diff --git a/src/library/archive_entry.cr b/src/library/archive_entry.cr
index ab229af..da313cd 100644
--- a/src/library/archive_entry.cr
+++ b/src/library/archive_entry.cr
@@ -107,4 +107,8 @@ class ArchiveEntry < Entry
   def examine : Bool
     File.exists? @zip_path
   end
+
+  def self.is_valid?(path : String) : Bool
+    is_supported_file path
+  end
 end
diff --git a/src/library/dir_entry.cr b/src/library/dir_entry.cr
index 59657af..82b7ccc 100644
--- a/src/library/dir_entry.cr
+++ b/src/library/dir_entry.cr
@@ -25,7 +25,7 @@ class DirEntry < Entry
       return
     end
 
-    unless DirEntry.validate_directory_entry @dir_path
+    unless DirEntry.is_valid? @dir_path
       @err_msg = "Directory #{@dir_path} is not valid directory entry."
       Logger.warn "#{@err_msg} Please make sure the " \
                   "directory has valid images."
@@ -129,7 +129,7 @@ class DirEntry < Entry
       .sort { |a, b| compare_numerically a, b }
   end
 
-  def self.validate_directory_entry(dir_path)
-    image_files(dir_path).size > 0
+  def self.is_valid?(path : String) : Bool
+    image_files(path).size > 0
   end
 end
diff --git a/src/library/entry.cr b/src/library/entry.cr
index 5f19d77..131e489 100644
--- a/src/library/entry.cr
+++ b/src/library/entry.cr
@@ -28,8 +28,7 @@ abstract class Entry
         if err_msg
           json.field "err_msg", err_msg
         end
-        # for API backward compatability
-        json.field "zip_path", path
+        json.field "zip_path", path # for API backward compatability
         json.field "title_id", @book.id
         json.field "title_title", @book.title
         json.field "sort_title", sort_title
@@ -220,6 +219,16 @@ abstract class Entry
     date_added.not_nil! # is it ok to set not_nil! here?
   end
 
+  # Hack to have abstract class methods
+  # https://github.com/crystal-lang/crystal/issues/5956
+  private module ClassMethods
+    abstract def is_valid?(path : String) : Bool
+  end
+
+  macro inherited
+    extend ClassMethods
+  end
+
   abstract def path : String
 
   abstract def read_page(page_num)
diff --git a/src/library/title.cr b/src/library/title.cr
index 3f30490..e9873f2 100644
--- a/src/library/title.cr
+++ b/src/library/title.cr
@@ -53,7 +53,7 @@ class Title
           Library.default.title_hash[title.id] = title
           @title_ids << title.id
         end
-        if DirEntry.validate_directory_entry path
+        if DirEntry.is_valid? path
           entry = DirEntry.new path, self
           @entries << entry if entry.pages > 0 || entry.err_msg
         end
@@ -146,7 +146,7 @@ class Title
       path = File.join dir, fn
       if File.directory? path
         unless remained_entry_paths.includes? path
-          if DirEntry.validate_directory_entry path
+          if DirEntry.is_valid? path
             entry = DirEntry.new path, self
             if entry.pages > 0 || entry.err_msg
               @entries << entry
diff --git a/src/util/signature.cr b/src/util/signature.cr
index 6f67a58..f0f68bd 100644
--- a/src/util/signature.cr
+++ b/src/util/signature.cr
@@ -64,7 +64,7 @@ class Dir
         path = File.join dirname, fn
         if File.directory? path
           signatures << Dir.contents_signature path, cache
-          if DirEntry.validate_directory_entry path
+          if DirEntry.is_valid? path
             signatures << fn
           end
         else