src/main/kotlin/filesystem/FSCreator.kt

@@ -1,8 +1,10 @@
 package filesystem
 
 import java.nio.file.FileAlreadyExistsException
-import java.nio.file.Files
 import java.nio.file.Path
+import kotlin.io.path.createDirectory
+import kotlin.io.path.createFile
+import kotlin.io.path.writeText
 
 class FSCreator {
     /**
@@ -31,13 +33,13 @@ class FSCreator {
             val path = dest.resolve(entry.name)
             try {
                 when (entry) {
-                    is FSFile -> Files.createFile(path)
-                    is FSFolder -> Files.createDirectory(path)
+                    is FSFile -> path.createFile()
+                    is FSFolder -> path.createDirectory()
                 }
             } catch (_: FileAlreadyExistsException) {
             } // Allow files/folders to already exist.
             when (entry) {
-                is FSFile -> Files.write(path, entry.content.toByteArray())
+                is FSFile -> path.writeText(entry.content)
                 is FSFolder -> queue.addAll(entry.entries.map { it to path })
             }
         }

src/main/kotlin/filesystem/FSEntry.kt

@@ -1,8 +1,18 @@
 package filesystem
 
+import kotlin.io.path.Path
+
 // Note sealed allows for simpler logic in FSCreator by guaranteeing FSFile and FSFolder are the only possible FSEntries
 // (as we expect), and it also implicitly makes the class abstract as required.
-sealed class FSEntry(val name: String)
+sealed class FSEntry(val name: String) {
+    init {
+        val p = Path(name)
+        // Only allow single filenames (no paths or relative references (e.g. ".."))
+        if (p.toList().size != 1 || p.fileName != p.toFile().canonicalFile.toPath().fileName) {
+            throw InvalidEntryNameException(name)
+        }
+    }
+}
 
 class FSFile(name: String, val content: String) : FSEntry(name)
 
@@ -24,4 +34,6 @@ class FSFolder(name: String, val entries: List<FSEntry>) : FSEntry(name) {
         }
         return false
     }
-}
+}
+
+class InvalidEntryNameException(name: String) : Exception("Invalid FSEntry name: '$name'")