Flux: This is Reactor’s reactive data flow type of 0 or more data units, each coming at some point in the future.

just(): Reactor’s way to create an initial collection of Flux’d elements.

filter(): Similar to Java 8 Stream’s filter() method, it only allows data elements from the earlier Flux through if they satisfy the predicate clause. In this case, does it contain the "spring" string?

map(): Similar to Java 8 Stream’s map() method, it allows you to transform each data element into something else, even a different type. In this scenario, it converts the string into uppercase.