* Rename NullAdapter to SimpleAdapter

* Introduce abstract Adapter class
* Organaze test structure to match convemtions

test/adapter/simple_adapter_test.rb

@@ -3,15 +3,15 @@ require 'test_helper'
 module ActiveModel
   class Serializer
     class Adapter
-      class NullAdapterTest < Minitest::Test
+      class SimpleAdapterTest < Minitest::Test
         def setup
           @profile = Profile.new({ name: 'Name 1', description: 'Description 1', comments: 'Comments 1' })
           @profile_serializer = ProfileSerializer.new(@profile)
 
-          @adapter = NullAdapter.new(@profile_serializer)
+          @adapter = SimpleAdapter.new(@profile_serializer)
         end
 
-        def test_null_adapter
+        def test_simple_adapter
           assert_equal('{"name":"Name 1","description":"Description 1"}',
                        @adapter.to_json)
 

test/adapter_test.rb

@@ -0,0 +1,25 @@
+require 'test_helper'
+
+module ActiveModel
+  class Serializer
+    class AdapterTest < Minitest::Test
+      def setup
+        profile = Profile.new
+        serializer = ProfileSerializer.new(profile)
+        @adapter = ActiveModel::Serializer::Adapter.new(serializer)
+      end
+
+      def test_serializable_hash_is_abstract_method
+        assert_raises(NotImplementedError) do
+          @adapter.serializable_hash(only: [:name])
+        end
+      end
+
+      def test_serializable_hash_is_abstract_method
+        assert_raises(NotImplementedError) do
+          @adapter.to_json(only: [:name])
+        end
+      end
+    end
+  end
+end