Creating a dictionary class

Dictionary

class Dictionary
    attr_reader :entries
    def initialize()
        @entries = {}
    end
    
    def add(words)
        if words.class == String        
            @entries.merge!({words => nil})
        else
            @entries.merge!(words)
        end
    end
    
    def keywords
        @entries.keys.sort
    end
    
    def include?(keyword)
        @entries.include?(keyword)
    end
    
    def find(str) 
       #@entries.inject({}) do |found_words, (word, definition)|  
       #  found_words.update word => definition if word[0,str.length] == str
       #end
        
        l = str.length
        found_words = {}
        @entries.each do |word, definition|  
            if word[0,l] == str
                found_words.update word => definition
            end
        end
        found_words
    end    
end