fix: ignore static fields and add Kotlin interop tests

Static fields (other than serialVersionUID, which had a name-based
filter) previously leaked into config declarations because no general
isStatic() filter existed. Most visibly this broke Kotlin compatibility:
companion objects emit a synthetic public static final Companion field
on the enclosing class, causing save() to crash with "cannot simplify
type ...$Companion".

Adds the missing Modifier.isStatic() check in FieldDeclaration.of and
removes the now-redundant serialVersionUID name check (since it is
always static).

Adds a Java regression test (StaticFieldFilterTest) covering static
final, static mutable, serialVersionUID, and all-static-fields cases.

Adds a new core-test-kotlin module with broader Kotlin interop coverage:
companion objects (including named, empty, const val, @JvmStatic),
plain Kotlin classes, var/val properties, nullable types, and default
value handling.

Supersedes #54 (which lacked tests).

Author: sandrwich

core-test-kotlin/pom.xml

@@ -0,0 +1,134 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xmlns="http://maven.apache.org/POM/4.0.0"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>eu.okaeri</groupId>
+        <artifactId>okaeri-configs</artifactId>
+        <version>6.1.0-beta.1</version>
+    </parent>
+
+    <artifactId>okaeri-configs-core-test-kotlin</artifactId>
+    <name>okaeri-configs (core-test-kotlin)</name>
+    <description>Kotlin interop tests for okaeri-configs</description>
+
+    <properties>
+        <maven.compiler.source>21</maven.compiler.source>
+        <maven.compiler.target>21</maven.compiler.target>
+        <kotlin.jvmTarget>21</kotlin.jvmTarget>
+    </properties>
+
+    <dependencies>
+        <!-- Core library -->
+        <dependency>
+            <groupId>eu.okaeri</groupId>
+            <artifactId>okaeri-configs-core</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+
+        <!-- Test commons (shared test utilities) -->
+        <dependency>
+            <groupId>eu.okaeri</groupId>
+            <artifactId>okaeri-configs-test-commons</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+
+        <!-- YAML SnakeYAML for serialization round-trip testing -->
+        <dependency>
+            <groupId>eu.okaeri</groupId>
+            <artifactId>okaeri-configs-yaml-snakeyaml</artifactId>
+            <version>${project.version}</version>
+        </dependency>
+
+        <!-- Kotlin -->
+        <dependency>
+            <groupId>org.jetbrains.kotlin</groupId>
+            <artifactId>kotlin-stdlib</artifactId>
+            <version>${kotlin.version}</version>
+        </dependency>
+
+        <!-- JUnit 5 -->
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-api</artifactId>
+            <version>${junit.version}</version>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter-engine</artifactId>
+            <version>${junit.version}</version>
+            <scope>test</scope>
+        </dependency>
+
+        <!-- AssertJ -->
+        <dependency>
+            <groupId>org.assertj</groupId>
+            <artifactId>assertj-core</artifactId>
+            <version>${assertj.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <testSourceDirectory>src/test/kotlin</testSourceDirectory>
+        <plugins>
+            <plugin>
+                <groupId>org.jetbrains.kotlin</groupId>
+                <artifactId>kotlin-maven-plugin</artifactId>
+                <version>${kotlin.version}</version>
+                <executions>
+                    <execution>
+                        <id>test-compile</id>
+                        <phase>test-compile</phase>
+                        <goals>
+                            <goal>test-compile</goal>
+                        </goals>
+                    </execution>
+                </executions>
+                <configuration>
+                    <jvmTarget>${kotlin.jvmTarget}</jvmTarget>
+                </configuration>
+            </plugin>
+            <plugin>
+                <artifactId>maven-source-plugin</artifactId>
+                <configuration>
+                    <skipSource>true</skipSource>
+                </configuration>
+            </plugin>
+            <plugin>
+                <artifactId>maven-javadoc-plugin</artifactId>
+                <configuration>
+                    <skip>true</skip>
+                </configuration>
+            </plugin>
+            <plugin>
+                <artifactId>maven-deploy-plugin</artifactId>
+                <configuration>
+                    <skip>true</skip>
+                </configuration>
+            </plugin>
+            <plugin>
+                <groupId>org.jacoco</groupId>
+                <artifactId>jacoco-maven-plugin</artifactId>
+                <executions>
+                    <execution>
+                        <id>report-aggregate</id>
+                        <phase>verify</phase>
+                        <goals>
+                            <goal>report-aggregate</goal>
+                        </goals>
+                        <configuration>
+                            <excludes>
+                                <exclude>eu/okaeri/configs/test/**/*</exclude>
+                            </excludes>
+                        </configuration>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
+
+</project>

core-test-kotlin/src/test/kotlin/eu/okaeri/configs/kotlin/KotlinClassRoundTripTest.kt

@@ -0,0 +1,97 @@
+package eu.okaeri.configs.kotlin
+
+import eu.okaeri.configs.ConfigManager
+import eu.okaeri.configs.OkaeriConfig
+import eu.okaeri.configs.yaml.snakeyaml.YamlSnakeYamlConfigurer
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
+
+/**
+ * Verifies that Kotlin classes extending OkaeriConfig can be saved and loaded
+ * correctly, preserving values through round-trip.
+ *
+ * Kotlin idiomatic configs use plain classes with `var` properties because
+ * OkaeriConfig requires a no-arg constructor (incompatible with Kotlin
+ * `data class` primary-constructor parameters).
+ */
+class KotlinClassRoundTripTest {
+
+    class SimpleKotlinConfig : OkaeriConfig() {
+        var stringField: String = "default"
+        var intField: Int = 42
+        var booleanField: Boolean = false
+        var doubleField: Double = 3.14
+    }
+
+    class CollectionKotlinConfig : OkaeriConfig() {
+        var stringList: MutableList<String> = mutableListOf("a", "b", "c")
+        var intMap: MutableMap<String, Int> = mutableMapOf("one" to 1, "two" to 2)
+    }
+
+    @Test
+    fun `simple config round-trip preserves all values`(@TempDir tempDir: Path) {
+        val file = tempDir.resolve("simple.yml").toFile()
+
+        val original = ConfigManager.create(SimpleKotlinConfig::class.java) { it ->
+            it.configure { opt ->
+                opt.configurer(YamlSnakeYamlConfigurer())
+                opt.bindFile(file)
+            }
+        }
+        original.stringField = "modified"
+        original.intField = 999
+        original.booleanField = true
+        original.doubleField = 2.718
+        original.save()
+
+        val loaded = ConfigManager.create(SimpleKotlinConfig::class.java) { it ->
+            it.configure { opt ->
+                opt.configurer(YamlSnakeYamlConfigurer())
+                opt.bindFile(file)
+            }
+            it.load()
+        }
+
+        assertThat(loaded.stringField).isEqualTo("modified")
+        assertThat(loaded.intField).isEqualTo(999)
+        assertThat(loaded.booleanField).isTrue()
+        assertThat(loaded.doubleField).isEqualTo(2.718)
+    }
+
+    @Test
+    fun `collections round-trip preserves contents`(@TempDir tempDir: Path) {
+        val file = tempDir.resolve("collections.yml").toFile()
+
+        val original = ConfigManager.create(CollectionKotlinConfig::class.java) { it ->
+            it.configure { opt ->
+                opt.configurer(YamlSnakeYamlConfigurer())
+                opt.bindFile(file)
+            }
+        }
+        original.stringList = mutableListOf("x", "y", "z")
+        original.intMap = mutableMapOf("alpha" to 10, "beta" to 20)
+        original.save()
+
+        val loaded = ConfigManager.create(CollectionKotlinConfig::class.java) { it ->
+            it.configure { opt ->
+                opt.configurer(YamlSnakeYamlConfigurer())
+                opt.bindFile(file)
+            }
+            it.load()
+        }
+
+        assertThat(loaded.stringList).containsExactly("x", "y", "z")
+        assertThat(loaded.intMap).containsEntry("alpha", 10).containsEntry("beta", 20)
+    }
+
+    @Test
+    fun `default values are picked up by declaration`() {
+        val config = ConfigManager.create(SimpleKotlinConfig::class.java)
+        val fieldNames = config.declaration.fields.map { it.name }
+        assertThat(fieldNames).containsExactlyInAnyOrder(
+            "stringField", "intField", "booleanField", "doubleField"
+        )
+    }
+}

core-test-kotlin/src/test/kotlin/eu/okaeri/configs/kotlin/KotlinCompanionObjectTest.kt

@@ -0,0 +1,130 @@
+package eu.okaeri.configs.kotlin
+
+import eu.okaeri.configs.ConfigManager
+import eu.okaeri.configs.OkaeriConfig
+import eu.okaeri.configs.configurer.InMemoryConfigurer
+import eu.okaeri.configs.yaml.snakeyaml.YamlSnakeYamlConfigurer
+import org.assertj.core.api.Assertions.assertThat
+import org.junit.jupiter.api.Test
+import org.junit.jupiter.api.io.TempDir
+import java.nio.file.Path
+
+/**
+ * Verifies that Kotlin companion objects do not interfere with config serialization.
+ * The Kotlin compiler synthesizes a `public static final Companion` field on the
+ * enclosing class. Without static field filtering, this would crash on save with
+ * "cannot simplify type ...$Companion".
+ */
+class KotlinCompanionObjectTest {
+
+    class ConfigWithCompanion : OkaeriConfig() {
+        var name: String = "default"
+        var count: Int = 0
+
+        companion object {
+            const val MAX_COUNT = 100
+            fun create(): ConfigWithCompanion = ConfigWithCompanion()
+        }
+    }
+
+    class ConfigWithEmptyCompanion : OkaeriConfig() {
+        var value: String = "hello"
+
+        companion object
+    }
+
+    class ConfigWithNamedCompanion : OkaeriConfig() {
+        var label: String = "n"
+
+        companion object Helper {
+            fun build(): ConfigWithNamedCompanion = ConfigWithNamedCompanion()
+        }
+    }
+
+    @Test
+    fun `companion field is not in declaration`() {
+        val config = ConfigManager.create(ConfigWithCompanion::class.java)
+        assertThat(config.declaration.getField("Companion").orElse(null)).isNull()
+    }
+
+    @Test
+    fun `const val from companion is not in declaration`() {
+        val config = ConfigManager.create(ConfigWithCompanion::class.java)
+        assertThat(config.declaration.getField("MAX_COUNT").orElse(null)).isNull()
+    }
+
+    @Test
+    fun `regular fields are still in declaration`() {
+        val config = ConfigManager.create(ConfigWithCompanion::class.java)
+        assertThat(config.declaration.getField("name").isPresent).isTrue()
+        assertThat(config.declaration.getField("count").isPresent).isTrue()
+    }
+
+    @Test
+    fun `declaration only contains instance fields`() {
+        val config = ConfigManager.create(ConfigWithCompanion::class.java)
+        val fieldNames = config.declaration.fields.map { it.name }
+        assertThat(fieldNames).containsExactlyInAnyOrder("name", "count")
+    }
+
+    @Test
+    fun `save with companion does not crash`() {
+        val config = ConfigManager.create(ConfigWithCompanion::class.java) { it ->
+            it.configure { opt -> opt.configurer(InMemoryConfigurer()) }
+        }
+        config.name = "saved"
+        config.count = 5
+
+        val data = config.asMap(InMemoryConfigurer(), true)
+
+        assertThat(data).containsOnlyKeys("name", "count")
+        assertThat(data["name"]).isEqualTo("saved")
+        assertThat(data["count"]).isEqualTo(5)
+    }
+
+    @Test
+    fun `save and load round-trip preserves values`(@TempDir tempDir: Path) {
+        val file = tempDir.resolve("config.yml").toFile()
+
+        val original = ConfigManager.create(ConfigWithCompanion::class.java) { it ->
+            it.configure { opt ->
+                opt.configurer(YamlSnakeYamlConfigurer())
+                opt.bindFile(file)
+            }
+        }
+        original.name = "persisted"
+        original.count = 42
+        original.save()
+
+        val loaded = ConfigManager.create(ConfigWithCompanion::class.java) { it ->
+            it.configure { opt ->
+                opt.configurer(YamlSnakeYamlConfigurer())
+                opt.bindFile(file)
+            }
+            it.load()
+        }
+
+        assertThat(loaded.name).isEqualTo("persisted")
+        assertThat(loaded.count).isEqualTo(42)
+    }
+
+    @Test
+    fun `companion still accessible from code`() {
+        assertThat(ConfigWithCompanion.MAX_COUNT).isEqualTo(100)
+        assertThat(ConfigWithCompanion.create()).isInstanceOf(ConfigWithCompanion::class.java)
+    }
+
+    @Test
+    fun `empty companion does not break declaration`() {
+        val config = ConfigManager.create(ConfigWithEmptyCompanion::class.java)
+        val fieldNames = config.declaration.fields.map { it.name }
+        assertThat(fieldNames).containsExactly("value")
+    }
+
+    @Test
+    fun `named companion is filtered too`() {
+        val config = ConfigManager.create(ConfigWithNamedCompanion::class.java)
+        val fieldNames = config.declaration.fields.map { it.name }
+        assertThat(fieldNames).containsExactly("label")
+    }
+}