Autumn SALE
/ Refactoring / Techniques / Composing Methods

Extract Method

Problem

You have a code fragment that can be grouped together.

Solution

Move this code to a separate new method (or function) and replace the old code with a call to the method.

Before
void printOwing() {
  printBanner();

  // Print details.
  System.out.println("name: " + name);
  System.out.println("amount: " + getOutstanding());
}
After
void printOwing() {
  printBanner();
  printDetails(getOutstanding());
}

void printDetails(double outstanding) {
  System.out.println("name: " + name);
  System.out.println("amount: " + outstanding);
}
Before
void PrintOwing() 
{
  this.PrintBanner();

  // Print details.
  Console.WriteLine("name: " + this.name);
  Console.WriteLine("amount: " + this.GetOutstanding());
}
After
void PrintOwing()
{
  this.PrintBanner();
  this.PrintDetails();
}

void PrintDetails()
{
  Console.WriteLine("name: " + this.name);
  Console.WriteLine("amount: " + this.GetOutstanding());
}
Before
function printOwing() {
  $this->printBanner();

  // Print details.
  print("name:  " . $this->name);
  print("amount " . $this->getOutstanding());
}
After
function printOwing() {
  $this->printBanner();
  $this->printDetails($this->getOutstanding());
}

function printDetails($outstanding) {
  print("name:  " . $this->name);
  print("amount " . $outstanding);
}
Before
def printOwing(self):
    self.printBanner()

    # print details
    print("name:", self.name)
    print("amount:", self.getOutstanding())
After
def printOwing(self):
    self.printBanner()
    self.printDetails(self.getOutstanding())

def printDetails(self, outstanding):
    print("name:", self.name)
    print("amount:", outstanding)
Before
printOwing(): void {
  printBanner();

  // Print details.
  console.log("name: " + name);
  console.log("amount: " + getOutstanding());
}
After
printOwing(): void {
  printBanner();
  printDetails(getOutstanding());
}

printDetails(outstanding: number): void {
  console.log("name: " + name);
  console.log("amount: " + outstanding);
}

Why Refactor

The more lines found in a method, the harder it’s to figure out what the method does. This is the main reason for this refactoring.

Besides eliminating rough edges in your code, extracting methods is also a step in many other refactoring approaches.

Benefits

  • More readable code! Be sure to give the new method a name that describes the method’s purpose: createOrder(), renderCustomerInfo(), etc.

  • Less code duplication. Often the code that’s found in a method can be reused in other places in your program. So you can replace duplicates with calls to your new method.

  • Isolates independent parts of code, meaning that errors are less likely (such as if the wrong variable is modified).

How to Refactor

  1. Create a new method and name it in a way that makes its purpose self-evident.

  2. Copy the relevant code fragment to your new method. Delete the fragment from its old location and put a call for the new method there instead.

    Find all variables used in this code fragment. If they’re declared inside the fragment and not used outside of it, simply leave them unchanged—they’ll become local variables for the new method.

  3. If the variables are declared prior to the code that you’re extracting, you will need to pass these variables to the parameters of your new method in order to use the values previously contained in them. Sometimes it’s easier to get rid of these variables by resorting to Replace Temp with Query.

  4. If you see that a local variable changes in your extracted code in some way, this may mean that this changed value will be needed later in your main method. Double-check! And if this is indeed the case, return the value of this variable to the main method to keep everything functioning.