How to Create a List of Callable of Mono in Java Using Generics

Learn how to efficiently manage different types of `Callable` of `Mono` in Java generics, enabling smoother code creation and functionality.
---

Visit these links for original content and any more details, such as alternate solutions, comments, revision history etc. For example, the original title of the Question was: Create list of Callables of Monos of different types

If anything seems off to you, please feel free to write me at vlogize [AT] gmail [DOT] com.
---
Introduction to Callables of Mono

When working with Java, especially in a reactive programming context with Mono, you may encounter scenarios where you need to handle a list that contains different types of Callable objects of Mono. This poses a challenge due to the way Java handles generics, specifically with covariance. If you’ve ever been stuck with errors while trying to integrate multiple Callable<Mono<...>>, don’t worry! This guide will guide you through the problem and provide clear solutions.

The Problem

When you attempt to compile a method that takes a list of Callable<Mono<?>>, you might find yourself facing errors due to incompatible types. For instance, a Callable<Mono<Integer>> is not a subtype of Callable<Mono<?>>. This can lead to frustration, especially when working with various data types.

You may have tried something like this:

[[See Video to Reveal this Text or Code Snippet]]

However, instead of success, you get compilation errors. Let’s break this down and explore how we can fix this issue effectively.

Understanding the Compilation Error

The error message you encounter is indicative of Java's strict type-checking in generics. It clarifies that:

Callable<Mono<Integer>> and Callable<Mono<String>> cannot be treated as Callable<Mono<?>>.

This is due to the fact that Java generics are invariant, meaning they do not support type substitution in a way that accommodates the required structure.

Solutions to the Problem

Fortunately, there are a couple of approaches you can take to overcome this challenge. Let’s explore both options in detail.

Option 1: Change the Type in Your Method

The simplest way to address the issue is to change the type of Callable in yourMethod1 to accommodate the wildcard type. Here’s how you can adjust your code:

[[See Video to Reveal this Text or Code Snippet]]

Option 2: Use Wildcards in the Method Parameter

Alternatively, you can keep the specific types in your method and use an upper bounded wildcard in the mymethod parameter. Here’s how that looks:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Both approaches allow you to pass different types of Callable containing Mono into your methods without encountering compilation errors. The choice between the two depends on your specific requirements and coding style.

By handling generics correctly, you can leverage Java's powerful type system while maintaining clear and manageable code. Experiment with these solutions and see which one works best for your application!