---
id: java-generics
title: "Java Generics"
category: "Programming_Language"
status: "draft"
verification_status: "conceptual"
canonical_id: ""
aliases: ["자바 제네릭"]
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", "generics"]
raw_sources: ["https://www.w3schools.com/java/java_generics.asp"]
applied_in: []
github_commit: ""
---

# [[Java Generics]]

## 🎯 한 줄 통찰 (One-line insight)
`<T extends Number>` bounds a generic to only numeric wrapper types specifically so `.doubleValue()` becomes callable inside the class — without that bound, `T` could be any type (including one with no `doubleValue()` method), so the bound isn't just a restriction, it's what makes the generic method's body compile at all. [S1]

## 🧠 핵심 개념 (Core concepts)
- **Generics** — write classes/interfaces/methods that work across data types without fixing the type in advance. [S1]
- **Benefits** — code reusability (one class/method, many types), type safety (compile-time error catching), cleaner code (no manual casting). [S1]
- **Type parameter `<T>`** — a placeholder; becomes concrete (String, Integer, etc.) at usage site. [S1]
- **Generic class** — `class Box<T> { T value; ... }`. [S1]
- **Generic method** — `public static <T> void printArray(T[] array) { ... }`. [S1]
- **Bounded types** — `<T extends Number>` restricts T to Number subclasses, enabling numeric methods like `.doubleValue()` inside the class body. [S1]
- **Used internally by Collections** — `ArrayList<String>`, `HashMap<K,V>` are themselves generic. [S1]

## 📖 세부 내용 (Details)
- Generic class: `class Box<T> { T value; void set(T value) { this.value = value; } T get() { return value; } } ... Box<String> stringBox = new Box<>(); Box<Integer> intBox = new Box<>();`. [S1]
- Generic method: `public static <T> void printArray(T[] array) { for (T item : array) { System.out.println(item); } } ... printArray(names); printArray(numbers);`. [S1]
- Bounded generic: `class Stats<T extends Number> { T[] nums; double average() { double sum = 0; for (T num : nums) { sum += num.doubleValue(); } return sum / nums.length; } }` — works for both `Integer[]` and `Double[]`. [S1]

## ⚖️ 모순 및 업데이트 (Contradictions & updates)
소스에서 모순되는 정보는 발견되지 않음.

## 🛠️ 적용 사례 (Applied in summary)
현재 발견된 실제 적용 사례가 없습니다 — Integer/Double 배열 모두에 동작하는 평균 계산(Stats<T extends Number>)이 경계 제네릭의 대표 실전 사례다. [S1]

## 💻 코드 패턴 (Code patterns)
Bounded generic class computing average for any Number subtype (Java):
```java
class Stats<T extends Number> {
    T[] nums;
    Stats(T[] nums) { this.nums = nums; }
    double average() {
        double sum = 0;
        for (T num : nums) { sum += num.doubleValue(); }
        return sum / nums.length;
    }
}
```

## ✅ 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 Wrapper Classes]], [[Java ArrayList]], [[Java Annotations]]
- **참조 맥락:** 타입 안전한 재사용 가능 코드 — Annotations 챕터로 이어짐.

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

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