Kotlin API Guidelines: Creating High-Quality APIs

Kotlin|Apr 9, 2023|Last edited: May 4, 2023
  • Kotlin
  • Best Practices
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  • Kotlin API Guidelines: Creating High-Quality APIs
    Apr 9, 2023
    Optimize your Kotlin API development with our comprehensive guide, covering backward compatibility, debuggability, predictability, and readability.
    Best Practices

    Mastering the Art of API Design with Kotlin: Best Practices and Guidelines for Backward Compatibility, Debuggability, Predictability, and Readability

    Kotlin is a popular programming language used for developing Android apps. It is a statically typed language that is designed to be concise, expressive, and safe. With the release of the Kotlin API guidelines, Kotlin developers can now have a comprehensive set of guidelines to follow when developing APIs for their libraries.
    The Kotlin API guidelines are a set of best practices and ideas that can help developers create high-quality APIs that are backward compatible, debuggable, predictable, and readable. In this blog post, we will explore some of the key topics covered in the guidelines.
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    Key takeaways from the "jvm-api-guidelines" series

    1. Debuggability: Always provide a toString() method to facilitate debugging and make it easier for users to understand the object state.
    1. Predictability: Use sealed interfaces to provide a well-defined set of subtypes, and use sealed classes to hide implementation details. Validate inputs and state to prevent errors and avoid using arrays and varargs in public APIs.
    1. Backward Compatibility: Define what backward compatibility means for your API, and use annotations such as @PublishedApi and @RequiresOptIn to provide clear guidance to users. Consider using explicit API mode and tools to enforce backward compatibility.
    1. Readability: Use consistent naming conventions and formatting to ensure API consistency. Consider using builder DSL and constructor-like functions where applicable, and use member and extension functions appropriately. Avoid using Boolean arguments in functions.

    Good Library Design

    When developing a Kotlim library, it's important to create an API that is easy to understand and use. This can be achieved by following a set of best practices that prioritize features like backward compatibility, understandable documentation, low cognitive complexity, and consistent API design. In this blog, we will explore these features and how to implement them effectively.

    Backward compatibility

    One of the key features of a good library is backward compatibility. This means that the library should maintain compatibility with previous versions, ensuring that developers can continue to use the library without needing to update their code. This feature is essential as it provides a sense of stability and consistency for users of the library.

    Understandable and complete documentation

    A library's documentation should be complete and easy to understand. This means that the documentation should provide comprehensive information about how to use the library, including its features and functionality. Providing clear examples and use cases can also help users to better understand how the library works.

    Low cognitive complexity

    A library's API should be designed with low cognitive complexity in mind. This means that the API should be easy to understand and use, with a clear and logical structure. One way to achieve this is by breaking the API down into smaller, more manageable components, each with its own clear purpose.

    Readability and Consistent API

    A reliable and comprehensively-documented application programming interface (API) is paramount in ensuring a seamless development process. This assertion holds true for factors such as argument order, nomenclature conventions, and overloads. Furthermore, documenting all conventions is a worthwhile endeavor.


    A library's API should be predictable, meaning that the behavior of the API should be consistent and intuitive. This can be achieved by adhering to established design patterns and conventions, and by avoiding unexpected behaviors or edge cases.


    Debugging is an essential part of software development, and a library's API should be designed with debuggability in mind. This means that the API should provide clear error messages and logging, making it easy to identify and fix issues when they arise.


    In conclusion, creating a good Kotlin library requires careful consideration of the API design. By prioritizing features like backward compatibility, understandable documentation, low cognitive complexity, and consistent API design, developers can create libraries that are easy to use and maintain. By following best practices like those outlined in this guide, developers can create high-quality libraries that are a joy to work with.
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