chapter four

4 Make type codes work

 

This chapter covers

  • Eliminating early binding with Never use if with else and Never use switch
  • Removing if statements with Replace type code with classes and Push code into classes
  • Removing bad generalization with Specialize method
  • Preventing coupling with Only inherit from interfaces
  • Removing methods with Inline method and Try delete then compile

At the end of the last chapter, we had just introduced a handleInput function that we could not use Extract method (P3.2.1) on because we did not want to break up the else if chain. Unfortunately, handleInput is not compliant with our fundamental Five lines (R3.1.1) rule, so we cannot leave it as is.

Here’s the function.

Listing 4.1 Initial
function handleInput(input: Input) {
  if (input === Input.LEFT) moveHorizontal(-1);
  else if (input === Input.RIGHT) moveHorizontal(1);
  else if (input === Input.UP) moveVertical(-1);
  else if (input === Input.DOWN) moveVertical(1);
}

4.1 Refactoring a simple if statement

We are stuck. To show how we deal with else if chains like this, we start by introducing a new rule.

4.1.1 Rule: Never use if with else

Statement

Never use if with else, unless we are checking against a data type we do not control.

Explanation

4.1.2 Applying the rule

4.1.3 Refactoring pattern: Replace type code with classes

4.1.4 Pushing code into classes

4.1.5 Refactoring pattern: Push code into classes

4.1.6 Inlining a superfluous method

4.1.7 Refactoring pattern: Inline method

4.2 Refactoring a large if statement

Summary