Skip to main content

Duplicated Code

"Duplicate Code" is often the root of all evil in software. It sneakily creeps into your project when you copy-paste a snippet from Stack Overflow or even from your own file because "it's just a few lines."

The problem is that every time you duplicate code, you double your maintenance burden. If you find a bug in one copy, you have to remember to fix it in the other copies too. And let's be honest: you will forget.

"Duplication is the primary enemy of a well-designed system." — Robert C. Martin

Signs of the Smell​

  • The same code structure appears in more than one place.
  • You see two classes that perform almost the same task but with different variable names.
  • You find yourself copying a block of code and making minor tweaks.

Reasons of the Smell​

Laziness: It’s faster to copy-paste than to think about how to abstract the logic. Multiple Authors: Two developers implement the same feature in different places without talking to each other. Rush: "I'll clean this up later" (which usually means "never").

Refactoring Recipe​

  • Extract Method
  • Pull Up Method
  • Form Template Method

Extract Method​

The easiest fix for duplication within the same class.

Before:

void printOwing() {
    printBanner();
    System.out.println("name: " + name);
    System.out.println("amount: " + getOutstanding());
}

void printCredit() {
    printBanner(); // Duplicated!
    System.out.println("name: " + name);
    System.out.println("credit: " + getCredit());
}

After:

void printOwing() {
    printDetails(getOutstanding(), "amount");
}

void printCredit() {
    printDetails(getCredit(), "credit");
}

void printDetails(double val, String label) {
    printBanner();
    System.out.println("name: " + name);
    System.out.println(label + ": " + val);
}

Pull Up Method​

If two subclasses share the same method, move it up to the superclass.

Before:

class Car extends Vehicle {
    void turn() { ... }
}
class Truck extends Vehicle {
    void turn() { ... } // Identical to Car.turn()
}

After:

class Vehicle {
    void turn() { ... }
}

References​