Refactoring & Code Quality

Refactoring Catalog: Core Mechanics

Refactoring is the disciplined technique of restructuring code without changing its observable behavior, in small, independently verifiable steps. A handful of mechanical moves — Extract, Inline, Rename, Move — cover the large majority of real refactoring work.

Extract Method/Function: pull a cohesive fragment out into a well-named method, replacing it with a call
Extract Variable: name an expression so the code reads as intent rather than arithmetic
Inline Method/Variable: the reverse move — when the indirection adds no clarity, fold it back in
Rename: the highest-leverage, lowest-risk refactoring there is — a name is documentation that never goes stale if it stays accurate
Move Method/Field: relocate behavior or data to the class it is most cohesive with (the fix for Feature Envy, see Code Smells)
Every refactoring is reversible — Extract has an Inline, Move has a Move-back — which is what makes "try it and see" a legitimate way to explore a design

Core refactorings and their intent
Refactoring	Intent
Extract Method	Turn an unnamed fragment into a named, reusable, independently testable unit
Extract Variable	Replace an opaque expression with a name that explains what it means
Inline Method / Variable	Remove indirection that has stopped paying for itself
Rename	Make the name match what the thing actually is, now
Move Method / Field	Put behavior or data next to what it is most cohesive with

Extract Method, before and after

// Before: one method, two responsibilities buried inside it.
void printInvoice(Invoice invoice) {
    double total = 0;
    for (LineItem item : invoice.getItems()) {
        total += item.getPrice() * item.getQuantity();
    }
    System.out.println("Total: 
quot; + total);
    for (LineItem item : invoice.getItems()) {
        System.out.println(item.getName() + " x" + item.getQuantity());
    }
}

// After: each cohesive fragment gets a name.
void printInvoice(Invoice invoice) {
    System.out.println("Total: 
quot; + calculateTotal(invoice));
    printLineItems(invoice);
}
double calculateTotal(Invoice invoice) { /* the sum, extracted */ }
void printLineItems(Invoice invoice) { /* the loop, extracted */ }