rest of the comments

packages/shared_preferences/shared_preferences_android/android/src/main/java/io/flutter/plugins/sharedpreferences/LegacySharedPreferencesPlugin.java

@@ -32,6 +32,7 @@
 public class LegacySharedPreferencesPlugin implements FlutterPlugin, SharedPreferencesApi {
   private static final String TAG = "SharedPreferencesPlugin";
   private static final String SHARED_PREFERENCES_NAME = "FlutterSharedPreferences";
+  // All identifiers must match the SharedPreferencesPlugin.kt file, as well as the strings.dart file.
   private static final String JSON_LIST_IDENTIFIER = "VGhpcyBpcyB0aGUgcHJlZml4IGZvciBhIGxpc3Qu!";
   private static final String LIST_IDENTIFIER = "VGhpcyBpcyB0aGUgcHJlZml4IGZvciBhIGxpc3Qu";
   private static final String BIG_INTEGER_PREFIX = "VGhpcyBpcyB0aGUgcHJlZml4IGZvciBCaWdJbnRlZ2Vy";
@@ -102,6 +103,7 @@ public void onDetachedFromEngine(@NonNull FlutterPlugin.FlutterPluginBinding bin
   }
 
   // Deprecated, for testing purposes only.
+  @Deprecated
   @Override
   public @NonNull Boolean setStringList(@NonNull String key, @NonNull List<String> value)
       throws RuntimeException {
@@ -150,10 +152,14 @@ public void onDetachedFromEngine(@NonNull FlutterPlugin.FlutterPluginBinding bin
   private Object transformPref(@NonNull String key, @NonNull Object value) {
     if (value instanceof String) {
       String stringValue = (String) value;
-      if (stringValue.startsWith(JSON_LIST_IDENTIFIER)) {
-        return value;
-      } else if (stringValue.startsWith(LIST_IDENTIFIER)) {
-        return listEncoder.decode(stringValue.substring(LIST_IDENTIFIER.length()));
+
+      if (value.startsWith(LIST_PREFIX)) {
+        // The newer JSON-encoded lists use an extended prefix to distinguish them.
+        if (value.startsWith(JSON_LIST_PREFIX)) {
+          return value;
+        } else {
+          return listEncoder.decode(stringValue.substring(LIST_IDENTIFIER.length()));
+        }
       } else if (stringValue.startsWith(BIG_INTEGER_PREFIX)) {
         // TODO (tarrinneal): Remove all BigInt code.
         // https://github.com/flutter/flutter/issues/124420

packages/shared_preferences/shared_preferences_android/android/src/main/java/io/flutter/plugins/sharedpreferences/Messages.java

@@ -94,13 +94,16 @@ public interface SharedPreferencesApi {
     /** Adds property to shared preferences data set of type double. */
     @NonNull
     Boolean setDouble(@NonNull String key, @NonNull Double value);
+    /** Adds property to shared preferences data set of type List<String>. */
+    @NonNull
+    Boolean setStringList(@NonNull String key, @NonNull String value);
     /**
      * Adds property to shared preferences data set of type List<String>.
      *
      * <p>Deprecated, this is only here for testing purposes.
      */
     @NonNull
-    Boolean setStringList(@NonNull String key, @NonNull List<String> value);
+    Boolean setDeprecatedStringList(@NonNull String key, @NonNull List<String> value);
     /** Removes all properties from shared preferences data set with matching prefix. */
     @NonNull
     Boolean clear(@NonNull String prefix, @Nullable List<String> allowList);
@@ -280,7 +283,7 @@ static void setUp(
                 ArrayList<Object> wrapped = new ArrayList<>();
                 ArrayList<Object> args = (ArrayList<Object>) message;
                 String keyArg = (String) args.get(0);
-                List<String> valueArg = (List<String>) args.get(1);
+                String valueArg = (String) args.get(1);
                 try {
                   Boolean output = api.setStringList(keyArg, valueArg);
                   wrapped.add(0, output);
@@ -293,6 +296,34 @@ static void setUp(
           channel.setMessageHandler(null);
         }
       }
+      {
+        BinaryMessenger.TaskQueue taskQueue = binaryMessenger.makeBackgroundTaskQueue();
+        BasicMessageChannel<Object> channel =
+            new BasicMessageChannel<>(
+                binaryMessenger,
+                "dev.flutter.pigeon.shared_preferences_android.SharedPreferencesApi.setDeprecatedStringList"
+                    + messageChannelSuffix,
+                getCodec(),
+                taskQueue);
+        if (api != null) {
+          channel.setMessageHandler(
+              (message, reply) -> {
+                ArrayList<Object> wrapped = new ArrayList<>();
+                ArrayList<Object> args = (ArrayList<Object>) message;
+                String keyArg = (String) args.get(0);
+                List<String> valueArg = (List<String>) args.get(1);
+                try {
+                  Boolean output = api.setDeprecatedStringList(keyArg, valueArg);
+                  wrapped.add(0, output);
+                } catch (Throwable exception) {
+                  wrapped = wrapError(exception);
+                }
+                reply.reply(wrapped);
+              });
+        } else {
+          channel.setMessageHandler(null);
+        }
+      }
       {
         BinaryMessenger.TaskQueue taskQueue = binaryMessenger.makeBackgroundTaskQueue();
         BasicMessageChannel<Object> channel =

packages/shared_preferences/shared_preferences_android/android/src/main/kotlin/io/flutter/plugins/sharedpreferences/MessagesAsync.g.kt

@@ -94,12 +94,18 @@ interface SharedPreferencesAsyncApi {
   fun setInt(key: String, value: Long, options: SharedPreferencesPigeonOptions)
   /** Adds property to shared preferences data set of type double. */
   fun setDouble(key: String, value: Double, options: SharedPreferencesPigeonOptions)
+  /** Adds property to shared preferences data set of type List<String>. */
+  fun setStringList(key: String, value: String, options: SharedPreferencesPigeonOptions)
   /**
    * Adds property to shared preferences data set of type List<String>.
    *
    * Deprecated, this is only here for testing purposes.
    */
-  fun setStringList(key: String, value: List<String>, options: SharedPreferencesPigeonOptions)
+  fun setDeprecatedStringList(
+      key: String,
+      value: List<String>,
+      options: SharedPreferencesPigeonOptions
+  )
   /** Gets individual String value stored with [key], if any. */
   fun getString(key: String, options: SharedPreferencesPigeonOptions): String?
   /** Gets individual void value stored with [key], if any. */
@@ -252,7 +258,7 @@ interface SharedPreferencesAsyncApi {
           channel.setMessageHandler { message, reply ->
             val args = message as List<Any?>
             val keyArg = args[0] as String
-            val valueArg = args[1] as List<String>
+            val valueArg = args[1] as String
             val optionsArg = args[2] as SharedPreferencesPigeonOptions
             val wrapped: List<Any?> =
                 try {
@@ -267,6 +273,33 @@ interface SharedPreferencesAsyncApi {
           channel.setMessageHandler(null)
         }
       }
+      run {
+        val taskQueue = binaryMessenger.makeBackgroundTaskQueue()
+        val channel =
+            BasicMessageChannel<Any?>(
+                binaryMessenger,
+                "dev.flutter.pigeon.shared_preferences_android.SharedPreferencesAsyncApi.setDeprecatedStringList$separatedMessageChannelSuffix",
+                codec,
+                taskQueue)
+        if (api != null) {
+          channel.setMessageHandler { message, reply ->
+            val args = message as List<Any?>
+            val keyArg = args[0] as String
+            val valueArg = args[1] as List<String>
+            val optionsArg = args[2] as SharedPreferencesPigeonOptions
+            val wrapped: List<Any?> =
+                try {
+                  api.setDeprecatedStringList(keyArg, valueArg, optionsArg)
+                  listOf(null)
+                } catch (exception: Throwable) {
+                  wrapError(exception)
+                }
+            reply.reply(wrapped)
+          }
+        } else {
+          channel.setMessageHandler(null)
+        }
+      }
       run {
         val taskQueue = binaryMessenger.makeBackgroundTaskQueue()
         val channel =

packages/shared_preferences/shared_preferences_android/android/src/main/kotlin/io/flutter/plugins/sharedpreferences/SharedPreferencesPlugin.kt

@@ -30,6 +30,8 @@ import kotlinx.coroutines.runBlocking
 
 const val TAG = "SharedPreferencesPlugin"
 const val SHARED_PREFERENCES_NAME = "FlutterSharedPreferences"
+// All identifiers must match the LegacySharedPreferencesPlugin.java file, as well as the
+// strings.dart file.
 const val LIST_PREFIX = "VGhpcyBpcyB0aGUgcHJlZml4IGZvciBhIGxpc3Qu"
 const val JSON_LIST_PREFIX = "VGhpcyBpcyB0aGUgcHJlZml4IGZvciBhIGxpc3Qu!"
 const val DOUBLE_PREFIX = "VGhpcyBpcyB0aGUgcHJlZml4IGZvciBEb3VibGUu"
@@ -411,10 +413,13 @@ internal fun preferencesFilter(key: String, value: Any?, allowList: Set<String>?
 /** Transforms preferences that are stored as Strings back to original type. */
 internal fun transformPref(value: Any?, listEncoder: SharedPreferencesListEncoder): Any? {
   if (value is String) {
-    if (value.startsWith(JSON_LIST_PREFIX)) {
-      return value
-    } else if (value.startsWith(LIST_PREFIX)) {
-      return listEncoder.decode(value.substring(LIST_PREFIX.length))
+    if (value.startsWith(LIST_PREFIX)) {
+      // The newer JSON-encoded lists use an extended prefix to distinguish them.
+      if (value.startsWith(JSON_LIST_PREFIX)) {
+        return value
+      } else {
+        return listEncoder.decode(value.substring(LIST_PREFIX.length))
+      }
     } else if (value.startsWith(DOUBLE_PREFIX)) {
       return value.substring(DOUBLE_PREFIX.length).toDouble()
     }

packages/shared_preferences/shared_preferences_android/example/integration_test/shared_preferences_test.dart

@@ -799,10 +799,10 @@ void main() {
         await clearPreferences(preferences, options);
         await preferences.setStringListLegacyForTesting(
             listKey, testList, options);
-        final List<String>? oldSystemList =
+        final List<String>? platformEncodedList =
             await preferences.getStringList(listKey, options);
-        expect(oldSystemList, testList);
-        await preferences.setStringList(listKey, oldSystemList!, options);
+        expect(platformEncodedList, testList);
+        await preferences.setStringList(listKey, platformEncodedList!, options);
         expect(await preferences.getStringList(listKey, options), testList);
       });
 

packages/shared_preferences/shared_preferences_android/lib/src/messages.g.dart

@@ -204,12 +204,42 @@ class SharedPreferencesApi {
     }
   }
 
+  /// Adds property to shared preferences data set of type List<String>.
+  Future<bool> setStringList(String key, String value) async {
+    final String pigeonVar_channelName =
+        'dev.flutter.pigeon.shared_preferences_android.SharedPreferencesApi.setStringList$pigeonVar_messageChannelSuffix';
+    final BasicMessageChannel<Object?> pigeonVar_channel =
+        BasicMessageChannel<Object?>(
+      pigeonVar_channelName,
+      pigeonChannelCodec,
+      binaryMessenger: pigeonVar_binaryMessenger,
+    );
+    final List<Object?>? pigeonVar_replyList =
+        await pigeonVar_channel.send(<Object?>[key, value]) as List<Object?>?;
+    if (pigeonVar_replyList == null) {
+      throw _createConnectionError(pigeonVar_channelName);
+    } else if (pigeonVar_replyList.length > 1) {
+      throw PlatformException(
+        code: pigeonVar_replyList[0]! as String,
+        message: pigeonVar_replyList[1] as String?,
+        details: pigeonVar_replyList[2],
+      );
+    } else if (pigeonVar_replyList[0] == null) {
+      throw PlatformException(
+        code: 'null-error',
+        message: 'Host platform returned null value for non-null return value.',
+      );
+    } else {
+      return (pigeonVar_replyList[0] as bool?)!;
+    }
+  }
+
   /// Adds property to shared preferences data set of type List<String>.
   ///
   /// Deprecated, this is only here for testing purposes.
-  Future<bool> setStringList(String key, List<String> value) async {
+  Future<bool> setDeprecatedStringList(String key, List<String> value) async {
     final String pigeonVar_channelName =
-        'dev.flutter.pigeon.shared_preferences_android.SharedPreferencesApi.setStringList$pigeonVar_messageChannelSuffix';
+        'dev.flutter.pigeon.shared_preferences_android.SharedPreferencesApi.setDeprecatedStringList$pigeonVar_messageChannelSuffix';
     final BasicMessageChannel<Object?> pigeonVar_channel =
         BasicMessageChannel<Object?>(
       pigeonVar_channelName,

packages/shared_preferences/shared_preferences_android/lib/src/messages_async.g.dart

@@ -189,13 +189,39 @@ class SharedPreferencesAsyncApi {
     }
   }
 
+  /// Adds property to shared preferences data set of type List<String>.
+  Future<void> setStringList(
+      String key, String value, SharedPreferencesPigeonOptions options) async {
+    final String pigeonVar_channelName =
+        'dev.flutter.pigeon.shared_preferences_android.SharedPreferencesAsyncApi.setStringList$pigeonVar_messageChannelSuffix';
+    final BasicMessageChannel<Object?> pigeonVar_channel =
+        BasicMessageChannel<Object?>(
+      pigeonVar_channelName,
+      pigeonChannelCodec,
+      binaryMessenger: pigeonVar_binaryMessenger,
+    );
+    final List<Object?>? pigeonVar_replyList = await pigeonVar_channel
+        .send(<Object?>[key, value, options]) as List<Object?>?;
+    if (pigeonVar_replyList == null) {
+      throw _createConnectionError(pigeonVar_channelName);
+    } else if (pigeonVar_replyList.length > 1) {
+      throw PlatformException(
+        code: pigeonVar_replyList[0]! as String,
+        message: pigeonVar_replyList[1] as String?,
+        details: pigeonVar_replyList[2],
+      );
+    } else {
+      return;
+    }
+  }
+
   /// Adds property to shared preferences data set of type List<String>.
   ///
   /// Deprecated, this is only here for testing purposes.
-  Future<void> setStringList(String key, List<String> value,
+  Future<void> setDeprecatedStringList(String key, List<String> value,
       SharedPreferencesPigeonOptions options) async {
     final String pigeonVar_channelName =
-        'dev.flutter.pigeon.shared_preferences_android.SharedPreferencesAsyncApi.setStringList$pigeonVar_messageChannelSuffix';
+        'dev.flutter.pigeon.shared_preferences_android.SharedPreferencesAsyncApi.setDeprecatedStringList$pigeonVar_messageChannelSuffix';
     final BasicMessageChannel<Object?> pigeonVar_channel =
         BasicMessageChannel<Object?>(
       pigeonVar_channelName,

packages/shared_preferences/shared_preferences_android/lib/src/shared_preferences_android.dart

@@ -50,9 +50,9 @@ class SharedPreferencesAndroid extends SharedPreferencesStorePlatform {
       case 'Double':
         return _api.setDouble(key, value as double);
       case 'StringList':
-        return _api.setString(key, '$jsonListPrefix${jsonEncode(value)}');
+        return _api.setStringList(key, '$jsonListPrefix${jsonEncode(value)}');
       case 'PlatformEncodedStringListForTesting':
-        return _api.setStringList(key, value as List<String>);
+        return _api.setDeprecatedStringList(key, value as List<String>);
     }
     // TODO(tarrinneal): change to ArgumentError across all platforms.
     throw PlatformException(

packages/shared_preferences/shared_preferences_android/lib/src/shared_preferences_async_android.dart

@@ -137,8 +137,8 @@ base class SharedPreferencesAsyncAndroid
     return api.setString(key, stringValue, pigeonOptions);
   }
 
-  /// Adds a `List<String>` to preferences using the no longer used system to test
-  /// moving from the old system to the new one.
+  /// Adds a `List<String>` to preferences using platform encoding to test
+  /// moving from platform encoding to json encoding.
   @visibleForTesting
   Future<void> setStringListLegacyForTesting(
     String key,
@@ -148,7 +148,7 @@ base class SharedPreferencesAsyncAndroid
     final SharedPreferencesPigeonOptions pigeonOptions =
         _convertOptionsToPigeonOptions(options);
     final SharedPreferencesAsyncApi api = _getApiForBackend(pigeonOptions);
-    return api.setStringList(key, value, pigeonOptions);
+    return api.setDeprecatedStringList(key, value, pigeonOptions);
   }
 
   @override

packages/shared_preferences/shared_preferences_android/pigeons/messages.dart

@@ -35,11 +35,15 @@ abstract class SharedPreferencesApi {
   @TaskQueue(type: TaskQueueType.serialBackgroundThread)
   bool setDouble(String key, double value);
 
+  /// Adds property to shared preferences data set of type List<String>.
+  @TaskQueue(type: TaskQueueType.serialBackgroundThread)
+  bool setStringList(String key, String value);
+
   /// Adds property to shared preferences data set of type List<String>.
   ///
   /// Deprecated, this is only here for testing purposes.
   @TaskQueue(type: TaskQueueType.serialBackgroundThread)
-  bool setStringList(String key, List<String> value);
+  bool setDeprecatedStringList(String key, List<String> value);
 
   /// Removes all properties from shared preferences data set with matching prefix.
   @TaskQueue(type: TaskQueueType.serialBackgroundThread)

packages/shared_preferences/shared_preferences_android/pigeons/messages_async.dart

@@ -54,11 +54,19 @@ abstract class SharedPreferencesAsyncApi {
     SharedPreferencesPigeonOptions options,
   );
 
+  /// Adds property to shared preferences data set of type List<String>.
+  @TaskQueue(type: TaskQueueType.serialBackgroundThread)
+  void setStringList(
+    String key,
+    String value,
+    SharedPreferencesPigeonOptions options,
+  );
+
   /// Adds property to shared preferences data set of type List<String>.
   ///
   /// Deprecated, this is only here for testing purposes.
   @TaskQueue(type: TaskQueueType.serialBackgroundThread)
-  void setStringList(
+  void setDeprecatedStringList(
     String key,
     List<String> value,
     SharedPreferencesPigeonOptions options,

packages/shared_preferences/shared_preferences_android/test/shared_preferences_android_test.dart

@@ -343,11 +343,17 @@ class _FakeSharedPreferencesApi implements SharedPreferencesApi {
   }
 
   @override
-  Future<bool> setStringList(String key, List<String?> value) async {
+  Future<bool> setStringList(String key, String value) async {
     items[key] = '$jsonListPrefix${jsonEncode(value)}';
     return true;
   }
 
+  @override
+  Future<bool> setDeprecatedStringList(String key, List<String> value) async {
+    items[key] = value;
+    return true;
+  }
+
   @override
   // ignore: non_constant_identifier_names
   BinaryMessenger? get pigeonVar_binaryMessenger => throw UnimplementedError();