---
id: java-hashmap
title: "Java HashMap"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["자바 HashMap"]
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", "collections-framework", "hashmap"]
raw_sources: ["https://www.w3schools.com/java/java_hashmap.asp"]
applied_in: []
github_commit: ""
---

# [[Java HashMap]]

## 🎯 한 줄 통찰 (One-line insight)
Re-putting an existing key doesn't throw or create a duplicate entry — it silently OVERWRITES the previous value (`put("Norway", "Oslo")` twice just keeps one "Norway"→"Oslo" entry), which is the HashMap-specific version of the "keys must be unique" rule, distinct from HashSet's simple duplicate-ignore behavior since here the VALUE could actually change on overwrite. [S1]

## 🧠 핵심 개념 (Core concepts)
- **`HashMap`** — key/value storage; access by key instead of index. [S1]
- **`put(key, value)`** — adds a new pair OR overwrites the value if the key already exists. [S1]
- **`get(key)`**, **`remove(key)`**, **`clear()`**, **`size()`** — standard operations, all key-based. [S1]
- **`keySet()`** vs. **`values()`** — iterate keys only, or values only, in a for-each loop. [S1]
- **Wrapper classes for primitive values** — same rule as ArrayList/HashSet. [S1]

## 📖 세부 내용 (Details)
- Overwrite on duplicate key: `capitalCities.put("Norway", "Oslo"); capitalCities.put("Norway", "Oslo"); // still one Norway entry — value overwritten, not duplicated`. [S1]
- Loop keys: `for (String i : capitalCities.keySet()) { System.out.println(i); }`. [S1]
- Loop values: `for (String i : capitalCities.values()) { System.out.println(i); }`. [S1]
- Loop keys+values: `for (String i : capitalCities.keySet()) { System.out.println("key: " + i + " value: " + capitalCities.get(i)); }`. [S1]
- Integer values: `HashMap<String, Integer> people = new HashMap<String, Integer>(); people.put("John", 32);`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
- **키 중복 시 값 덮어쓰기**: 같은 키로 다시 put()하면 이전 값이 최신 값으로 덮어써진다는 점이 명시됨(키는 반드시 유일해야 함). [S1]

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — 국가-수도 매핑처럼 키로 값을 조회하는 것이 실전 활용의 대표 예시다. [S1]

## 💻 코드 패턴 (Code patterns)
Looping keys and values together (Java):
```java
HashMap<String, String> capitalCities = new HashMap<String, String>();
capitalCities.put("England", "London");
capitalCities.put("Norway", "Oslo");
for (String i : capitalCities.keySet()) {
    System.out.println("key: " + i + " value: " + capitalCities.get(i));
}
```

## ✅ 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 Map]], [[Java TreeMap]], [[Java LinkedHashMap]]
- **참조 맥락:** Map의 대표 구현체 — 정렬된 TreeMap 챕터로 이어짐.

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

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