---
name: kotlin-tooling-java-to-kotlin
description: >
  Use when converting Java source files to idiomatic Kotlin, when user mentions
  "java to kotlin", "j2k", "convert java", "migrate java to kotlin", or when
  working with .java files that need to become .kt files. Handles framework-aware
  conversion for Spring, Lombok, Hibernate, Jackson, Micronaut, Quarkus, Dagger/Hilt,
  RxJava, JUnit, Guice, Retrofit, and Mockito.
license: Apache-2.0
metadata:
  author: JetBrains
  version: "1.0.0"
---

# Java to Kotlin Conversion

Convert Java source files to idiomatic Kotlin using a disciplined 4-step conversion
methodology with 5 invariants checked at each step. Supports framework-aware conversion
that handles annotation site targets, library idioms, and API preservation.

## Workflow

```dot
digraph j2k_workflow {
  rankdir=TB;
  "User specifies files" -> "Step 0: Scan & Detect";
  "Step 0: Scan & Detect" -> "Load framework guides";
  "Load framework guides" -> "Step 1: Convert";
  "Step 1: Convert" -> "Step 2: Write .kt";
  "Step 2: Write .kt" -> "Step 3: Git rename";
  "Step 3: Git rename" -> "Step 4: Verify";
  "Step 4: Verify" -> "Next file?" [label="pass"];
  "Step 4: Verify" -> "Fix issues" [label="fail"];
  "Fix issues" -> "Step 1: Convert";
  "Next file?" -> "Step 0: Scan & Detect" [label="batch: yes"];
  "Next file?" -> "Done" [label="no more files"];
}
```

## Step 0: Scan & Detect Frameworks

Before converting, scan the Java file's import statements to detect which frameworks
are in use. Load ONLY the matching framework reference files to keep context focused.

### Framework Detection Table

| Import prefix | Framework guide |
|---|---|
| `org.springframework.*` | [SPRING.md](references/frameworks/SPRING.md) |
| `lombok.*` | [LOMBOK.md](references/frameworks/LOMBOK.md) |
| `javax.persistence.*`, `jakarta.persistence.*`, `org.hibernate.*` | [HIBERNATE.md](references/frameworks/HIBERNATE.md) |
| `com.fasterxml.jackson.*` | [JACKSON.md](references/frameworks/JACKSON.md) |
| `io.micronaut.*` | [MICRONAUT.md](references/frameworks/MICRONAUT.md) |
| `io.quarkus.*`, `javax.enterprise.*`, `jakarta.enterprise.*` | [QUARKUS.md](references/frameworks/QUARKUS.md) |
| `dagger.*`, `dagger.hilt.*` | [DAGGER-HILT.md](references/frameworks/DAGGER-HILT.md) |
| `io.reactivex.*`, `rx.*` | [RXJAVA.md](references/frameworks/RXJAVA.md) |
| `org.junit.*`, `org.testng.*` | [JUNIT.md](references/frameworks/JUNIT.md) |
| `com.google.inject.*` | [GUICE.md](references/frameworks/GUICE.md) |
| `retrofit2.*`, `okhttp3.*` | [RETROFIT.md](references/frameworks/RETROFIT.md) |
| `org.mockito.*` | [MOCKITO.md](references/frameworks/MOCKITO.md) |

If `javax.inject.*` is detected, check for Dagger/Hilt vs Guice by looking for other
imports from those frameworks. If ambiguous, load both guides.

## Step 1: Convert

Apply the conversion methodology from [CONVERSION-METHODOLOGY.md](references/CONVERSION-METHODOLOGY.md).

This is a 4-step chain-of-thought process:
1. **Faithful 1:1 translation** — exact semantics preserved
2. **Nullability & mutability audit** — val/var, nullable types
3. **Collection type conversion** — Java mutable → Kotlin types
4. **Idiomatic transformations** — properties, string templates, lambdas

Five invariants are checked after each step. If any invariant is violated, revert
to the previous step and redo.

Apply any loaded framework-specific guidance during step 4 (idiomatic transformations).

## Step 2: Write Output

Write the converted Kotlin code to a `.kt` file with the same name as the original
Java file, in the same directory.

## Step 3: Preserve Git History

To preserve `git blame` history, use a two-phase approach:

```bash
# Phase 1: Rename (creates rename tracking)
git mv src/main/java/com/example/Foo.java src/main/kotlin/com/example/Foo.kt
git commit -m "Rename Foo.java to Foo.kt"

# Phase 2: Replace content (tracked as modification, not new file)
# Write the converted Kotlin content to Foo.kt
git commit -m "Convert Foo from Java to Kotlin"
```

If the project keeps Java and Kotlin in the same source root (e.g., `src/main/java/`),
rename in place:

```bash
git mv src/main/java/com/example/Foo.java src/main/java/com/example/Foo.kt
```

If the project does not use Git, simply write the `.kt` file and delete the `.java` file.

## Step 4: Verify

After conversion, verify using [checklist.md](assets/checklist.md):
- Attempt to compile the converted file
- Run existing tests
- Check annotation site targets
- Confirm no behavioral changes

## Batch Conversion

When converting multiple files (a directory or package):

1. **List all `.java` files** in the target scope
2. **Sort by dependency order** — convert leaf dependencies first (files that don't
   import other files in the conversion set), then work up to files that depend on them
3. **Convert one file at a time** — apply the full workflow (steps 0-4) for each
4. **Track progress** — report which files are done, which remain
5. **Handle cross-references** — after converting a file, update imports in other Java
   files if needed (e.g., if a class moved packages)

For large batches, consider converting in packages (bottom-up from leaf packages).

## Common Pitfalls

See [KNOWN-ISSUES.md](references/KNOWN-ISSUES.md) for:
- Kotlin keyword conflicts (`when`, `in`, `is`, `object`)
- SAM conversion ambiguity
- Platform types from Java interop
- `@JvmStatic` / `@JvmField` / `@JvmOverloads` usage
- Checked exceptions and `@Throws`
- Wildcard generics → Kotlin variance