Replace Exception with Test

Problem

You throw an exception in a place where a simple test would do the job?

Solution

Replace the exception with a condition test.

Before
double getValueForPeriod(int periodNumber) {
  try {
    return values[periodNumber];
  } catch (ArrayIndexOutOfBoundsException e) {
    return 0;
  }
}
After
double getValueForPeriod(int periodNumber) {
  if (periodNumber >= values.length) {
    return 0;
  }
  return values[periodNumber];
}
Before
double GetValueForPeriod(int periodNumber) 
{
  try 
  {
    return values[periodNumber];
  } 
  catch (IndexOutOfRangeException e) 
  {
    return 0;
  }
}
After
double GetValueForPeriod(int periodNumber) 
{
  if (periodNumber >= values.Length) 
  {
    return 0;
  }
  return values[periodNumber];
}
Before
function getValueForPeriod($periodNumber) {
  try {
    return $this->values[$periodNumber];
  } catch (ArrayIndexOutOfBoundsException $e) {
    return 0;
  }
}
After
function getValueForPeriod($periodNumber) {
  if ($periodNumber >= count($this->values)) {
    return 0;
  }
  return $this->values[$periodNumber];
}
Before
def getValueForPeriod(periodNumber):
    try:
        return values[periodNumber]
    except IndexError:
        return 0
After
def getValueForPeriod(self, periodNumber):
    if periodNumber >= len(self.values):
        return 0
    return self.values[periodNumber]

Why Refactor

Exceptions should be used to handle irregular behavior related to an unexpected error. They should not serve as a replacement for testing. If an exception can be avoided by simply verifying a condition before running, then do so. Exceptions should be reserved for real errors.

For instance, you entered a minefield and triggered a mine there, resulting in an exception; the exception was successfully handled and you were lifted through the air to safety beyond the mine field. But you could have avoided this all by simply reading the warning sign in front of the minefield to begin with.

Benefits

  • A simple conditional can sometimes be more obvious than exception handling code.

How to Refactor

  1. Create a conditional for an edge case and move it before the try/catch block.

  2. Move code from the catch section inside this conditional.

  3. In the catch section, place the code for throwing a usual unnamed exception and run all the tests.

  4. If no exceptions were thrown during the tests, get rid of the try/catch operator.