diff --git a/lib/groonga/context.rb b/lib/groonga/context.rb
index af2a2787..11624e8f 100644
--- a/lib/groonga/context.rb
+++ b/lib/groonga/context.rb
@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
-#
-# Copyright (C) 2010-2016  Kouhei Sutou <kou@clear-code.com>
+# Copyright (C) 2010-2025  Sutou Kouhei <kou@clear-code.com>
 #
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -232,7 +230,7 @@ def execute_command(name, parameters={})
     # @yieldparam response [String] A response for a command.
     # @return [void]
     def restore(dumped_commands)
-      buffer = ""
+      buffer = +""
       continued = false
       dumped_commands.each_line do |line|
         line = line.chomp
diff --git a/lib/groonga/dumper.rb b/lib/groonga/dumper.rb
index 72d395ea..0de758be 100644
--- a/lib/groonga/dumper.rb
+++ b/lib/groonga/dumper.rb
@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
-#
-# Copyright (C) 2011-2016  Kouhei Sutou <kou@clear-code.com>
+# Copyright (C) 2011-2025  Sutou Kouhei <kou@clear-code.com>
 # Copyright (C) 2014  Masafumi Yokoyama <myokoym@gmail.com>
 #
 # This library is free software; you can redistribute it and/or
@@ -26,7 +24,7 @@ def default_output
     end
 
     def utf8_string
-      ""
+      +""
     end
   end
 
@@ -759,7 +757,7 @@ def resolve_value(record, column, value)
         ""
       else
         return value unless value.respond_to?(:valid_encoding?)
-        sanitized_value = ""
+        sanitized_value = +""
         value = fix_encoding(value)
         value.each_char do |char|
           if char.valid_encoding?
diff --git a/lib/groonga/patricia-trie.rb b/lib/groonga/patricia-trie.rb
index 44e98051..3b99289c 100644
--- a/lib/groonga/patricia-trie.rb
+++ b/lib/groonga/patricia-trie.rb
@@ -1,6 +1,4 @@
-# -*- coding: utf-8 -*-
-#
-# Copyright (C) 2009-2010  Kouhei Sutou <kou@clear-code.com>
+# Copyright (C) 2009-2025  Sutou Kouhei <kou@clear-code.com>
 #
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -55,7 +53,7 @@ def tag_keys(text, options={})
       options ||= {}
       other_text_handler = options[:other_text_handler]
       position = 0
-      result = ''
+      result = +""
       if text.respond_to?(:encoding)
         encoding = text.encoding
         bytes = text.dup.force_encoding("ascii-8bit")
diff --git a/test/test-database-inspector.rb b/test/test-database-inspector.rb
index f5042f91..60b305c6 100644
--- a/test/test-database-inspector.rb
+++ b/test/test-database-inspector.rb
@@ -1,4 +1,4 @@
-# Copyright (C) 2013  Kouhei Sutou <kou@clear-code.com>
+# Copyright (C) 2013-2025  Sutou Kouhei <kou@clear-code.com>
 #
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -131,7 +131,7 @@ def inspect_columns(columns)
         None
       INSPECTED
     else
-      inspected = "      Columns:\n"
+      inspected = +"      Columns:\n"
       columns.each do |column|
         inspected << inspect_column(column)
       end
@@ -270,7 +270,7 @@ def test_has_tables
 
       private
       def inspected(n_tables)
-        inspected_tables = "  Tables:\n"
+        inspected_tables = +"  Tables:\n"
         if @database.tables.empty?
           inspected_tables << "    None\n"
         else
diff --git a/test/test-schema-dumper.rb b/test/test-schema-dumper.rb
index cbf1e260..1b92fbe7 100644
--- a/test/test-schema-dumper.rb
+++ b/test/test-schema-dumper.rb
@@ -1,4 +1,4 @@
-# Copyright (C) 2009-2016  Kouhei Sutou <kou@clear-code.com>
+# Copyright (C) 2009-2025  Sutou Kouhei <kou@clear-code.com>
 # Copyright (C) 2014-2016  Masafumi Yokoyama <yokoyama@clear-code.com>
 #
 # This library is free software; you can redistribute it and/or
@@ -427,7 +427,7 @@ def test_double_array_trie
     class ColumnCompressionTest < self
       def test_zlib
         define_column_compression_zlib_schema
-        flags = "COLUMN_SCALAR"
+        flags = +"COLUMN_SCALAR"
         flags << "|COMPRESS_ZLIB" if context.support_zlib?
         assert_equal(<<-SCHEMA, dump)
 table_create Posts TABLE_NO_KEY
@@ -437,7 +437,7 @@ def test_zlib
 
       def test_lz4
         define_column_compression_lz4_schema
-        flags = "COLUMN_SCALAR"
+        flags = +"COLUMN_SCALAR"
         flags << "|COMPRESS_LZ4" if context.support_lz4?
         assert_equal(<<-SCHEMA, dump)
 table_create Posts TABLE_NO_KEY
@@ -447,7 +447,7 @@ def test_lz4
 
       def test_zstd
         define_column_compression_zstd_schema
-        flags = "COLUMN_SCALAR"
+        flags = +"COLUMN_SCALAR"
         flags << "|COMPRESS_ZSTD" if context.support_zstd?
         assert_equal(<<-SCHEMA, dump)
 table_create Posts TABLE_NO_KEY
@@ -457,7 +457,7 @@ def test_zstd
 
       def test_with_weight_vector
         define_column_compression_with_weight_vector_schema
-        flags = "COLUMN_VECTOR|WITH_WEIGHT"
+        flags = +"COLUMN_VECTOR|WITH_WEIGHT"
         flags << "|COMPRESS_ZLIB" if context.support_zlib?
         assert_equal(<<-SCHEMA, dump)
 table_create Posts TABLE_NO_KEY