Mono is a type of instant Publisher that represents a single or empty value. This means it can emit only one value at most for the onNext() request and then terminates with the onComplete() signal. In case of failure, it only emits a single onError() signal.Most Mono implementations are expected to immediately call onComplete on their Subscriber after having called onNext.a combination of onNext and onError is explicitly forbidden.

Overridden Lambda implementation available in mono

subscribe();  //1

subscribe(Consumer<? super T> consumer);  //2

subscribe(Consumer<? super T> consumer,
          Consumer<? super Throwable> errorConsumer);  //3

subscribe(Consumer<? super T> consumer,
          Consumer<? super Throwable> errorConsumer,
          Runnable completeConsumer);  //4

subscribe(Consumer<? super T> consumer,
          Consumer<? super Throwable> errorConsumer,
          Runnable completeConsumer,
          Consumer<? super Subscription> subscriptionConsumer); //5

//1

        Publisher<String> rctMono  = Mono.just("Hello React"); // Simple Mono Publisher using Just
        var subs = new SubscriberImpl();
        rctMono.subscribe(subs);
        subs.getSubscription().request(10);

Output

08:21:46.241 [main] INFO org.mugil.subscriber.SubscriberImpl -- Received Hello React
08:21:46.252 [main] INFO org.mugil.subscriber.SubscriberImpl -- Subscription ended

//5

       Mono<Integer> rctMono2 = Mono.just(1)
                                    .map(i -> i/0);
       rctMono2.subscribe(i -> System.out.println(i),   //Consumer
                          err -> System.out.println("Error Msg -" + err.getMessage()), //onError, Not Mandatory
                          () -> System.out.println("Completed"), //onComplete, Not Mandatory
                          subscription -> subscription.request(1)); //onRequest, Not Mandatory

Output

Error Msg -/ by zero

Simple code which returns Mono based on switch case

    public static Mono<String> getUserName(Integer num){
        return switch (num){
            case 1 -> Mono.just("How are you");
            case 2 -> Mono.empty();
            default -> Mono.error(new RuntimeException("Invalid Input"));
        };
    }

Using mono.just would invoke the sumOfNums as it always fetches value from JVM memory rather than streaming

public static void main(String[] args) {
  List<Integer> lstNums = List.of(1,2,3);
  Mono.just(sumOfNums(lstNums)); //Mono.just takes the value from memory so wont be suitable for streaming data incase large data should be handled
}

public static int sumOfNums(List<Integer> lstNums){
   System.out.println("Sum invoked");
   return lstNums.stream().mapToInt(num -> num).sum();
}

Output

Sum invoked

Using mono.fromSupplier would invoke the sumOfNums during Terminal Operation rather than Intermediate Operation

List<Integer> lstNums = List.of(1,2,3);
        Mono.fromSupplier(() -> sumOfNums(lstNums)); // Intermediate Operation                 


  public static int sumOfNums(List<Integer> lstNums){
        System.out.println("Sum invoked");
        return lstNums.stream().mapToInt(num -> num).sum();
  }

Output

Mono.fromSupplier vs Mono.fromCallable
fromCallable calls a checked exception where as fromSupplier doesnot throws checked exception. So if you substitute supplier in place of callable you should also write try catch block to handle exception