnull String param)

When I call it with param as null, I get

Answer 1

When you compile your project in IntelliJ IDEA, it instruments the bytecode of compiled classes to add runtime checks for various flavors of @Nonnull annotations. This behavior is controlled by the option:

Settings | Build, Execution, Deployment | Compiler | [x] Add runtime assertions for not-null-annotated methods and parameters.