2nd/10_Wiki/Topics/Architecture/Introduce Null Object (널 객체 도입하기).md

---
id: wiki-2026-0508-introduce-null-object-널-객체-도입하기
title: Introduce Null Object (널 객체 도입하기)
category: 10_Wiki/Topics
status: verified
canonical_id: self
aliases: [Null Object Pattern, Null Object Refactoring]
duplicate_of: none
source_trust_level: A
confidence_score: 0.9
verification_status: applied
tags: [refactoring, design-pattern, null-safety]
raw_sources: []
last_reinforced: 2026-05-10
github_commit: pending
tech_stack:
  language: typescript
  framework: none
---

# Introduce Null Object (널 객체 도입하기)

## 매 한 줄
> **"매 null check 의 polymorphism 으로 대체"**. Fowler 의 Refactoring 카탈로그 entry — `if (x == null)` scattered checks 를 default-behavior object 로 collapse. 2026 modern 에서는 Optional/Maybe monad, sealed class hierarchy, TypeScript discriminated union 으로 evolve 했지만 core idea 는 동일.

## 매 핵심

### 매 문제
- Repeated null guards: `customer == null ? "unknown" : customer.name()` 의 매 caller 마다 반복.
- Null-aware logic 의 매 client code 에 leak — encapsulation violation.
- NullPointerException / undefined error 의 매 production crash 의 top cause.

### 매 solution
- Null state 를 dedicated subclass 로 represent: `NullCustomer extends Customer`.
- Default behavior 를 polymorphic method 로 push down.
- Caller 는 `customer.name()` 의 null check 없이 호출.

### 매 응용
1. Fowler refactoring step-by-step (Replace Conditional with Polymorphism 의 sibling).
2. Domain-driven design 의 missing aggregate handling.
3. Strategy pattern 의 default no-op variant.

## 💻 패턴

### 1. Before (null check scattered)
```typescript
class Site {
  constructor(public customer: Customer | null) {}
}

const customer = site.customer;
const name = customer === null ? "occupant" : customer.name;
const plan = customer === null ? BillingPlan.basic() : customer.plan;
const history = customer === null ? new PaymentHistory([]) : customer.history;
```

### 2. After (Null Object)
```typescript
abstract class Customer {
  abstract get name(): string;
  abstract get plan(): BillingPlan;
  abstract get history(): PaymentHistory;
  isNull(): boolean { return false; }
}

class RealCustomer extends Customer {
  constructor(
    public name: string,
    public plan: BillingPlan,
    public history: PaymentHistory,
  ) { super(); }
}

class NullCustomer extends Customer {
  get name() { return "occupant"; }
  get plan() { return BillingPlan.basic(); }
  get history() { return new PaymentHistory([]); }
  isNull() { return true; }
}

class Site {
  constructor(public customer: Customer = new NullCustomer()) {}
}

// Caller — 매 null check 없이
const name = site.customer.name;
```

### 3. Special Case (richer variant)
```typescript
class UnknownCustomer extends Customer {
  get name() { return "unknown"; }
  // ... default
}

class DeceasedCustomer extends Customer {
  get name() { return "[deceased]"; }
  // ... distinct semantics
}

// Factory dispatch
function loadCustomer(id: string): Customer {
  const row = db.find(id);
  if (!row) return new UnknownCustomer();
  if (row.deceased) return new DeceasedCustomer(row);
  return new RealCustomer(row.name, row.plan, row.history);
}
```

### 4. Optional / Maybe (functional alternative)
```typescript
type Maybe<T> = { kind: "some"; value: T } | { kind: "none" };

const customer: Maybe<Customer> = loadCustomer(id);

const name = customer.kind === "some"
  ? customer.value.name
  : "occupant";

// Or with helper
function fold<T, R>(m: Maybe<T>, onSome: (v: T) => R, onNone: () => R): R {
  return m.kind === "some" ? onSome(m.value) : onNone();
}
```

### 5. Sealed class (Kotlin / TS discriminated union)
```typescript
type CustomerState =
  | { tag: "real"; data: RealCustomer }
  | { tag: "null"; reason: "missing" | "deceased" };

function displayName(s: CustomerState): string {
  switch (s.tag) {
    case "real": return s.data.name;
    case "null": return s.reason === "deceased" ? "[deceased]" : "occupant";
  }
}
```

### 6. Logger Null Object (classic example)
```typescript
interface Logger {
  log(msg: string): void;
}

class ConsoleLogger implements Logger {
  log(msg: string) { console.log(msg); }
}

class NullLogger implements Logger {
  log(_msg: string) { /* no-op */ }
}

class Service {
  constructor(private logger: Logger = new NullLogger()) {}
  // 매 logger null check 없이 항상 호출 가능
}
```

### 7. React/UI default component
```tsx
function UserBadge({ user }: { user: User | null }) {
  // 매 null branch 없이 — NullUser component 로 dispatch
  const u = user ?? NULL_USER;
  return <span>{u.displayName}</span>;
}

const NULL_USER: User = {
  id: "anonymous",
  displayName: "Anonymous",
  avatar: "/default.png",
};
```

## 매 결정 기준
| 상황 | Approach |
|---|---|
| OO codebase, 매 null check 의 3+ 위치 반복 | Null Object class. |
| Functional / TS strict, 매 type-level safety 필요 | Maybe / Optional / discriminated union. |
| Side-effect 의 default no-op (logger, audit) | NullLogger 의 dependency injection. |
| 매 distinct missing-state semantics (unknown vs deceased) | Special Case (multiple null subtypes). |
| 매 single-use null check | 매 그냥 inline `?? default`. |

**기본값**: TypeScript / modern stack 에서는 `?? default` 또는 discriminated union. OO heavy refactor target 에서는 Null Object class.

## 🔗 Graph
- 부모: [[Refactoring_Best_Practices|Refactoring]] · [[Design Patterns]]
- 응용: [[Dependency Injection]]
- Adjacent: [[Replace Conditional with Polymorphism]]

## 🤖 LLM 활용
**언제**: legacy codebase 의 null-check noise 제거 refactor, library API 의 default behavior design.
**언제 X**: 매 single null check, performance-critical hot path (allocation overhead), exception 이 의도된 signal 인 경우.

## ❌ 안티패턴
- **Hidden bugs**: NullCustomer 의 silent default 가 매 real bug 를 mask. Logging 또는 `isNull()` flag 로 trace 가능하게 유지.
- **God Null Object**: 매 모든 method 에 default 의 over-design. 매 진짜 polymorphic 사용처만 cover.
- **Equality confusion**: `nullCustomer === null` false 의 매 caller 혼란. 매 명확한 documentation + `isNull()` helper.
- **Mutation on Null Object**: 매 shared instance 의 mutation 이 cross-contamination 의 cause. 매 immutable 유지.

## 🧪 검증 / 중복
- Verified (Fowler, *Refactoring* 2nd ed., "Introduce Special Case" — Null Object 의 modern rename).
- 신뢰도 A.

## 🕓 Changelog
| 날짜 | 변경 |
|---|---|
| 2026-05-08 | Phase 1 |
| 2026-05-10 | Manual cleanup — Null Object refactoring + modern Maybe/sealed union variants |