Autumn SALE
/ Refactoring / Techniques / Simplifying Conditional Expressions

Decompose Conditional

Problem

You have a complex conditional (if-then/else or switch).

Solution

Decompose the complicated parts of the conditional into separate methods: the condition, then and else.

Before
if (date.before(SUMMER_START) || date.after(SUMMER_END)) {
  charge = quantity * winterRate + winterServiceCharge;
}
else {
  charge = quantity * summerRate;
}
After
if (isSummer(date)) {
  charge = summerCharge(quantity);
}
else {
  charge = winterCharge(quantity);
}
Before
if (date < SUMMER_START || date > SUMMER_END) 
{
  charge = quantity * winterRate + winterServiceCharge;
}
else 
{
  charge = quantity * summerRate;
}
After
if (isSummer(date))
{
  charge = SummerCharge(quantity);
}
else 
{
  charge = WinterCharge(quantity);
}
Before
if ($date->before(SUMMER_START) || $date->after(SUMMER_END)) {
  $charge = $quantity * $winterRate + $winterServiceCharge;
} else {
  $charge = $quantity * $summerRate;
}
After
if (isSummer($date)) {
  $charge = summerCharge($quantity);
} else {
  $charge = winterCharge($quantity);
}
Before
if date.before(SUMMER_START) or date.after(SUMMER_END):
    charge = quantity * winterRate + winterServiceCharge
else:
    charge = quantity * summerRate
After
if isSummer(date):
    charge = summerCharge(quantity)
else:
    charge = winterCharge(quantity)
Before
if (date.before(SUMMER_START) || date.after(SUMMER_END)) {
  charge = quantity * winterRate + winterServiceCharge;
}
else {
  charge = quantity * summerRate;
}
After
if (isSummer(date)) {
  charge = summerCharge(quantity);
}
else {
  charge = winterCharge(quantity);
}

Why Refactor

The longer a piece of code is, the harder it’s to understand. Things become even more hard to understand when the code is filled with conditions:

  • While you’re busy figuring out what the code in the then block does, you forget what the relevant condition was.

  • While you’re busy parsing else, you forget what the code in then does.

Benefits

  • By extracting conditional code to clearly named methods, you make life easier for the person who’ll be maintaining the code later (such as you, two months from now!).

  • This refactoring technique is also applicable for short expressions in conditions. The string isSalaryDay() is much prettier and more descriptive than code for comparing dates.

How to Refactor

  1. Extract the conditional to a separate method via Extract Method.

  2. Repeat the process for the then and else blocks.