reactor-core/src/main/java/reactor/core/publisher/Flux.java
public final Mono<T> next() {
		if(this instanceof Callable){
			@SuppressWarnings("unchecked")
			Callable<T> m = (Callable<T>)this;
			return convertToMono(m);
		}
		return Mono.onAssembly(new MonoNext<>(this));
	}
public final Mono<T> next() {
		if(this instanceof Callable){
			@SuppressWarnings("unchecked")
			Callable<T> m = (Callable<T>)this;
			return convertToMono(m);
		}
		return Mono.onAssembly(new MonoNext<>(this));
	}
Emit only the first item emitted by this {@link Flux}, into a new {@link Mono}.
<p>
<img class="marble" src="doc-files/marbles/next.svg" alt="">

@return a new {@link Mono} emitting the first value in this {@link Flux}