Object Mentor Blog: One Thing: Extract till you Drop.

One Thing: Extract till you Drop.

Uncle Bob — Fri, 11 Sep 2009 13:33:00 -0500

For years authors and consultants (like me) have been telling us that functions should do one thing. They should do it well. They should do it only.

The question is: What the hell does “one thing” mean?

After all, one man’s “one thing” might be someone else’s “two things”.

Consider this class:

  class SymbolReplacer {
    protected String stringToReplace;
    protected List<String> alreadyReplaced = new ArrayList<String>();

    SymbolReplacer(String s) {
      this.stringToReplace = s;
    }

    String replace() {
      Pattern symbolPattern = Pattern.compile("\\$([a-zA-Z]\\w*)");
      Matcher symbolMatcher = symbolPattern.matcher(stringToReplace);
      while (symbolMatcher.find()) {
        String symbolName = symbolMatcher.group(1);
        if (getSymbol(symbolName) != null && !alreadyReplaced.contains(symbolName)) {
          alreadyReplaced.add(symbolName);
          stringToReplace = stringToReplace.replace("$" + symbolName, translate(symbolName));
        }
      }
      return stringToReplace;
    }

    protected String translate(String symbolName) {
      return getSymbol(symbolName);
    }
  }

It’s not too hard to understand. The replace() function searches through a string looking for $NAME and replaces each instance with the appropriate translation of NAME. It also makes sure that it doesn’t replace a name more than once. Simple.

Of course the words “It also…” pretty much proves that this function does more than one thing. So we can probably split the function up into two functions as follows:

    String replace() {
      Pattern symbolPattern = Pattern.compile("\\$([a-zA-Z]\\w*)");
      Matcher symbolMatcher = symbolPattern.matcher(stringToReplace);
      while (symbolMatcher.find()) {
        String symbolName = symbolMatcher.group(1);
        replaceAllInstances(symbolName);
      }
      return stringToReplace;
    }

    private void replaceAllInstances(String symbolName) {
      if (getSymbol(symbolName) != null && !alreadyReplaced.contains(symbolName)) {
        alreadyReplaced.add(symbolName);
        stringToReplace = stringToReplace.replace("$" + symbolName, translate(symbolName));
      }
    }

OK, so now the replace() function simply finds all the symbols that need replacing, and the replaceAllInstances() function replaces them if they haven’t already been replaced. So do these function do one thing each?

Well, the replace() compiles the pattern and build the Matcher() Maybe those actions should be moved into the constructor?

  class SymbolReplacer {
    protected String stringToReplace;
    protected List<String> alreadyReplaced = new ArrayList<String>();
    private Matcher symbolMatcher;
    private final Pattern symbolPattern = Pattern.compile("\\$([a-zA-Z]\\w*)");

    SymbolReplacer(String s) {
      this.stringToReplace = s;
      symbolMatcher = symbolPattern.matcher(s);
    }

    String replace() {
      while (symbolMatcher.find()) {
        String symbolName = symbolMatcher.group(1);
        replaceAllInstances(symbolName);
      }
      return stringToReplace;
    }

    private void replaceAllInstances(String symbolName) {
      if (getSymbol(symbolName) != null && !alreadyReplaced.contains(symbolName)) {
        alreadyReplaced.add(symbolName);
        stringToReplace = stringToReplace.replace("$" + symbolName, translate(symbolName));
      }
    }

    protected String translate(String symbolName) {
      return getSymbol(symbolName);
    }
  }

OK, so now certainly the replace() function is doing one thing? Ah, but I see at least two. It loops, extracts the symbolName and then does the replace. OK, so how about this?

    String replace() {
      for (String symbolName = nextSymbol(); symbolName != null; symbolName = nextSymbol())
        replaceAllInstances(symbolName);

      return stringToReplace;
    }

    private String nextSymbol() {
      return symbolMatcher.find() ? symbolMatcher.group(1) : null;
    }

I had to restructure things a little bit. The loop is a bit ugly. I wish I could have said for (String symbolName : symbolMatcher) but I guess Matchers don’t work that way.

I kind of like the nextSymbol() function. It gets the Matcher nicely out of the way.

So now the replace() and nextSymbol() functions are certainly doing one thing. Aren’t they?

Well, I suppose I could separate the loop from the return in replace().

    String replace() {
      replaceAllSymbols();
      return stringToReplace;
    }

    private void replaceAllSymbols() {
      for (String symbolName = nextSymbol(); symbolName != null; symbolName = nextSymbol())
        replaceAllInstances(symbolName);
    }

I don’t see how I could make these functions smaller. They must be doing one thing. There’s no way to extract any other functions from them!

Uh… Wait. Is that the definition of one thing? Is a function doing one thing if, and only if, you simply cannot extract any other functions from it? What else could “one thing” mean? After all, If I can extract one function out of another, the original function must have been doing more than one thing.

So does that mean that for all these years the authors and consultants (like me) have been telling us to extract until you can’t extract anymore?

Let’s try that with the rest of this class and see what it looks like…

  class SymbolReplacer {
    protected String stringToReplace;
    protected List<String> alreadyReplaced = new ArrayList<String>();
    private Matcher symbolMatcher;
    private final Pattern symbolPattern = Pattern.compile("\\$([a-zA-Z]\\w*)");

    SymbolReplacer(String s) {
      this.stringToReplace = s;
      symbolMatcher = symbolPattern.matcher(s);
    }

    String replace() {
      replaceAllSymbols();
      return stringToReplace;
    }

    private void replaceAllSymbols() {
      for (String symbolName = nextSymbol(); symbolName != null; symbolName = nextSymbol())
        replaceAllInstances(symbolName);
    }

    private String nextSymbol() {
      return symbolMatcher.find() ? symbolMatcher.group(1) : null;
    }

    private void replaceAllInstances(String symbolName) {
      if (shouldReplaceSymbol(symbolName))
        replaceSymbol(symbolName);
    }

    private boolean shouldReplaceSymbol(String symbolName) {
      return getSymbol(symbolName) != null && !alreadyReplaced.contains(symbolName);
    }

    private void replaceSymbol(String symbolName) {
      alreadyReplaced.add(symbolName);
      stringToReplace = stringToReplace.replace(
        symbolExpression(symbolName), 
        translate(symbolName));
    }

    private String symbolExpression(String symbolName) {
      return "$" + symbolName;
    }

    protected String translate(String symbolName) {
      return getSymbol(symbolName);
    }
  }

Well, I think it’s pretty clear that each of these functions is doing one thing. I’m not sure how I’d extract anything further from any of them.

Perhaps you think this is taking things too far. I used to think so too. But after programming for over 40+ years, I’m beginning to come to the conclusion that this level of extraction is not taking things too far at all. In fact, to me, it looks just about right.

So, my advice: Extract till you just can’t extract any more. Extract till you drop.

After all, with modern tools it takes very little time. It makes each function almost trivial. The code reads very nicely. It forces you to put little snippets of code into nicely named functions. And, well gosh, extracting till you drop is kind of fun!

"One Thing: Extract till you Drop." by I really love reading your comments guys.I learn a lot from you.

Thank you so much

Regard,

Day Traders

Thu, 09 Feb 2012 05:43:51 -0600

I really love reading your comments guys.I learn a lot from you.

Thank you so much

Regard,

Day Traders

"One Thing: Extract till you Drop." by Microsoft Office 2010

Fri, 13 Jan 2012 20:04:12 -0600

This article is GREAT it can be EXCELLENT JOB and what a great tool!

"One Thing: Extract till you Drop." by Muffazal Lakdawala

Tue, 10 Jan 2012 04:20:08 -0600

Nice post. I will be back to read more of such post keep going.

"One Thing: Extract till you Drop." by iPhone contacts backup

Wed, 04 Jan 2012 06:20:29 -0600

Hi, all. It is a better idea to backup iPhone content such as contacts, sms and other media files to computer in case they lost by accidently.

"One Thing: Extract till you Drop." by Weight Loss Surgeon

Thu, 22 Dec 2011 03:54:03 -0600

This is really great i was expecting something like this form you great post keep going.

"One Thing: Extract till you Drop." by FLOWERS

Mon, 19 Dec 2011 01:54:18 -0600

The blog is very much interesting with the subject.FLOWERS

"One Thing: Extract till you Drop." by Agree with Angry Man

Thu, 08 Dec 2011 17:30:25 -0600

The refactored code is absolutely retarded. The first is the best. The second try is acceptable. The rest: fucking garbage.

"One Thing: Extract till you Drop." by mowing lawns

Fri, 02 Dec 2011 01:20:02 -0600

The blog is very much interesting with the subject.

"One Thing: Extract till you Drop." by I really love reading your comments guys.I learn a lot from you.

Thank you so much

Regard,

Epoyjun

Thu, 01 Dec 2011 07:26:16 -0600

I really love reading your comments guys.I learn a lot from you.

Thank you so much

Regard,

Epoyjun

"One Thing: Extract till you Drop." by most efficient space heater

Thu, 01 Dec 2011 03:56:24 -0600

I really enjoyed reading this. most efficient space heater

"One Thing: Extract till you Drop." by most efficient space heater

Thu, 01 Dec 2011 03:52:44 -0600

I really enjoyed reading this. most efficient space heater

"One Thing: Extract till you Drop." by crest printable coupons

Thu, 01 Dec 2011 03:49:56 -0600

I wanted to thanks for your time for this wonderful read!! crest printable coupons

"One Thing: Extract till you Drop." by tv show

Sat, 26 Nov 2011 09:54:38 -0600

great post! thanks a lot! keep it up;)

"One Thing: Extract till you Drop." by web design Hertfordshire

Thu, 24 Nov 2011 00:37:39 -0600

Function description seem really good and reliable.

"One Thing: Extract till you Drop." by cheap soccer jerseys

Thu, 17 Nov 2011 21:49:12 -0600

You make it entertaining and you still manage to keep it smart.

This is truly a great blog thanks for sharing…

In most cases I don’t make comments on blogs, but I want to mention that this post really forced me to do so. Really nice post!

Your posts are extremely helpful and informative.

"One Thing: Extract till you Drop." by Dr Dre beats Headphones Canada

Sat, 12 Nov 2011 02:04:36 -0600

The new DR-BT140QStereoBluetooth head set with comfortable ear hook design and introduction regarding black,

"One Thing: Extract till you Drop." by chiropractor virginia beach

Fri, 11 Nov 2011 01:46:15 -0600

I would like to say that this blog really convinced me, you give me best information! Thanks, very good post.

"One Thing: Extract till you Drop." by Cheap Beats By Dr.Dre

Tue, 08 Nov 2011 04:47:37 -0600

Today the market is flooded with greater variety and headphones

"One Thing: Extract till you Drop." by Custom Backpack

Tue, 01 Nov 2011 08:03:39 -0500

very nice post, good job

"One Thing: Extract till you Drop." by ysbearing/yisong@1stbearing.com

Wed, 19 Oct 2011 01:46:45 -0500

Slewing bearing called slewing ring bearings, is a comprehensive load to bear a large bearing, can bear large axial, radial load and overturning moment. http://www.1stbearing.com

"One Thing: Extract till you Drop." by DC Hats

Fri, 14 Oct 2011 03:43:19 -0500

mastering a terrific deal much more on this matter. If feasible, as you obtain knowledge, would you thoughts updating your web site with a wonderful deal more information? Its very helpful personally.

"One Thing: Extract till you Drop." by topwowgogo@gmail.com

Thu, 13 Oct 2011 03:15:06 -0500

WOW!!!!The first impression your blog give to me is so distinctive,very honored to you can share with us your blog. Have a lot of harvest,your masterpiece,thank you very much. In order to thank,I will also give you the way to share a resource Cheap Replica NFL Jerseys. I had visited this Wholesale NFL Jerseys China already,and have gained.Guarantee you will like Womens NFL Jerseys

"One Thing: Extract till you Drop." by Longchamp outlet

Sun, 09 Oct 2011 22:04:28 -0500

You are perfectly correct on that!

It was quite a good read. Good efforts from the webmaster.

"One Thing: Extract till you Drop." by juicy couture tracksuits

Sun, 09 Oct 2011 22:03:18 -0500

I admire the valuable information you offer in your articles. I will bookmark your blog and have my children check up here often. I am quite sure they will learn lots of new stuff here than anybody else!

Resources like the one you mentioned here will be very useful to me!

I am glad you have posted information on this topic.I hope you will keep this blog up to date with more informatio

"One Thing: Extract till you Drop." by Cheap Soccer Jerseys

Sun, 09 Oct 2011 22:01:42 -0500

This is exactly what i was looking for. thank you for the informative post and keep up the good work! Big thanks for the useful info i found on

Object Mentor Blog: One Thing: Extract till you Drop.

One Thing: Extract till you Drop.

"One Thing: Extract till you Drop." by I really love reading your comments guys.I learn a lot from you.Thank you so much Regard,Day Traders

"One Thing: Extract till you Drop." by Microsoft Office 2010

"One Thing: Extract till you Drop." by Muffazal Lakdawala

"One Thing: Extract till you Drop." by iPhone contacts backup

"One Thing: Extract till you Drop." by Weight Loss Surgeon

"One Thing: Extract till you Drop." by FLOWERS

"One Thing: Extract till you Drop." by Agree with Angry Man

"One Thing: Extract till you Drop." by mowing lawns

"One Thing: Extract till you Drop." by I really love reading your comments guys.I learn a lot from you.Thank you so much Regard,Epoyjun

"One Thing: Extract till you Drop." by most efficient space heater

"One Thing: Extract till you Drop." by most efficient space heater

"One Thing: Extract till you Drop." by crest printable coupons

"One Thing: Extract till you Drop." by tv show

"One Thing: Extract till you Drop." by web design Hertfordshire

"One Thing: Extract till you Drop." by cheap soccer jerseys

"One Thing: Extract till you Drop." by Dr Dre beats Headphones Canada

"One Thing: Extract till you Drop." by chiropractor virginia beach

"One Thing: Extract till you Drop." by Cheap Beats By Dr.Dre

"One Thing: Extract till you Drop." by Custom Backpack

"One Thing: Extract till you Drop." by ysbearing/yisong@1stbearing.com

"One Thing: Extract till you Drop." by DC Hats

"One Thing: Extract till you Drop." by topwowgogo@gmail.com

"One Thing: Extract till you Drop." by Longchamp outlet

"One Thing: Extract till you Drop." by juicy couture tracksuits

"One Thing: Extract till you Drop." by Cheap Soccer Jerseys

"One Thing: Extract till you Drop." by I really love reading your comments guys.I learn a lot from you.

Thank you so much

Regard,

Day Traders

"One Thing: Extract till you Drop." by I really love reading your comments guys.I learn a lot from you.

Thank you so much

Regard,

Epoyjun