2nd/10_Wiki/Topics/Topic_Programming/Topic_Java/Java_HashMap.md

id, title, category, status, verification_status, canonical_id, aliases, duplicate_of, source_trust_level, confidence_score, created_at, updated_at, review_reason, merge_history, tags, raw_sources, applied_in, github_commit

id	title	category	status	verification_status	canonical_id	aliases	duplicate_of	source_trust_level	confidence_score	created_at	updated_at	review_reason	merge_history	tags	raw_sources	applied_in	github_commit
java-hashmap	Java HashMap	Programming_Language	draft	conceptual		자바 HashMap		B	0.9	2026-07-04	2026-07-04			java programming w3schools collections-framework hashmap	https://www.w3schools.com/java/java_hashmap.asp

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):

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).