cover edge cases for file name extension comparison

Signed-off-by: alperozturk96 <alper_ozturk@proton.me>

Author: alperozturk96

app/src/androidTest/java/com/nextcloud/utils/FileExtensionTest.kt

@@ -0,0 +1,116 @@
+/*
+ * Nextcloud - Android Client
+ *
+ * SPDX-FileCopyrightText: 2026 Alper Ozturk <alper.ozturk@nextcloud.com>
+ * SPDX-License-Identifier: AGPL-3.0-or-later
+ */
+
+package com.nextcloud.utils
+
+import com.nextcloud.utils.fileNameValidator.FileNameValidator.isExtensionChanged
+import junit.framework.TestCase.assertFalse
+import junit.framework.TestCase.assertTrue
+import org.junit.Test
+
+class FileExtensionTest {
+
+    @Test
+    fun sameExtensionReturnsFalse() {
+        assertFalse(isExtensionChanged("file.txt", "other.txt"))
+    }
+
+    @Test
+    fun differentExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged("file.txt", "file.pdf"))
+    }
+
+    @Test
+    fun caseDifferenceDoesNotTriggerChange() {
+        assertFalse(isExtensionChanged("file.JPG", "file.jpg"))
+    }
+
+    @Test
+    fun bothWithoutExtensionReturnsFalse() {
+        assertFalse(isExtensionChanged("README", "LICENSE"))
+    }
+
+    @Test
+    fun noExtensionToExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged("README", "file.txt"))
+    }
+
+    @Test
+    fun extensionToNoExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged("file.txt", "README"))
+    }
+
+    @Test
+    fun hiddenFilesWithoutExtensionReturnFalse() {
+        assertFalse(isExtensionChanged(".gitignore", ".env"))
+    }
+
+    @Test
+    fun hiddenFileToNormalExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged(".gitignore", "file.txt"))
+    }
+
+    @Test
+    fun multipleDotsSameLastExtensionReturnsFalse() {
+        assertFalse(isExtensionChanged("archive.tar.gz", "backup.gz"))
+    }
+
+    @Test
+    fun multipleDotsDifferentLastExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged("archive.tar.gz", "archive.tar.zip"))
+    }
+
+    @Test
+    fun trailingDotTreatedAsNoExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged("file.", "file.txt"))
+    }
+
+    @Test
+    fun bothTrailingDotReturnFalse() {
+        assertFalse(isExtensionChanged("file.", "another."))
+    }
+
+    @Test
+    fun emptyStringsReturnFalse() {
+        assertFalse(isExtensionChanged("", ""))
+    }
+
+    @Test
+    fun emptyStringToExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged("", "file.txt"))
+    }
+
+    @Test
+    fun bothNullReturnFalse() {
+        assertFalse(isExtensionChanged(null, null))
+    }
+
+    @Test
+    fun previousNullNewNotNullReturnsTrue() {
+        assertTrue(isExtensionChanged(null, "file.txt"))
+    }
+
+    @Test
+    fun previousNotNullNewNullReturnsTrue() {
+        assertTrue(isExtensionChanged("file.txt", null))
+    }
+
+    @Test
+    fun singleDotFilenameReturnsFalse() {
+        assertFalse(isExtensionChanged(".", "."))
+    }
+
+    @Test
+    fun dotToExtensionReturnsTrue() {
+        assertTrue(isExtensionChanged(".", "file.txt"))
+    }
+
+    @Test
+    fun filenamesEndingWithDotReturnFalse() {
+        assertFalse(isExtensionChanged("test.", "another."))
+    }
+}

app/src/main/java/com/nextcloud/utils/extensions/StringExtensions.kt

@@ -35,6 +35,17 @@ fun String?.eTagChanged(eTagOnServer: String?): Boolean {
     return !this.equals(eTagOnServer, ignoreCase = true)
 }
 
+fun String.extension(): String {
+    val lastDot = lastIndexOf('.')
+
+    //  return empty string for filenames like ".gitignore"
+    if (lastDot <= 0 || lastDot == length - 1) {
+        return ""
+    }
+
+    return substring(lastDot + 1).lowercase()
+}
+
 fun String.truncateWithEllipsis(limit: Int) = take(limit) + if (length > limit) StringConstants.THREE_DOT else ""
 
 object StringConstants {

app/src/main/java/com/nextcloud/utils/fileNameValidator/FileNameValidator.kt

@@ -11,6 +11,7 @@ import android.content.Context
 import android.text.TextUtils
 import com.nextcloud.utils.extensions.StringConstants
 import com.nextcloud.utils.extensions.checkWCFRestrictions
+import com.nextcloud.utils.extensions.extension
 import com.nextcloud.utils.extensions.forbiddenFilenameBaseNames
 import com.nextcloud.utils.extensions.forbiddenFilenameCharacters
 import com.nextcloud.utils.extensions.forbiddenFilenameExtensions
@@ -19,7 +20,6 @@ import com.nextcloud.utils.extensions.removeFileExtension
 import com.owncloud.android.R
 import com.owncloud.android.datamodel.OCFile
 import com.owncloud.android.lib.resources.status.OCCapability
-import java.io.File
 
 object FileNameValidator {
 
@@ -148,17 +148,12 @@ object FileNameValidator {
         return null
     }
 
-    /**
-     * @return True, if the extension of both filenames is different. If either filename is null, function returns false
-     */
     fun isExtensionChanged(previousFileName: String?, newFileName: String?): Boolean {
         if (previousFileName == null || newFileName == null) {
-            return false
+            return previousFileName != newFileName
         }
-        val previousExtension = File(previousFileName).extension
-        val newExtension = File(newFileName).extension
 
-        return previousExtension != newExtension
+        return previousFileName.extension() != newFileName.extension()
     }
 
     fun isFileHidden(name: String): Boolean = !TextUtils.isEmpty(name) && name[0] == '.'