---
id: java-lambda
title: "Java Lambda"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["lambda expressions", "자바 람다"]
duplicate_of: ""
source_trust_level: "B"
confidence_score: 0.9
created_at: 2026-07-04
updated_at: 2026-07-04
review_reason: ""
merge_history: []
tags: ["java", "programming", "w3schools", "lambda", "functional-interface"]
raw_sources: ["https://www.w3schools.com/java/java_lambda.asp"]
applied_in: []
github_commit: ""
---

# [[Java Lambda]]

## 🎯 한 줄 통찰 (One-line insight)
A lambda can only replace an anonymous class when the target interface is "functional" (exactly ONE abstract method) — the tutorial's own rule of thumb draws the line precisely there: lambda for short single-method interfaces, anonymous class when you need multiple methods, extra fields, or to extend a class. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Lambda expression** (Java 8+) — a short, nameless block of code taking parameters and returning a value. [S1]
- **Syntax** — `parameter -> expression`; multiple parameters need parentheses; complex bodies need `{ }` + explicit `return`. [S1]
- **Functional interface requirement** — a lambda's target type must be an interface with EXACTLY one abstract method. [S1]
- **Common use** — passed as arguments to methods like `ArrayList.forEach()`. [S1]
- **Built-in functional interfaces** — e.g. `Consumer<T>` from `java.util.function`. [S1]
- **Custom functional interfaces** — user-defined single-method interfaces work as lambda targets too. [S1]
- **Lambda vs. anonymous class rule of thumb** — lambda for short single-method interfaces; anonymous class for multiple methods/fields/extending a class. [S1]

## 📖 세부 내용 (Details)
- forEach with lambda: `numbers.forEach((n) -> { System.out.println(n); });`. [S1]
- Lambda stored in a variable (Consumer): `Consumer<Integer> method = (n) -> { System.out.println(n); }; numbers.forEach(method);`. [S1]
- Lambda as method parameter (custom functional interface): `interface StringFunction { String run(String str); } ... StringFunction exclaim = (s) -> s + "!"; printFormatted("Hello", exclaim);`. [S1]
- Same task via anonymous class vs. lambda: `Greeting g = new Greeting() { public void sayHello() {...} };` vs. `Greeting g = () -> System.out.println("Hello from lambda");`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **람다 사용의 전제 조건**: 대상 인터페이스가 반드시 함수형 인터페이스(추상 메서드 1개)여야 익명 클래스를 람다로 대체할 수 있다는 점이 규칙으로 명시됨. [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — ArrayList.forEach()에 람다를 전달하는 패턴이 가장 흔한 실전 활용이다. [S1]

## 💻 코드 패턴 (Code patterns)
Lambda passed to forEach() (Java):
```java
ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(5); numbers.add(9); numbers.add(8); numbers.add(1);
numbers.forEach((n) -> { System.out.println(n); });
```

## ✅ 검증 상태 및 신뢰도
- **상태:** draft
- **검증 단계:** conceptual
- **출처 신뢰도:** B (W3Schools — widely used educational reference, not a primary standards body)
- **신뢰 점수:** 0.90
- **중복 검사 결과:** 신규 생성 (New discovery)

## 🔗 지식 그래프 (Knowledge Graph)
- **상위/루트:** [[Java Tutorial]]
- **관련 개념:** [[Java Anonymous]], [[Java Interface]], [[Java Threads]], [[Java Advanced Sorting]]
- **참조 맥락:** 익명 클래스의 축약형 — 고급 정렬(Comparator 람다) 챕터로 직접 이어짐.

## 📚 출처 (Sources)
- [S1] W3Schools — Java Lambda Expressions — https://www.w3schools.com/java/java_lambda.asp

## 📝 변경 이력 (Change history)
- 2026-07-04: Initial draft synthesized from the W3Schools "Java Lambda Expressions" page (Astra wiki-curation, P-Reinforce v3.1 format).