Refactoring Extract Variable

Problem

You have an expression that is hard to understand.

Solution

Place the result of the expression or its parts in separate variables that are self-explanatory.
Before
void renderBanner() {
  if ((platform.toUpperCase().indexOf("MAC") > -1) &&
       (browser.toUpperCase().indexOf("IE") > -1) &&
        wasInitialized() && resize > 0 )
  {
    // do something
  }
}
After
void renderBanner() {
  final boolean isMacOs = platform.toUpperCase().indexOf("MAC") > -1;
  final boolean isIE = browser.toUpperCase().indexOf("IE") > -1;
  final boolean wasResized = resize > 0;

  if (isMacOs && isIE && wasInitialized() && wasResized) {
    // do something
  }
}
Before
void RenderBanner() 
{
  if ((platform.ToUpper().IndexOf("MAC") > -1) &&
       (browser.ToUpper().IndexOf("IE") > -1) &&
        wasInitialized() && resize > 0 )
  {
    // do something
  }
}
After
void RenderBanner() 
{
  readonly bool isMacOs = platform.ToUpper().IndexOf("MAC") > -1;
  readonly bool isIE = browser.ToUpper().IndexOf("IE") > -1;
  readonly bool wasResized = resize > 0;

  if (isMacOs && isIE && wasInitialized() && wasResized) 
  {
    // do something
  }
}
Before
if (($platform->toUpperCase()->indexOf("MAC") > -1) &&
     ($browser->toUpperCase()->indexOf("IE") > -1) &&
      $this->wasInitialized() && $this->resize > 0 )
{
  // do something
}
After
$isMacOs = $platform->toUpperCase()->indexOf("MAC") > -1;
$isIE = $browser->toUpperCase()->indexOf("IE")  > -1;
$wasResized = $this->resize > 0;

if ($isMacOs && $isIE && $this->wasInitialized() && $wasResized) {
  // do something
}
Before
def renderBanner(self):
    if (self.platform.toUpperCase().indexOf("MAC") > -1) and \
       (self.browser.toUpperCase().indexOf("IE") > -1) and \
       self.wasInitialized() and (self.resize > 0):
        # do something
After
def renderBanner(self):
    isMacOs = self.platform.toUpperCase().indexOf("MAC") > -1
    isIE = self.browser.toUpperCase().indexOf("IE") > -1
    wasResized = self.resize > 0

    if isMacOs and isIE and self.wasInitialized() and wasResized:
        # do something

Why Refactor

The main reason for extracting variables is to make a complex expression more understandable, by dividing it into its intermediate parts.

These could be:

  • Condition of the if() operator or a part of the ?: operator in C-based languages
  • A long arithmetic expression without intermediate results
  • Long multipart lines

Extracting a variable may be the first step towards performing Extract Method if you see that the extracted expression is used in other places in your code.

Benefits

  • More readable code! Try to give the extracted variables good names that announce the variable's purpose loud and clear. More readability, fewer long-winded comments. Go for names like customerTaxValue, cityUnemploymentRate, clientSalutationString, etc.

Drawbacks

  • More variables are present in your code. But this is counterbalanced by the ease of reading your code.

How to Refactor

  1. Insert a new line before the relevant expression and declare a new variable there. Assign part of the complex expression to this variable.

  2. Replace that part of the expression with the new variable.

  3. Repeat the process for all complex parts of the expression.

Tired of reading?

No wonder, there are 7 hours worth of the text on this website.

Try out something different. We've just launched the interactive learning course on refactoring. It has more content and much more fun than a boring text.

Learn more...

Live Example

First time here? No worries!

Everything is quite simple. This example is like video (but much cooler):

  1. After pressing the Play button, you will see floating messages, which will guide you through the example. You can proceed by clicking on them.
  2. You can fast-forward or return to previous steps by pressing arrow buttons on the left.
  3. Also, you can take a look at the code diff window, which will show what has changed in code during the example ()
  4. To check the code for errors, you can press "Compile and test" button ()