ContentFolder.kt

package org.geepawhill.admin

import org.apache.commons.io.FileUtils
import java.nio.file.Files
import java.nio.file.Path

class ContentFolder(val root: Path) {
    operator fun get(first: String, vararg parts: String): String {
        val resolvedPath = root.resolve(Path.of(first, *parts))
        return this[resolvedPath]
    }

    operator fun get(path: Path): String {
        return Files.readString(path)
    }

    operator fun set(first: String, vararg parts: String, value: String) {
        val resolvedPath = root.resolve(Path.of(first, *parts))
        this[resolvedPath] = value
    }

    operator fun set(path: Path, value: String) {
        FileUtils.createParentDirectories(path.toFile())
        Files.writeString(path, value)
    }
}

ContentFolderTest.kt

package org.geepawhill.admin.content

import org.apache.commons.io.FileUtils
import org.assertj.core.api.Assertions.assertThat
import org.geepawhill.admin.ContentFolder
import org.junit.jupiter.api.AfterEach
import org.junit.jupiter.api.Test
import java.nio.file.Path

class ContentFolderTest {
    val testPath = Path.of("testData")
    val folder = ContentFolder(testPath)

    @AfterEach
    fun afterEach() {
        FileUtils.deleteDirectory(testPath.toFile())
    }

    @Test
    fun `assignment writes file`() {
        folder["index.html"] = "A string"
        assertThat(folder["index.html"]).isEqualTo("A string")
    }

    @Test
    fun `works using path`() {
        folder[Path.of("myfile")] = "A string"
        assertThat(folder[Path.of("myfile")]).isEqualTo("A string")
    }
}