Code Refactor Master

When to use this Skill

Use this Skill when:

• Refactoring existing code for better quality
• Eliminating code smells
• Improving code readability and maintainability
• Optimizing performance without changing behavior
• Applying design patterns
• Reducing complexity
• Cleaning up technical debt

Refactoring Principles

1. Core Rules

Golden Rule: Make code changes that improve internal structure without altering external behavior

Key Principles:

• One refactoring at a time
• Run tests after each refactoring
• Commit frequently with clear messages
• Keep refactoring separate from feature work
• Maintain backwards compatibility unless explicitly changing API

Red Flags to Refactor:

• Code duplication (DRY principle)
• Long methods (>20-30 lines)
• Large classes (>300-500 lines)
• Long parameter lists (>3-4 parameters)
• Deeply nested conditionals (>3 levels)
• Comments explaining what code does (code should be self-explanatory)

2. Common Code Smells

Bloaters:

• Long Method
• Large Class
• Primitive Obsession
• Long Parameter List
• Data Clumps

Object-Orientation Abusers:

• Switch Statements (consider polymorphism)
• Temporary Field
• Refused Bequest
• Alternative Classes with Different Interfaces

Change Preventers:

• Divergent Change (one class changes for many reasons)
• Shotgun Surgery (one change requires many small changes)
• Parallel Inheritance Hierarchies

Dispensables:

• Comments (where code should be self-explanatory)
• Duplicate Code
• Lazy Class
• Dead Code
• Speculative Generality

Couplers:

• Feature Envy
• Inappropriate Intimacy
• Message Chains
• Middle Man

3. Refactoring Catalog

Method-Level Refactorings:

Extract 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);
}

Inline Method

// Before - method too simple
int getRating() {
    return moreThanFiveLateDeliveries() ? 2 : 1;
}

boolean moreThanFiveLateDeliveries() {
    return numberOfLateDeliveries > 5;
}

// After
int getRating() {
    return numberOfLateDeliveries > 5 ? 2 : 1;
}

Replace Temp with Query

// Before
double calculateTotal() {
    double basePrice = quantity * itemPrice;
    if (basePrice > 1000) {
        return basePrice * 0.95;
    }
    return basePrice * 0.98;
}

// After
double calculateTotal() {
    if (basePrice() > 1000) {
        return basePrice() * 0.95;
    }
    return basePrice() * 0.98;
}

double basePrice() {
    return quantity * itemPrice;
}

Variable-Level Refactorings:

Rename Variable

# Before
d = 10  # elapsed time in days

# After
elapsed_time_in_days = 10

Split Temporary Variable

// Before
double temp = 2 * (height + width);
System.out.println(temp);
temp = height * width;
System.out.println(temp);

// After
double perimeter = 2 * (height + width);
System.out.println(perimeter);
double area = height * width;
System.out.println(area);

Class-Level Refactorings:

Extract Class

// Before
class Person {
    String name;
    String officeAreaCode;
    String officeNumber;

    String getTelephoneNumber() {
        return "(" + officeAreaCode + ") " + officeNumber;
    }
}

// After
class Person {
    String name;
    TelephoneNumber officeTelephone = new TelephoneNumber();

    String getTelephoneNumber() {
        return officeTelephone.getTelephoneNumber();
    }
}

class TelephoneNumber {
    String areaCode;
    String number;

    String getTelephoneNumber() {
        return "(" + areaCode + ") " + number;
    }
}

Replace Conditional with Polymorphism

# Before
class Bird:
    def get_speed(self):
        if self.type == "EUROPEAN":
            return self.get_base_speed()
        elif self.type == "AFRICAN":
            return self.get_base_speed() - self.get_load_factor()
        elif self.type == "NORWEGIAN_BLUE":
            return 0 if self.is_nailed else self.get_base_speed()

# After
class Bird:
    def get_speed(self):
        pass  # Abstract

class European(Bird):
    def get_speed(self):
        return self.get_base_speed()

class African(Bird):
    def get_speed(self):
        return self.get_base_speed() - self.get_load_factor()

class NorwegianBlue(Bird):
    def get_speed(self):
        return 0 if self.is_nailed else self.get_base_speed()

4. Java-Specific Refactorings

Use Modern Java Features:

Replace Anonymous Class with Lambda

// Before
list.sort(new Comparator<String>() {
    @Override
    public int compare(String a, String b) {
        return a.compareTo(b);
    }
});

// After
list.sort((a, b) -> a.compareTo(b));
// Or even better
list.sort(String::compareTo);

Use Streams API

// Before
List<String> result = new ArrayList<>();
for (String s : list) {
    if (s.length() > 3) {
        result.add(s.toUpperCase());
    }
}

// After
List<String> result = list.stream()
    .filter(s -> s.length() > 3)
    .map(String::toUpperCase)
    .collect(Collectors.toList());

Replace String Concatenation

// Before - inefficient in loops
String result = "";
for (String s : list) {
    result += s + ", ";
}

// After
StringBuilder result = new StringBuilder();
for (String s : list) {
    result.append(s).append(", ");
}
// Or better
String result = String.join(", ", list);

Use Optional

// Before
public User findUser(int id) {
    User user = database.getUser(id);
    return user != null ? user : new User();
}

// After
public Optional<User> findUser(int id) {
    return Optional.ofNullable(database.getUser(id));
}

Replace Type Code with Enum

// Before
public static final int TYPE_A = 1;
public static final int TYPE_B = 2;

// After
public enum Type {
    A, B
}

5. Python-Specific Refactorings

Use List Comprehensions

# Before
result = []
for item in items:
    if item > 0:
        result.append(item * 2)

# After
result = [item * 2 for item in items if item > 0]

Use Collections Module

# Before
counts = {}
for item in items:
    if item in counts:
        counts[item] += 1
    else:
        counts[item] = 1

# After
from collections import Counter
counts = Counter(items)

Use Context Managers

# Before
file = open('file.txt')
data = file.read()
file.close()

# After
with open('file.txt') as file:
    data = file.read()

Use f-strings

# Before
message = "Hello, %s! You are %d years old." % (name, age)

# After
message = f"Hello, {name}! You are {age} years old."

Use Type Hints

# Before
def process(data):
    return [x * 2 for x in data]

# After
def process(data: list[int]) -> list[int]:
    return [x * 2 for x in data]

Use Dataclasses

# Before
class Point:
    def __init__(self, x, y):
        self.x = x
        self.y = y

    def __repr__(self):
        return f"Point(x={self.x}, y={self.y})"

# After
from dataclasses import dataclass

@dataclass
class Point:
    x: int
    y: int

6. Clean Code Principles

Naming:

// Bad
int d; // elapsed time in days

// Good
int elapsedTimeInDays;

// Bad
public List<int[]> getThem() {
    List<int[]> list1 = new ArrayList<>();
    for (int[] x : theList) {
        if (x[0] == 4) {
            list1.add(x);
        }
    }
    return list1;
}

// Good
public List<Cell> getFlaggedCells() {
    List<Cell> flaggedCells = new ArrayList<>();
    for (Cell cell : gameBoard) {
        if (cell.isFlagged()) {
            flaggedCells.add(cell);
        }
    }
    return flaggedCells;
}

Functions:

• Small (20-30 lines max)
• Do one thing
• One level of abstraction
• Descriptive names
• Few arguments (0-3 ideal)

Comments:

// Bad - comment explains what code does
// Check to see if employee is eligible for full benefits
if ((employee.flags & HOURLY_FLAG) && (employee.age > 65))

// Good - code is self-explanatory
if (employee.isEligibleForFullBenefits())

7. Performance Refactorings

Algorithm Optimization:

# Before - O(n²)
def has_duplicates(arr):
    for i in range(len(arr)):
        for j in range(i + 1, len(arr)):
            if arr[i] == arr[j]:
                return True
    return False

# After - O(n)
def has_duplicates(arr):
    return len(arr) != len(set(arr))

Lazy Evaluation:

// Before - computes all values
List<Integer> results = list.stream()
    .map(this::expensiveOperation)
    .collect(Collectors.toList());
return results.get(0);

// After - computes only what's needed
return list.stream()
    .map(this::expensiveOperation)
    .findFirst()
    .orElse(null);

Memoization:

# Before
def fibonacci(n):
    if n <= 1:
        return n
    return fibonacci(n-1) + fibonacci(n-2)

# After
from functools import lru_cache

@lru_cache(maxsize=None)
def fibonacci(n):
    if n <= 1:
        return n
    return fibonacci(n-1) + fibonacci(n-2)

8. Refactoring Workflow

Step-by-step process:

1. Identify: Find code smell or improvement opportunity
2. Plan: Decide which refactoring to apply
3. Test: Ensure tests exist and pass
4. Refactor: Make one small change
5. Test: Run tests again
6. Commit: Save working state
7. Repeat: Continue with next refactoring

Example workflow:

# 1. Create feature branch
git checkout -b refactor/improve-user-service

# 2. Identify issue (e.g., long method)
# Read code, find UserService.processUser() is 150 lines

# 3. Run existing tests
mvn test  # or pytest

# 4. Extract method
# Break processUser into smaller methods

# 5. Run tests
mvn test  # Ensure still passing

# 6. Commit
git commit -m "refactor: extract validateUser method"

# 7. Continue
# Extract next method, repeat

9. Refactoring Checklist

Before refactoring:

• Tests exist and pass
• Understand the code behavior
• Have a clear goal
• Know which refactoring to apply

During refactoring:

• Make small, incremental changes
• Run tests after each change
• Keep code working at all times
• Commit frequently

After refactoring:

• All tests pass
• Code is more readable
• Complexity reduced
• No behavioral changes
• Documentation updated if needed

10. LeetCode-Specific Refactoring

Optimize brute force:

// Before - Brute force O(n²)
public int[] twoSum(int[] nums, int target) {
    for (int i = 0; i < nums.length; i++) {
        for (int j = i + 1; j < nums.length; j++) {
            if (nums[i] + nums[j] == target) {
                return new int[]{i, j};
            }
        }
    }
    return new int[]{};
}

// After - Hash map O(n)
public int[] twoSum(int[] nums, int target) {
    Map<Integer, Integer> map = new HashMap<>();
    for (int i = 0; i < nums.length; i++) {
        int complement = target - nums[i];
        if (map.containsKey(complement)) {
            return new int[]{map.get(complement), i};
        }
        map.put(nums[i], i);
    }
    return new int[]{};
}

Extract helper methods:

# Before - monolithic
def solve(self, grid):
    # 50 lines of code mixing concerns

# After - modular
def solve(self, grid):
    if not self.is_valid(grid):
        return []

    processed = self.preprocess(grid)
    result = self.compute(processed)
    return self.format_output(result)

def is_valid(self, grid):
    # validation logic

def preprocess(self, grid):
    # preprocessing logic

def compute(self, data):
    # core algorithm

def format_output(self, result):
    # formatting logic

Refactoring Anti-Patterns

Don't:

• Refactor without tests
• Change behavior during refactoring
• Make multiple changes at once
• Refactor and add features simultaneously
• Over-engineer simple code
• Prematurely optimize

Project Context

For CS_basics repository:

• Refactor solutions in leetcode_java/ and leetcode_python/
• Maintain consistency across language implementations
• Keep algorithm explanations updated
• Follow project conventions
• Test refactored code with example inputs
• Document complexity improvements

Tools and Resources

IDE Refactoring Tools:

• IntelliJ IDEA: Refactor menu (Ctrl+Alt+Shift+T)
• VS Code: Python refactoring extensions
• PyCharm: Refactoring actions

Use automated refactoring when available:

• Rename: Safe renaming across project
• Extract method/variable: Automatic extraction
• Inline: Safe inlining with preview
• Move: Restructure packages/modules