Refactoring

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What :

The first version of the code you write may not be of production quality. It is OK to first concentrate on making the code work, rather than worry over the quality of the code, as long as you improve the quality later. This process of improving a program's internal structure in small steps without modifying its external behavior is called refactoring.

Refactoring is not rewriting: Discarding poorly-written code entirely and re-writing it from scratch is not refactoring because refactoring needs to be done in small steps.
Refactoring is not bug fixing: By definition, refactoring is different from bug fixing or any other modifications that alter the external behavior (e.g. adding a feature) of the component in concern.

Improving code structure can have many secondary benefits: e.g.

hidden bugs become easier to spot
improve performance (sometimes, simpler code runs faster than complex code because simpler code is easier for the compiler to optimize).

Given below are two common refactorings (more).

Refactoring Name: Consolidate Duplicate Conditional Fragments

Situation: The same fragment of code is in all branches of a conditional expression.

Method: Move it outside of the expression.

Example:

if (isSpecialDeal()) {
    total = price * 0.95;
    send();
} else {
    total = price * 0.98;
    send();
}

→

if (isSpecialDeal()) {
    total = price * 0.95;
} else {
    total = price * 0.98;
}
send();

if is_special_deal:
    total = price * 0.95
    send()
else:
    total = price * 0.98
    send()

→

if is_special_deal:
    total = price * 0.95
else:
    total = price * 0.98
    
send()

Refactoring Name: Extract Method

Situation: You have a code fragment that can be grouped together.

Method: Turn the fragment into a method whose name explains the purpose of the method.

Example:

void printOwing() {
    printBanner();

    // print details
    System.out.println("name:	" + name);
    System.out.println("amount	" + getOutstanding());
}

void printOwing() {
    printBanner();
    printDetails(getOutstanding());
}

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

def print_owing():
    print_banner()

    # print details
    print("name:	" + name)
    print("amount	" + get_outstanding())

def print_owing():
    print_banner()
    print_details(get_outstanding())

def print_details(amount):
    print("name:	" + name)
    print("amount	" + amount)

Some IDEs have builtin support for basic refactorings such as automatically renaming a variable/method/class in all places it has been used.

Refactoring, even if done with the aid of an IDE, may still result in regressions. Therefore, each small refactoring should be followed by regression testing.

How :

Given below are some more commonly used refactorings. A more comprehensive list is available at refactoring-catalog.