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10_Wiki/Dev/Topic_Java/Java_Print_Variables.md

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+---
+id: java-print-variables
+title: "Java Print Variables"
+category: "Programming_Language"
+status: "draft"
+verification_status: "conceptual"
+canonical_id: ""
+aliases: ["자바 변수 출력"]
+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", "variables", "concatenation"]
+raw_sources: ["https://www.w3schools.com/python/python_variables_output.asp", "https://www.w3schools.com/java/java_variables_print.asp"]
+applied_in: []
+github_commit: ""
+---
+
+# [[Java Print Variables]]
+
+## 🎯 한 줄 통찰 (One-line insight)
+The `+` operator is dangerously overloaded — without parentheses, `"The sum is " + x + y` concatenates left-to-right as strings (`"56"`), while `"The sum is " + (x + y)` forces numeric addition first (`11`) — the exact same expression differing only in parenthesization. [S1]
+
+## 🧠 핵심 개념 (Core concepts)
+- **`+` for display** — combines text and a variable: `"Hello " + name`. [S1]
+- **`+` on two variables** — concatenates or adds depending on type. [S1]
+- **`+` dual meaning** — string concatenation for text, arithmetic addition for numbers. [S1]
+- **Left-to-right evaluation without parentheses** — mixing string and numeric `+` operands evaluates strictly left to right, so once a string appears, everything after becomes string concatenation. [S1]
+- **Parentheses force numeric evaluation first** — `(x + y)` computes the sum before concatenating with the string. [S1]
+
+## 🧩 추출된 패턴 (Extracted patterns)
+- **The `"sum is " + x + y` footgun** — this is a well-known Java gotcha: without parentheses around `x + y`, the output silently becomes string concatenation ("56") instead of the intended numeric sum ("11"), with no compiler warning. [S1]
+
+## 📖 세부 내용 (Details)
+- Text + variable: `String name = "John"; System.out.println("Hello " + name);`. [S1]
+- Variable + variable (string concat): `String fullName = firstName + lastName;`. [S1]
+- Numeric addition: `int x = 5; int y = 6; System.out.println(x + y); // 11`. [S1]
+- Mixing footgun: `System.out.println("The sum is " + x + y); // "The sum is 56"` vs. `System.out.println("The sum is " + (x + y)); // "The sum is 11"`. [S1]
+
+## ⚖️ 모순 및 업데이트 (Contradictions & updates)
+소스에서 모순되는 정보는 발견되지 않음. 괄호 유무로 인한 결과 차이가 소스에서 직접 경고됨. [S1]
+
+## 🛠️ 적용 사례 (Applied in summary)
+현재 발견된 실제 적용 사례가 없습니다 — 텍스트와 숫자를 함께 출력할 때 항상 괄호로 연산 순서를 명시하는 습관이 필요하다. [S1]
+
+## 💻 코드 패턴 (Code patterns)
+The parenthesization footgun (Java):
+```java
+int x = 5;
+int y = 6;
+System.out.println("The sum is " + x + y);   // "The sum is 56"
+System.out.println("The sum is " + (x + y)); // "The sum is 11"
+```
+
+## ✅ 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 Variables]], [[Java Strings Concat]], [[Java Strings Numbers]]
+- **참조 맥락:** + 연산자의 이중 의미(연결/덧셈)를 다루는 핵심 문서 — 괄호로 순서를 명시하는 습관이 중요하다.
+
+## 📚 출처 (Sources)
+- [S1] W3Schools — Java Print Variables — https://www.w3schools.com/java/java_variables_print.asp
+
+## 📝 변경 이력 (Change history)
+- 2026-07-04: Initial draft synthesized from the W3Schools "Java Print Variables" page (Astra wiki-curation, P-Reinforce v3.1 format).