How to Format Kotlin: A Practical Guide

Formatting Kotlin code means applying consistent indentation, spacing, line breaks, braces, and source layout so that Kotlin programs are easier to read and maintain.

Kotlin has established coding conventions and is commonly formatted automatically through IDE tooling or dedicated formatters. Automated formatting is especially useful when several developers contribute to the same project.

Formatting affects how source code is presented. It does not replace compilation, testing, debugging, or code review.

Kotlin developers commonly encounter formatting through IntelliJ IDEA and Android Studio, both of which provide Kotlin-aware formatting capabilities. Kotlin projects may also use tools such as ktlint to enforce formatting and style rules.

The exact formatter used by a project can depend on its build system, IDE configuration, and team conventions. Before changing formatting, check whether the repository already defines a specific workflow.

For projects that use automated checks, consistency between local formatting and the project's CI configuration is more important than manually choosing a different style.

Consider this intentionally compressed Kotlin code:

The same example can be written with conventional spacing and indentation:

1. Open the Kotlin file or project you want to format.

2. Check whether the project already specifies formatting rules or a formatter such as ktlint.

3. Use your Kotlin-aware IDE's formatting command or the formatter configured by the project.

4. Review the resulting changes and make sure formatting has not obscured an important source-code difference.

5. If the project uses automated formatting checks, run the same check locally before committing.

6. Compile and test the project after significant formatting changes.

7. Commit the formatting changes according to the project's normal workflow.

Kotlin functions can be written as block-bodied functions or expression-bodied functions. Formatting helps make both forms readable and makes longer parameter lists easier to inspect.

Spaces around parameter types, return types, operators, and default values should remain consistent.

Kotlin constructors can appear directly in the class declaration, which makes consistent formatting important when a class has several properties or constructor parameters.

Long primary constructors can be formatted across multiple lines to improve readability.

Data classes are commonly used for models and value-like objects. Keeping constructor properties consistently arranged makes these classes easy to scan.

Kotlin supports val and var declarations for immutable and mutable variables. Consistent spacing around assignments makes declarations easier to read.

When a property has a long type or initializer, formatting tools can help preserve a clear visual structure.

Kotlin collection literals are often short enough to remain on one line, but larger lists and maps are easier to maintain when their entries are separated clearly.

Automatic formatting is particularly useful when collection entries contain nested objects or function calls.

Lambdas are heavily used in Kotlin collection operations and functional-style code. Short lambdas can remain compact, while larger lambda bodies benefit from consistent indentation.

Formatting helps distinguish the lambda body from surrounding chained operations.

Kotlin's null-safety features include nullable types, safe calls, the Elvis operator, and explicit null checks. Formatting can make these expressions easier to scan, particularly when several operations are chained.

Do not confuse formatting with null-safety analysis. A formatter changes presentation; the Kotlin compiler remains responsible for enforcing Kotlin's type rules.

Kotlin allows if and when to be used as expressions, meaning they can return values. This makes readable indentation especially important when branches contain multiple statements.

A formatter can consistently lay out branches while preserving the expression structure.

Extension functions allow Kotlin developers to add callable functionality to existing types. Keeping the receiver type, function name, parameters, and return type clearly separated improves readability.

Interfaces define contracts while classes can provide implementations. Consistent indentation and spacing make interface members and implementation methods easy to compare.

Generic Kotlin declarations can become visually dense when they contain multiple type parameters, bounds, or nested generic types.

Consistent formatting makes generic functions and classes easier to understand without requiring developers to manually adjust every line.

Kotlin coroutines frequently involve suspend functions, coroutine builders, and nested blocks. Formatting helps distinguish coroutine scopes and their enclosed operations.

Formatting does not validate coroutine behavior. Concurrency correctness still requires appropriate testing and review.

IntelliJ IDEA and Android Studio provide Kotlin-aware code formatting through their code-style and reformatting features. Developers can generally invoke the IDE's reformat action on the current file or selected source.

Projects can also share code-style settings so that different developers receive more consistent formatting results. The exact keyboard shortcut can vary by operating system and IDE configuration.

If a repository uses ktlint or another automated formatter, treat the project's configured formatter as the source of truth rather than relying only on personal IDE settings.

ktlint is a Kotlin linter and formatter commonly used to enforce Kotlin coding conventions. It can be integrated into development workflows so that formatting and style checks can run automatically.

Because projects can configure their tooling differently, check the version and configuration used by the repository before assuming that another ktlint setup will produce identical results.

Formatting and linting are related but different tasks. Formatting focuses on source-code layout, while linting can identify style violations and other patterns that deserve attention.

A formatter cannot determine whether business logic is correct, whether an API is being used appropriately, or whether an implementation is secure. Those concerns require compilation, tests, code review, and specialized analysis.

An online formatter can be useful when you need to quickly clean up or inspect supported Kotlin code without configuring a local IDE.

Paste the Kotlin source into the formatter, select Kotlin if the interface requires a language choice, run the formatting operation, and review the output before copying it back into your project.

For production code, compare important formatting changes against the formatter configured by the project. Online tools may not use exactly the same version or configuration as your local Kotlin development environment.

One common mistake is manually formatting large amounts of Kotlin code instead of using the project's automated formatter. This can produce inconsistent results and unnecessary work.

Another mistake is mixing IDE formatting settings with repository-level formatting rules. If different developers use conflicting configurations, the same file may repeatedly change during commits.

It is also easy to mistake formatting for code quality. Well-formatted Kotlin can still contain bugs, inefficient operations, incorrect null handling, or flawed application logic.

Finally, avoid using a formatter intended for another programming language. Language-aware formatters understand syntax and can make substantially different formatting decisions.

Format Kotlin code during development, before committing changes, before code review, and whenever inconsistent formatting appears in a project.

Teams often automate formatting through IDE configuration, build plugins, lint checks, or continuous integration. The exact workflow depends on the project, but automation helps prevent formatting from becoming a recurring source of review noise.

MartTools includes a Code Formatter for cleaning up supported source code directly in the browser. If Kotlin is supported by the current formatter configuration, you can use it to inspect and normalize Kotlin source.

For a production Kotlin project, use the formatter configured by your repository as the final reference. This is particularly important when your project relies on ktlint, IDE code-style settings, or another specific formatting configuration.

When submitting source code to an online service, consider whether the code contains proprietary logic, credentials, private APIs, or other information that should not be shared externally.

Before committing formatted Kotlin code, check that imports remain correct, functions and constructors are readable, classes and data classes have a clear structure, collection expressions are easy to scan, lambdas are properly indented, null-safe expressions remain understandable, and the project still compiles and passes its tests.

Use the same formatting workflow consistently across the project. Automated formatting allows developers to focus code reviews on behavior and design instead of repeatedly debating minor whitespace differences.