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Kotlin Tooling Java To Kotlin

JetBrainsOfficialCreated by JetBrainsDistributed by JetBrains

  • Skill
  • Workflow
  • Software development
  • Apache-2.0
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From the author

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.
— the description by JetBrains

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a disciplined, four-step methodology for migrating Java code to Kotlin. It includes framework-aware guidance for common JVM libraries such as Spring, Hibernate, Lombok, and others. The process emphasizes maintaining behavioral parity through a chain-of-thought approach, checking five specific invariants at each stage to ensure the resulting Kotlin code is idiomatic, safe, and maintainable.

What it does

  • Scans Java files to detect and load relevant framework-specific conversion guides.
  • Executes a four-step conversion process: 1:1 translation, nullability audit, collection conversion, and idiomatic transformation.
  • Validates code against five core invariants at every step to prevent regressions.
  • Provides instructions for preserving Git history during file renaming.
  • Offers a checklist for verifying compilation and behavioral consistency.
  • Handles common pitfalls like keyword conflicts, SAM ambiguity, and platform types.

When to use it

  • When migrating existing Java projects or individual files to Kotlin.
  • When you need to ensure converted code follows idiomatic Kotlin patterns.
  • When working with Java codebases that utilize frameworks like Spring, Hibernate, or Dagger.
  • When you need to maintain Git blame history while refactoring Java to Kotlin.

Advantages

  • Reduces manual effort by providing a systematic, repeatable conversion workflow.
  • Ensures framework-specific idioms are applied correctly through targeted reference guides.
  • Maintains code integrity by enforcing strict invariants throughout the transformation.
  • Minimizes common interop issues by addressing platform types and JVM annotations.

Usage examples

“Convert this Java file to idiomatic Kotlin.”
The agent will scan the file for frameworks, perform the four-step conversion, and output the resulting .kt file.
“Migrate my Java project to Kotlin.”
The agent will list all .java files, sort them by dependency order, and perform a batch conversion following the defined workflow.

Install

  1. Download the skill as a ZIP:

    https://mp-staging.autogineer.com/agent-skills/jetbrains/kotlin-tooling-java-to-kotlin/download
  2. Unzip it into the cross-client skills folder of your project:

    mkdir -p .agents/skills && unzip kotlin-tooling-java-to-kotlin.zip -d .agents/skills

Documentation by JetBrains

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

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 prefixFramework guide
org.springframework.*SPRING.md
lombok.*LOMBOK.md
javax.persistence.*, jakarta.persistence.*, org.hibernate.*HIBERNATE.md
com.fasterxml.jackson.*JACKSON.md
io.micronaut.*MICRONAUT.md
io.quarkus.*, javax.enterprise.*, jakarta.enterprise.*QUARKUS.md
dagger.*, dagger.hilt.*DAGGER-HILT.md
io.reactivex.*, rx.*RXJAVA.md
org.junit.*, org.testng.*JUNIT.md
com.google.inject.*GUICE.md
retrofit2.*, okhttp3.*RETROFIT.md
org.mockito.*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.

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:

# 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:

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:

  • 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 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

Files

references/frameworks/MOCKITO.md

Mockito to MockK Conversion Guide

When This Applies

Detected when imports match org.mockito.*.

While Mockito works in Kotlin, MockK is the idiomatic Kotlin mocking library. Convert to MockK when the project already uses MockK or is doing a full Kotlin migration. If the project wants to keep Mockito, convert only the Java syntax to Kotlin syntax using the mockito-kotlin helper library.

Key Rules

1. MockK conversion table
MockitoMockK
Mockito.mock(Foo.class)mockk<Foo>()
@Mock Foo foo@MockK lateinit var foo: Foo (with @ExtendWith(MockKExtension::class))
when(foo.bar()).thenReturn(x)every { foo.bar() } returns x
when(foo.bar()).thenThrow(e)every { foo.bar() } throws e
when(foo.bar()).thenAnswer { }every { foo.bar() } answers { }
doNothing().when(foo).bar()justRun { foo.bar() }
verify(foo).bar()verify { foo.bar() }
verify(foo, times(2)).bar()verify(exactly = 2) { foo.bar() }
verify(foo, never()).bar()verify(exactly = 0) { foo.bar() }
ArgumentCaptor<T>slot<T>() and capture(slot)
any()any()
eq(x)eq(x) (often not needed — MockK matches exact values by default)
Mockito.spy(obj)spyk(obj)
@InjectMocksNo direct equivalent — use constructor injection
verifyNoMoreInteractions(foo)confirmVerified(foo)
2. Coroutine support in MockK

For suspending functions, use coEvery and coVerify instead of every and verify:

coEvery { foo.suspendBar() } returns x
coVerify { foo.suspendBar() }
3. Keeping Mockito (syntax-only conversion)

If keeping Mockito, use the mockito-kotlin library (org.mockito.kotlin) for Kotlin-friendly wrappers:

  • mock<Foo>() instead of Mockito.mock(Foo::class.java) — uses reified generics.

  • whenever(foo.bar()) instead of Mockito.`when`(foo.bar()) — avoids backtick- escaping when (it is a Kotlin keyword).

  • argumentCaptor<T>() — type-safe captor via reified generics.

  • any() — properly handles Kotlin's non-null types.

4. Relaxed mocks

MockK supports relaxed mocks that return default values without explicit stubbing: mockk<Foo>(relaxed = true). This has no direct Mockito equivalent (Mockito's RETURNS_DEFAULTS is the closest).


Example 1: Converting to MockK

Java Input
package com.acme.service;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;
import static org.mockito.ArgumentMatchers.anyLong;

import org.junit.Before;
import org.junit.Test;
import org.mockito.ArgumentCaptor;

/**
 * Tests for OrderService using Mockito mocks.
 */
public class OrderServiceTest {

    private UserRepository userRepository;
    private OrderRepository orderRepository;
    private OrderService orderService;

    @Before
    public void setUp() {
        userRepository = mock(UserRepository.class);
        orderRepository = mock(OrderRepository.class);
        orderService = new OrderService(userRepository, orderRepository);
    }

    @Test
    public void testCreateOrderForUser() {
        User user = new User(1L, "Alice");
        when(userRepository.findById(1L)).thenReturn(user);

        orderService.createOrder(1L, "ITEM-100");

        ArgumentCaptor<Order> captor = ArgumentCaptor.forClass(Order.class);
        verify(orderRepository).save(captor.capture());
        assertEquals("ITEM-100", captor.getValue().getItemCode());
        assertEquals(1L, captor.getValue().getUserId());
    }

    @Test
    public void testGetOrderCount() {
        when(orderRepository.countByUserId(anyLong())).thenReturn(5);

        int count = orderService.getOrderCount(1L);

        assertEquals(5, count);
        verify(orderRepository).countByUserId(1L);
    }
}
Kotlin Output (MockK)
package com.acme.service

import io.mockk.every
import io.mockk.mockk
import io.mockk.slot
import io.mockk.verify
import org.junit.jupiter.api.BeforeEach
import org.junit.jupiter.api.Test
import kotlin.test.assertEquals

/**
 * Tests for OrderService using MockK mocks.
 */
class OrderServiceTest {

    private val userRepository = mockk<UserRepository>()
    private val orderRepository = mockk<OrderRepository>()
    private val orderService = OrderService(userRepository, orderRepository)

    @Test
    fun `should create order for user`() {
        val user = User(1L, "Alice")
        every { userRepository.findById(1L) } returns user
        every { orderRepository.save(any()) } returns Unit

        orderService.createOrder(1L, "ITEM-100")

        val orderSlot = slot<Order>()
        verify { orderRepository.save(capture(orderSlot)) }
        assertEquals("ITEM-100", orderSlot.captured.itemCode)
        assertEquals(1L, orderSlot.captured.userId)
    }

    @Test
    fun `should return order count`() {
        every { orderRepository.countByUserId(any()) } returns 5

        val count = orderService.getOrderCount(1L)

        assertEquals(5, count)
        verify { orderRepository.countByUserId(1L) }
    }
}

Key points:

  • mock(Foo.class) → mockk<Foo>() using reified generics.

  • @Before setUp is eliminated — mocks are initialized inline with property declarations. This works because MockK mocks do not require a runner.

  • when(...).thenReturn(...) → every { ... } returns ....

  • ArgumentCaptor → slot<T>() with capture(slot), accessed via slot.captured.

  • anyLong() → any() (MockK's any() handles all types).

  • verify(foo).bar() → verify { foo.bar() }.


Example 2: Keeping Mockito (mockito-kotlin syntax)

Java Input
package com.acme.service;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

import org.junit.Before;
import org.junit.Test;

/**
 * Tests for PricingService using Mockito.
 */
public class PricingServiceTest {

    private PriceRepository priceRepository;
    private PricingService pricingService;

    @Before
    public void setUp() {
        priceRepository = mock(PriceRepository.class);
        pricingService = new PricingService(priceRepository);
    }

    @Test
    public void testGetPrice() {
        when(priceRepository.findPriceByItemCode("ITEM-1")).thenReturn(9.99);
        double price = pricingService.getPrice("ITEM-1");
        assertEquals(9.99, price, 0.001);
        verify(priceRepository).findPriceByItemCode("ITEM-1");
    }
}
Kotlin Output (mockito-kotlin)
package com.acme.service

import org.junit.jupiter.api.BeforeEach
import org.junit.jupiter.api.Test
import org.mockito.kotlin.mock
import org.mockito.kotlin.verify
import org.mockito.kotlin.whenever
import kotlin.test.assertEquals

/**
 * Tests for PricingService using Mockito.
 */
class PricingServiceTest {

    private val priceRepository = mock<PriceRepository>()
    private val pricingService = PricingService(priceRepository)

    @Test
    fun `should return price for item`() {
        whenever(priceRepository.findPriceByItemCode("ITEM-1")).thenReturn(9.99)

        val price = pricingService.getPrice("ITEM-1")

        assertEquals(9.99, price, 0.001)
        verify(priceRepository).findPriceByItemCode("ITEM-1")
    }
}

Key points:

  • mock(Foo.class) → mock<Foo>() from org.mockito.kotlin (reified generics).

  • when(...) → whenever(...) to avoid backtick-escaping the when keyword.

  • verify stays the same — org.mockito.kotlin.verify wraps Mockito's verify.

  • The setUp method is eliminated — mocks are initialized inline.

  • assertEquals with a delta parameter works the same way from kotlin.test.

Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Needs a command-line tool

Security

No issues found by our static scan· No issues found

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Attribution and licence

Created by
JetBrains
Publisher
JetBrains (Official)
Distributed by
JetBrains
Source
JetBrains/skills · kotlin-tooling-java-to-kotlin · commit 3f53782
Licence
Apache License 2.0 (redistribution allowed)
  • LICENSE (written by AutoGineer)
  • ATTRIBUTION.md (written by AutoGineer)
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