Today, I got feedback from a code review and learned something new. It was about "magic numbers", which are unique values with unexplained meaning or multiple occurrences which could (preferably) be replaced with named constants.

I've known about the concept of using constants rather than hard-coding values, but I didn't think of that for "magic number" situations. It's a pretty simple concept but I just wanted to share what I learned.

Magic Numbers are Bad

Magic numbers are language agnostic, but below, you'll see a code snippet in Java WITH magic numbers (the bad way).

class MagicNumber {

    String pipeDelimitedString = "This|is|an|example|of|Magic|Numbers|123 Parker Road|cityline@gmail.com";
    String [] splitString = pipeDelimitedString.split("\\|");

    obj.doSomething(splitString[7]);
    obj.doSomething(splitString[8]);           
}

As you can see, I am simply splitting a pipe delimited string that contains some useful data (Home Address and Email Address).

splitString[7] refers to the 8th element, or 123 Parker Road, which is a Home Address

splitString[8] refers to the 9th element, or cityline@gmail.com, which is an Email Address

Why is that bad? We’ll it’s not easy to read.

splitString[n], where n is a number, may not mean anything from an outsider looking at your code. The numbers 7 & 8 have no meaning!

Now, you’ll see a code snippet in Java WITHOUT magic numbers (the good way).

class MagicNumber {

  String pipeDelimitedString = "This|is|an|example|of|Magic|Numbers|123 Parker Road|cityline@gmail.com";
  String [] splitString = pipeDelimitedString.split("\\|");

  obj.doSomething(splitString[Constants.HOME_ADDRESS]);
  obj.doSomething(splitString[Constants.EMAIL_ADDRESS]);           
}

class Constants {

  public static final int HOME_ADDRESS = 7;
  public static final int EMAIL_ADDRESS = 8;
}

I’ve created a constants class, with some constants (HOME_ADDRESS = 7 and EMAIL_ADDRESS = 8).

Constants.HOME_ADDRESS & Constants.EMAIL_ADDRESS are much easier to understand and now have meaning!

If you aren’t convinced, what’s easier to read:
splitString[Constants.HOME_ADDRESS] or splitString[7]?

Again, this is just ONE example of magic numbers but there are many more. Thanks for reading!

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