DRY Don’t Repeat Yourself 

The 4 Rules of Simple Design • Runs all the tests • No duplication • Expresses developer intent • Minimizes the number of classes and methods 

The 4 Rules of Simple Design • No duplication = DRY 

Easiest way to boost your coding skills 

Don’t take my word 

Spot the Duplication 

Plain Old Duplication if @unparsed_geminstaller_output_flags flags = @unparsed_geminstaller_output_flags.split(',') flags.delete_if {|flag| flag == nil or flag == ''} flags.map! {|flag| flag.downcase} flags.sort! flags.uniq! flags.map! {|flag| flag.to_sym} geminstaller_output_valid = true flags.each do |flag| unless VALID_GEMINSTALLER_OUTPUT_FLAGS.include?(flag) @output = "Invalid geminstaller-output flag: #{flag}\n" geminstaller_output_valid = false end end @options[:geminstaller_output] = flags if geminstaller_output_valid end if @unparsed_rubygems_output_flags flags = @unparsed_rubygems_output_flags.split(',') flags.delete_if {|flag| flag == nil or flag == ''} flags.map! {|flag| flag.downcase} flags.sort! flags.uniq! flags.map! {|flag| flag.to_sym} rubygems_output_valid = true flags.each do |flag| unless VALID_RUBYGEMS_OUTPUT_FLAGS.include?(flag) @output = "Invalid rubygems-output flag: #{flag}\n" rubygems_output_valid = false end end @options[:rubygems_output] = flags if rubygems_output_valid end 

Intention Duplication if (list.isEmpty()) { .... } .... if (list.size() == 0) { .... } 

Name Duplication TestRunner.runTests() def handle_web_response(web_response) ProductSales productSales = new ProductSales() 

Comment Duplication // Restart the process process.restart() 

Structure Duplication if isinstance(StringHandler, o): o.updateString(context) elif isinstance(IntHandler, o): o.updateInt(context) 

Let’s get it on! • The Serializer Kata http://bit.ly/KataDRY • Do not move to next step until code duplication ≤ 0