Replace Nested Conditional with Guard Clauses

Problem

You have a group of nested conditionals and it is hard to determine the normal flow of code execution.

Solution

Isolate all special checks and edge cases into separate clauses and place them before the main checks. Ideally, you should have a "flat" list of conditionals, one after the other.

Before
public double getPayAmount() {
  double result;
  if (isDead){
    result = deadAmount();
  }
  else {
    if (isSeparated){
      result = separatedAmount();
    }
    else {
      if (isRetired){
        result = retiredAmount();
      }
      else{
        result = normalPayAmount();
      }
    }
  }
  return result;
}
After
public double getPayAmount() {
  if (isDead){
    return deadAmount();
  }
  if (isSeparated){
    return separatedAmount();
  }
  if (isRetired){
    return retiredAmount();
  }
  return normalPayAmount();
}
Before
public double GetPayAmount()
{
  double result;
  
  if (isDead)
  {
    result = DeadAmount();
  }
  else 
  {
    if (isSeparated)
    {
      result = SeparatedAmount();
    }
    else 
    {
      if (isRetired)
      {
        result = RetiredAmount();
      }
      else
      {
        result = NormalPayAmount();
      }
    }
  }
  
  return result;
}
After
public double GetPayAmount() 
{
  if (isDead)
  {
    return DeadAmount();
  }
  if (isSeparated)
  {
    return SeparatedAmount();
  }
  if (isRetired)
  {
    return RetiredAmount();
  }
  return NormalPayAmount();
}
Before
public function getPayAmount() {
  if ($this->isDead)
    $result = $this->deadAmount();
  else {
    if ($this->isSeparated)
      $result = $this->separatedAmount();
    else {
      if ($this->isRetired)
        $result = $this->retiredAmount();
      else
        $result = $this->normalPayAmount();
    }
  }
  return $result;
}
After
public function getPayAmount() {
  if ($this->isDead)
    return $this->deadAmount();
  if ($this->isSeparated)
    return $this->separatedAmount();
  if ($this->isRetired)
    return $this->retiredAmount();
  return $this->normalPayAmount();
}
Before
def getPayAmount(self):
    if self.isDead:
        result = deadAmount()
    else:
        if self.isSeparated:
            result = separatedAmount()
        else:
            if self.isRetired:
                result = retiredAmount()
            else:
                result = normalPayAmount()
    return result
After
def getPayAmount(self):
    if self.isDead:
        return deadAmount()
    if self.isSeparated:
        return separatedAmount()
    if self.isRetired:
        return retiredAmount()
    return normalPayAmount()

Why Refactor

Spotting the "conditional from hell" is fairly easy. The indentations of each level of nestedness form an arrow, pointing to the right in the direction of pain and woe:

if () {
    if () {
        do {
            if () {
                if () {
                    if () {
                        ...
                    }
                }
                ...
            }
            ...
        }
        while ();
        ...
    }
    else {
        ...
    }
}

It is difficult to figure out what each conditional does and how, since the "normal" flow of code execution is not immediately obvious. These conditionals indicate helter-skelter evolution, with each condition added as a stopgap measure without any thought paid to optimizing the overall structure.

To simplify the situation, isolate the special cases into separate conditions that immediately end execution and return a null value if the guard clauses are true. In effect, your mission here is to make the structure flat.

How to Refactor

Try to rid the code of side effects – Separate Query from Modifier may be helpful for the purpose. This solution will be necessary for the reshuffling described below.

  1. Isolate all guard clauses that lead to calling an exception or immediate return of a value from the method. Place these conditions at the beginning of the method.

  2. After rearrangement is complete and all tests are successfully completed, see whether you can use Consolidate Conditional Expression for guard clauses that lead to the same exceptions or returned values.