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    • Jenkins μ‹œμž‘ν•˜κΈ°
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  • 개발 방법둠
    • TDD
  • Spring
    • IoC
    • Is Spring Bean Thread-Safe?
    • Spring Singleton
    • Component Scan
    • Spring Annotation
    • 의쑴 관계 μ£Όμž…(DI)
    • Lombok ν™œμš©ν•˜κΈ°
    • Bean 생λͺ…주기와 콜백
    • Bean Scope
    • AOP(1) - AOPλž€
    • AOP(2) - Aop Proxy
    • AOP(3) - Dynamic Proxy
    • AOP(4) - AspectJ
    • POJO
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    • Transaction
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    • JPA Entity
    • Spring Data JPA
    • Spring Data Specification
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    • Cache
    • restTemplate
    • YAML 파일 μ„€μ •
    • Spring Boot
      • H2 DB μ„€μ •
      • 닀쀑 λ°μ΄ν„°λ² μ΄μŠ€ μ„€μ •
      • Mybatis μ—°λ™ν•˜κΈ°
    • Spring Batch
      • Batch μ‹œμž‘ν•΄λ³΄κΈ°
      • Batch Job Flow
      • Job
      • Step
      • Batch Scope & Job Parameter
      • JobRepository와 λ©”νƒ€ν…Œμ΄λΈ”
      • Chunk μ§€ν–₯ ν”„λ‘œκ·Έλž˜λ°
      • ItemReader
      • ItemProcessor
      • ItemWriter
      • Batch Schedular
      • Job별 Beanλ“±λ‘ν•˜κΈ°
      • Batch κ΅¬ν˜„μ‹œ λ°œμƒν•œ 였λ₯˜ 정리
      • Spring Batch Scaling
        • Multithread Jobκ΅¬ν˜„μ‹œ μ΄μŠˆμ‚¬ν•­
    • Spring test
      • Junit5
        • ν…ŒμŠ€νŠΈ 이름 ν‘œκΈ°
        • ν…ŒμŠ€νŠΈ κ·Έλ£Ή μ‚¬μ΄μ˜ 관계
        • νƒœκ·Έμ™€ 필터링
        • 동적 ν…ŒμŠ€νŠΈ
        • ν…ŒμŠ€νŠΈ LifeCycle
        • ν…ŒμŠ€νŠΈ λ©”μ„œλ“œ
        • ν…ŒμŠ€νŠΈ μˆœμ„œ
        • AssertJ
        • ν…ŒμŠ€νŠΈ 병렬 μ‹€ν–‰
        • AssertJ
        • Mock
      • Spring Boot Test DB 뢄리
      • Spring Batch Test
  • Web Application
    • Web Server & WAS
    • κ΄€λ ¨ κ°œλ… - HTTP API, HTML, CSR, SSR
    • Servlet
    • JSP
    • Cookie And Session
    • μ˜ˆμ™ΈνŽ˜μ΄μ§€
    • Java Bean
    • JDBC
    • Connection Pool
    • 파일 μ—…λ‘œλ“œ
    • Expression Language
    • JSTL
    • FrontControllerνŒ¨ν„΄ Command νŒ¨ν„΄
    • Forwarding
    • MVC
    • νšŒμ›κ°€μž…μ˜ˆμ œ
    • μ°Έκ³ 
      • κ°œλ°œν™˜κ²½μ„€μ •
  • Java+
    • SOAP/WSDL vs REST
    • WSDL을 JAVA둜 λ³€ν™˜ν•˜κΈ°
    • SOAP 톡신 OPEN API둜 κ°œλ°œν•΄λ³΄κΈ°
  • Java
    • Basic
      • λ³€μˆ˜μ™€ νƒ€μž…
      • μ—°μ‚°μž
      • 쑰건문과 반볡문
      • μ°Έμ‘° νƒ€μž…
      • 클래슀
      • 상속(Inheritance)
      • μΈν„°νŽ˜μ΄μŠ€(Interface)
      • 쀑첩 ν΄λž˜μŠ€μ™€ 쀑첩 μΈν„°νŽ˜μ΄μŠ€
      • μ˜ˆμ™Έ 처리
      • API - Object, System, Class, Math, Wrapper
      • API - String, StringBuffer, StringBuilder
      • Thread
      • Generic
      • Lambda
      • Collection - List, Set
      • Collection - Map
      • Collection - Tree
      • Collection - Stack, Queue
      • Stream
      • Reflection
      • μ •κ·œν‘œν˜„μ‹
      • GUI
      • UML
      • Serializable
    • Advanced
      • OutOfMemoryError
      • AutoValue
      • meta-annotation
        • @Retention
        • @Target
        • @Repeatable
    • Effective Java 3/E
      • ITEM 1: Static Factory Method(정적 λ©”μ†Œλ“œ)
      • ITEM 2: Builder Pattern
      • ITEM 3: Singleton
      • ITEM 4: Private Constructor
      • ITEM 5: Dependency Injection
      • ITEM 6: Avoid Unnecessary Object
      • ITEM 7: Eliminate Object Reference
      • ITEM 8: Avoid finalizer and cleaner
      • ITEM 9: try-with-resources
      • ITEM 10: The gerneral contract when overriding equlas
      • ITEM 11: Overriding hashCode
      • ITEM 12: overriding toString
      • ITEM 13: overriding clone judiciously
      • ITEM 14: Consider implementing comparable
      • ITEM 15: ν΄λž˜μŠ€μ™€ λ©€λ²„μ˜ 접근을 μ΅œμ†Œν™”ν•΄λΌ
      • ITEM 16: Use Accessor methods
      • ITEM 17: λ³€κ²½ κ°€λŠ₯성을 μ΅œμ†Œν™”ν•΄λΌ(λΆˆλ³€ 클래슀)
      • ITEM 18: 상속보단 μ»΄ν¬μ§€μ…˜μ„ μ‚¬μš©ν•΄λΌ
      • ITEM 19: 상속을 κ³ λ €ν•΄ μ„€κ³„ν•˜κ³  λ¬Έμ„œν™”ν•΄λΌ
      • ITEM 20: 좔상 ν΄λž˜μŠ€λ³΄λ‹€ μΈν„°νŽ˜μ΄μŠ€λ₯Ό μš°μ„ ν•˜λΌ
      • ITEM 21: μΈν„°νŽ˜μ΄μŠ€λŠ” κ΅¬ν˜„ν•˜λŠ” μͺ½μ„ 생각해 섀계해라.
      • ITEM 22: μΈν„°νŽ˜μ΄μŠ€λŠ” νƒ€μž…μ„ μ •μ˜ν•˜λŠ” μš©λ„λ‘œλ§Œ μ‚¬μš©ν•΄λΌ
      • ITEM 23: νƒœκ·Έ 달린 ν΄λž˜μŠ€λ³΄λ‹€ 클래슀 계측ꡬ쑰λ₯Ό ν™œμš©ν•΄λΌ
      • ITEM 24: 멀버 ν΄λž˜μŠ€λŠ” λ˜λ„λ‘ static으둜 κ΅¬ν˜„ν•΄λΌ
      • ITEM 25: ν†±λ ˆλ²¨ ν΄λž˜μŠ€λŠ” ν•œ νŒŒμΌμ— ν•˜λ‚˜λ§Œ 생성해라.
      • ITEM 26: Raw type은 μ‚¬μš©ν•˜μ§€ 마라
      • ITEM 27: 비검사 κ²½κ³ λ₯Ό μ œκ±°ν•΄λΌ
      • ITEM 28: λ°°μ—΄λ³΄λ‹€λŠ” 리슀트λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 29: 이왕이면 μ œλ„€λ¦­ νƒ€μž…μœΌλ‘œ λ§Œλ“€μ–΄λΌ
      • ITEM 30: 이왕이면 μ œλ„€λ¦­ λ©”μ„œλ“œλ‘œ λ§Œλ“€μ–΄λΌ
      • ITEM 31 : ν•œμ •μ  μ™€μΌλ“œμΉ΄λ“œλ₯Ό μ‚¬μš©ν•΄ API μœ μ—°μ„±μ„ 높여라
      • ITEM 32: μ œλ„€λ¦­κ³Ό κ°€λ³€μΈμˆ˜λ₯Ό ν•¨κ»˜ μ“Έ λ•ŒλŠ” 신쀑해라
      • ITEM 33: νƒ€μž… μ•ˆμ „ 이쒅 μ»¨ν…Œμ΄λ„ˆλ₯Ό 고렀해라
      • ITEM 34: int μƒμˆ˜ λŒ€μ‹  μ—΄κ±° νƒ€μž…μ„ μ‚¬μš©ν•΄λΌ
      • ITEM 35: ordinal λ©”μ„œλ“œ λŒ€μ‹  μΈμŠ€ν„΄μŠ€ ν•„λ“œλ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 36: λΉ„νŠΈ ν•„λ“œ λŒ€μ‹  EnumSet을 μ‚¬μš©ν•΄λΌ
      • ITEM 37: ordinal 인덱싱 λŒ€μ‹  EnumMap을 μ‚¬μš©ν•΄λΌ
      • TEM 38 : ν™•μž₯ν•  수 μžˆλŠ” μ—΄κ±°νƒ€μž…μ΄ ν•„μš”ν•˜λ©΄ μΈν„°νŽ˜μ΄μŠ€λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 39: λͺ…λͺ… νŒ¨ν„΄λ³΄λ‹€ μ• λ„ˆν…Œμ΄μ…˜μ„ μ‚¬μš©ν•΄λΌ
      • ITEM 40: @Override μ–΄λ…Έν…Œμ΄μ…˜μ„ μΌκ΄€λ˜κ²Œ μ‚¬μš©ν•΄λΌ
      • ITEM 41: μ •μ˜ν•˜λ €λŠ” 것이 νƒ€μž…μ΄λΌλ©΄ 마컀 μΈν„°νŽ˜μ΄μŠ€λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 42: 읡λͺ… ν΄λž˜μŠ€λ³΄λ‹€λŠ” λžŒλ‹€λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 43: λžŒλ‹€λ³΄λ‹€λŠ” λ©”μ„œλ“œ μ°Έμ‘°λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 44: ν‘œμ€€ ν•¨μˆ˜ν˜• μΈν„°νŽ˜μ΄μŠ€λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 45: μŠ€νŠΈλ¦Όμ€ μ£Όμ˜ν•΄μ„œ μ‚¬μš©ν•΄λΌ
      • ITEM 46: μŠ€νŠΈλ¦Όμ—μ„œ λΆ€μž‘μš© μ—†λŠ” ν•¨μˆ˜λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 47: λ°˜ν™˜ νƒ€μž…μœΌλ‘œλŠ” μŠ€νŠΈλ¦Όλ³΄λ‹€ μ»¬λ ‰μ…˜μ΄ λ‚«λ‹€.
      • ITEM 48: 슀트림 λ³‘λ ¬ν™”λŠ” μ£Όμ˜ν•΄μ„œ μ‚¬μš©ν•΄λΌ
      • ITEM 49: λ§€κ°œλ³€μˆ˜κ°€ μœ νš¨ν•œμ§€ 검사해라
      • ITEM 50: μ μ‹œμ— 방어적 볡사본을 λ§Œλ“€μ–΄λΌ
      • ITEM 51: λ©”μ„œλ“œ μ‹œκ·Έλ‹ˆμ²˜λ₯Ό μ‹ μ€‘νžˆ 섀계해라
      • ITEM 52: λ‹€μ€‘μ •μ˜λŠ” μ‹ μ€‘νžˆ μ‚¬μš©ν•΄λΌ
      • ITEM 53: κ°€λ³€μΈμˆ˜λŠ” μ‹ μ€‘νžˆ μ‚¬μš©ν•΄λΌ
      • ITEM 54: null이 μ•„λ‹Œ, 빈 μ»¬λ ‰μ…˜μ΄λ‚˜ 배열을 λ°˜ν™˜ν•΄λΌ
      • ITEM 55: Optional λ°˜ν™˜μ€ μ‹ μ€‘ν•˜κ²Œ 해라
      • ITEM 56: 곡개된 API μš”μ†Œμ—λŠ” 항상 주석을 μž‘μ„±ν•΄λΌ
      • ITEM 57: μ§€μ—­λ³€μˆ˜μ˜ λ²”μœ„λ₯Ό μ΅œμ†Œν™”ν•΄λΌ
      • ITEM 58: 전톡적인 for λ¬Έλ³΄λ‹€λŠ” for-each문을 μ‚¬μš©ν•΄λΌ
      • ITEM 59: 라이브러리λ₯Ό 읡히고 μ‚¬μš©ν•΄λΌ
      • ITEM 60: μ •ν™•ν•œ 닡이 ν•„μš”ν•˜λ‹€λ©΄ float와 double은 피해라
      • ITEM 61: λ°•μ‹±λœ κΈ°λ³Έ νƒ€μž…λ³΄λ‹€λŠ” κΈ°λ³Έ νƒ€μž…μ„ μ‚¬μš©ν•΄λΌ
      • ITEM 62: λ‹€λ₯Έ νƒ€μž…μ΄ μ μ ˆν•˜λ‹€λ©΄ λ¬Έμžμ—΄ μ‚¬μš©μ„ 피해라
      • ITEM 63: λ¬Έμžμ—΄ 연결은 λŠλ¦¬λ‹ˆ μ£Όμ˜ν•΄λΌ
      • ITEM 64: κ°μ²΄λŠ” μΈν„°νŽ˜μ΄μŠ€λ₯Ό μ‚¬μš©ν•΄ 참쑰해라
      • ITEM 65: λ¦¬ν”Œλ ‰μ…˜λ³΄λ‹€λŠ” μΈν„°νŽ˜μ΄μŠ€λ₯Ό μ‚¬μš©ν•΄λΌ
      • ITEM 66: λ„€μ΄ν‹°λΈŒ λ©”μ„œλ“œλŠ” μ‹ μ€‘νžˆ μ‚¬μš©ν•΄λΌ
      • ITEM 67: μ΅œμ ν™”λŠ” μ‹ μ€‘νžˆ 해라
      • ITEM 68: 일반적으둜 ν†΅μš©λ˜λŠ” λͺ…λͺ… κ·œμΉ™μ„ 따라라
    • 객체지ν–₯ 섀계 원칙(SOLID)
    • λ””μžμΈνŒ¨ν„΄
      • Strategy Pattern
      • Template Method Pattern
      • Factory Method Pattern
      • Singleton
      • Delegation
      • Proxy
      • Adapter Pattern
    • μ‹€μŠ΅
      • μΈν„°νŽ˜μ΄μŠ€ μ‹€μŠ΅ - Vehicle
      • μΈν„°νŽ˜μ΄μŠ€ μ‹€μŠ΅ - Remote
      • GUI μ‹€μŠ΅ - Calculator
      • GUI μ‹€μŠ΅ - button
      • GUI μ‹€μŠ΅ - lotto
      • Thread μ‹€μŠ΅ - μ’Œμ„μ˜ˆμ•½, 메세지보내기
    • Jar vs War
  • λ°μ΄ν„°λ² μ΄μŠ€
    • KEY
    • Index
    • Transaction
    • Trigger
    • Procedure / Function
    • Package
    • λ°μ΄ν„°λ² μ΄μŠ€ 배움터
      • λ°μ΄ν„°λ² μ΄μŠ€ μ‹œμŠ€ν…œ
      • 관계데이터 λͺ¨λΈ
      • κ΄€κ³„λŒ€μˆ˜μ™€ SQL
    • MySQL
      • Databaseλž€
      • MySQL μ‹œμž‘ν•˜κΈ°
      • MySQL Database
      • MySQL Table
      • CRUD
      • κ΄€κ³„ν˜• λ°μ΄ν„°λ² μ΄μŠ€
      • Server와 Client
    • PostgreSQL
    • NoSQL
      • Install Cassandra on mac
      • Cassandraλž€?
      • NiFiλž€
  • Algorithm
    • String
    • Recursion
    • Dynamic Programming
    • Array, Struct, Pointer
    • Math
    • Sort
    • List
    • Stack
    • Queue
    • Graph
    • Tree
    • Maze
    • AVL
    • μ΄μ§„νƒμƒ‰νŠΈλ¦¬(Binary Search Tree)
    • DFS와 BFS
    • λ‹€μ΅μŠ€νŠΈλΌ μ•Œκ³ λ¦¬μ¦˜(Dijkstra's Algorithm)
    • Red-Black 트리
    • A* μ•Œκ³ λ¦¬μ¦˜
    • Heap
    • Huffman Coding
    • Priority Queue
    • Bellman-Ford μ•Œκ³ λ¦¬μ¦˜
    • C++
      • Class
      • STL
        • STL pair
        • STL Container - Associate Container
        • STL Container - Sequence Container
        • STL Container - Container Adapter
  • JavaScript
    • JABASCRIPT BASIC
    • Shallow Copy vs Deep Copy
    • OBJECT MODEL
    • NODE
    • 동기 처리 vs 비동기 처리
    • AJAX
    • CALLBACK
    • PROMISE
    • DEFERRER
    • UNDERSCORE
    • WEBPACK
    • SCOPE
    • EXECUTION CONTEXT
    • Image Object
    • BFCacheλž€?
    • history.scrollRestoration
    • Intersection Observer
    • JWT - JSON Web Token
    • HTML vs JSON
  • Vue.js
    • ν™˜κ²½μ„€μ •
    • Vue.jsλž€?
    • Vue Instance
    • Vue Component
    • Vue Router
    • HTTP 톡신
    • Template
    • Single File Component
    • Vue Animation
    • Vuex
    • Djnago와 μ—°λ™ν•˜κΈ°
  • Backbone.js
    • Model
    • Collection
    • Sync
    • view
  • Node.js
    • Doit! - λ…Έλ“œλ‘œ λ§Œλ“€ 수 μžˆλŠ” λŒ€ν‘œμ μΈ μ„œλ²„μ™€ μš©λ„
    • Doit! - λ…Έλ“œμ— λŒ€ν•΄ μ•Œμ•„λ³΄κ³  개발 도ꡬ μ„€μΉ˜ν•˜κΈ°
    • Doit! - λ…Έλ“œ κ°„λ‹¨ν•˜κ²Œ μ‚΄νŽ΄λ³΄κΈ°
    • Doit! - λ…Έλ“œμ˜ μžλ°”μŠ€ν¬λ¦½νŠΈμ™€ μΉœν•΄μ§€κΈ°
    • Doit! - λ…Έλ“œμ˜ κΈ°λ³Έ κΈ°λŠ₯ μ•Œμ•„λ³΄κΈ°
    • Doit! - μ›Ή μ„œλ²„ λ§Œλ“€κΈ°
    • Doit! - λ°μ΄ν„°λ² μ΄μŠ€ μ‚¬μš©ν•˜κΈ°
    • Doit! - μ΅μŠ€ν”„λ ˆμŠ€ ν”„λ‘œμ νŠΈλ₯Ό λͺ¨λ“ˆν™”ν•˜κΈ°
    • Doit! - λ·° ν…œν”Œλ¦Ώ μ μš©ν•˜κΈ°
    • Doit! - 패슀포트둜 μ‚¬μš©μž μΈμ¦ν•˜κΈ°
    • Doit! - μ±„νŒ…μ„œλ²„ λ§Œλ“€κΈ°
    • Doit! - JSON-RPC μ„œλ²„ λ§Œλ“€κΈ°
  • Python
    • Warning-Could not import the lzma module
    • Pandas
      • Pandas 자료ꡬ쑰
      • Pandas 데이터 μž…μΆœλ ₯
      • DataFrame Data μ‚΄νŽ΄λ³΄κΈ°
      • μ‹œκ°ν™” 도ꡬ - Matplotlib
  • ML
    • μΆ”μ²œ μ‹œμŠ€ν…œ
      • Collaborative Filtering
      • Matrix Factorization
  • Django
    • Basic
      • ν™˜κ²½μ„€μ •
      • About Django
      • Start Django Project
      • Secret Key κ΄€λ¦¬ν•˜κΈ°
      • Settings λΆ„λ¦¬ν•˜κΈ°
      • Django App
      • Django View & URL (1)
      • Django Model
        • MySQL 연동
      • Django Admin
      • Django View & URL (2)
      • Django Template
      • Django Template & View & URL
      • Django Static
      • Django form
    • Advanced
      • Django Generic View
      • Django Automated Testing
      • Django Extenstion Template
      • Django Model Package
      • Django OpenSSL setting
    • REST framework
      • Rest API
      • Serializers
      • ViewSet
    • Error
      • ν™˜κ²½μ„€μ • zlib 였λ₯˜λ°œμƒ
      • ModuleNotFoundError
    • νŒ¨ν‚€μ§€
      • django-debug-toolbar
    • Vue.js μ—°λ™ν•˜κΈ°
  • Ruby
    • variable & input/output
    • 쑰건문
    • 반볡문
    • Array & Hash
    • Method
    • Proc&Lamda
    • Class
  • Ruby on Rails
    • Scaffolding
    • Controller
    • Model
    • Model-M:N relation
    • Model Validation
    • 멋사 10μ£Όμ°¨ μˆ˜μ—…(Tip)
  • HTML/CSS
    • Udacity - Intro to HTML/CSS
    • Udacity - Responsive Web Design
    • Udacity - Responsive Images
    • HTML Basic
    • CSS Basic
    • HTML5 Sementic Tag
    • HTML ν…μŠ€νŠΈ κ΄€λ ¨ νƒœκ·Έλ“€
    • HTML5 λ©€ν‹°λ―Έλ””μ–΄
    • HTML 폼 κ΄€λ ¨ νƒœκ·Έλ“€
    • ν…μŠ€νŠΈ κ΄€λ ¨ μŠ€νƒ€μΌ
    • 색상과 배경을 μœ„ν•œ μŠ€νƒ€μΌ
    • λ ˆμ΄μ•„μ›ƒμ„ μœ„ν•œ μŠ€νƒ€μΌ
    • CSS 포지셔닝
    • λ‹€μž¬λ‹€λŠ₯ν•œ CSS3 μ„ νƒμž
    • CSS와 μ• λ‹ˆλ©”μ΄μ…˜
    • λ°˜μ‘ν˜• μ›Ήμ΄λž€?
  • OS(운영체제)
    • Linux
      • Daemon
      • Cron
      • ν”„λ‘œμ„ΈμŠ€ κ΄€λ ¨ λͺ…λ Ήμ–΄
      • ν…μŠ€νŠΈ 파일 λͺ…λ Ήμ–΄
  • Network
    • λ„€νŠΈμ›Œν¬ κΈ°λ³Έ κ°œλ…
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  • equals λ©”μ„œλ“œ κ·œμ•½
  • λ°˜μ‚¬μ„±
  • λŒ€μΉ­μ„±
  • 좔이성
  • 일관성
  • NULL-μ•„λ‹˜
  • equals κ΅¬ν˜„ 주의 사항
  • @AutoValue

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  1. Java
  2. Effective Java 3/E

ITEM 10: The gerneral contract when overriding equlas

PreviousITEM 9: try-with-resourcesNextITEM 11: Overriding hashCode

Last updated 4 years ago

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equals λ©”μ„œλ“œμ˜ μž¬μ •μ˜μ—λŠ” μ—¬λŸ¬κ°€μ§€ 함정이 μžˆλ‹€. κ·ΈλŸ¬λ―€λ‘œ λ‹€μŒκ³Ό 같은 μƒν™©μ—μ„œλŠ” μž¬μ •μ˜ ν•˜μ§€ μ•ŠλŠ” 것이 μ’‹λ‹€.

  • 각 μΈμŠ€ν„΄μŠ€κ°€ 본질적으둜 κ³ μœ ν•œ 경우( λ™μž‘ν•˜λŠ” 개체λ₯Ό ν‘œν˜„ν•˜λŠ” 클래슀 ex-Thread )

  • μΈμŠ€ν„΄μŠ€μ˜ 논리적 λ™μΉ˜μ„±(logical equality)을 검사할 일이 μ—†λŠ” 경우

  • μƒμœ„ ν΄λž˜μŠ€μ—μ„œ μž¬μ •μ˜ν•œ equalsκ°€ ν•˜μœ„ ν΄λž˜μŠ€μ—λ„ μ ν•©ν•œ 경우(AbstractSet, AbstractList)

  • ν΄λž˜μŠ€κ°€ private or package-private이고 equals λ©”μ„œλ“œλ₯Ό ν˜ΈμΆœν•  일이 μ—†λŠ” 경우

κ·Έλ ‡λ‹€λ©΄ equals λ₯Ό μž¬μ •μ˜ν•΄μ•Όν•  λ•ŒλŠ” μ–Έμ œμΌκΉŒ?

객체 식별성(두 객체가 물리적으둜 같은지)이 μ•„λ‹ˆλΌ 논리적 λ™μΉ˜μ„±(logical equality)을 ν™•μΈν•΄μ•Όν•˜μ§€λ§Œ, μƒμœ„ 클래슀의 equalsκ°€ μž¬μ •μ˜λ˜μ§€ μ•Šμ€ κ²½μš°μ΄λ‹€. 주둜, κ°’ 클래슀(Integer, String)κ°€ ν•΄λ‹Ήλœλ‹€.

κ°’ ν΄λž˜μŠ€μ΄λ”λΌλ„ 라면 equalsλ₯Ό μž¬μ •μ˜ν•˜μ§€ μ•Šμ•„λ„ λœλ‹€. (Enum 포함)

equals λ©”μ„œλ“œ κ·œμ•½

equals λ©”μ„œλ“œλŠ” λ™μΉ˜κ΄€κ³„(equivalence relation)λ₯Ό κ΅¬ν˜„ν•˜λ©°, λ‹€μŒμ„ λ§Œμ‘±ν•œλ‹€.

κ·œμ•½

μ„€λͺ…

λ°˜μ‚¬μ„±(reflexivity)

null이 μ•„λ‹Œ λͺ¨λ“  μ°Έμ‘° κ°’ x에 λŒ€ν•΄, x.equals(x)λŠ” true이닀.

λŒ€μΉ˜μ„±(symmetry)

null이 μ•„λ‹Œ λͺ¨λ“  μ°Έμ‘° κ°’ x,y에 λŒ€ν•΄, x.equals(y)κ°€ true이면, y.equals(x)도 true이닀.

좔이성(transitivity)

null이 μ•„λ‹Œ λͺ¨λ“  μ°Έμ‘° κ°’ x, y, z에 λŒ€ν•΄ x.equals(y)κ°€ true이고 y.equals(z)κ°€ true이면, x.eqauls(z)도 true이닀.

일관성(consistency)

null이 μ•„λ‹Œ λͺ¨λ“  μ°Έμ‘° κ°’ x,y에 λŒ€ν•΄ x.equals(y)λ₯Ό λ°˜λ³΅ν•΄μ„œ ν˜ΈμΆœν•˜λ©΄ 항상 true or flaseλ₯Ό λ°˜ν™˜ν•œλ‹€.

null μ•„λ‹˜

null이 μ•„λ‹Œ λͺ¨λ“  μ°Έμ‘° κ°’ x에 λŒ€ν•΄, x.equalsλŠ” flase이닀.

equals κ·œμ•½μ„ μ–΄κΈ°λ©΄ κ·Έ 객체λ₯Ό μ‚¬μš©ν•˜λŠ” λ‹€λ₯Έ 객체듀이 μ–΄λ–»κ²Œ λ°˜μ‘ν• μ§€ μ•Œ 수 μ—†λ‹€.

λ°˜μ‚¬μ„±

λ‹¨μˆœνžˆ λ§ν•˜λ©΄ κ°μ²΄λŠ” 자기 μžμ‹ κ³Ό κ°™μ•„μ•Ό ν•œλ‹€λŠ” λœ»μ΄λ‹€.

λŒ€μΉ­μ„±

두 κ°μ²΄λŠ” μ„œλ‘œμ— λŒ€ν•œ λ™μΉ˜μ—¬λΆ€μ— λŒ€ν•΄ λ˜‘κ°™μ€ κ²°κ³Όκ°€ λ‚˜μ™€μ•Όν•œλ‹€.

public final class CaseInsensitiveString {

    private final String s;

    public CaseInsensitiveString(String s){
        this.s = Objects.requireNonNull(s);
    }

    // λŒ€μΉ­μ„± μœ„λ°˜
    @Override
    public boolean equals(Object o){
        if(o instanceof CaseInsensitiveString)
            return s.equalsIgnoreCase(((CaseInsensitiveString) o).s);
        if(o instanceof String)
            return s.equalsIgnoreCase((String) o);
        return false;
    }
}
CaseInsensitiveString cis = new CaseInsensitiveString("Test");
String s = "test";

System.out.println(s.equals(cis)); // false
System.out.println(cis.equals(s)); // true

μ—¬κΈ°μ„œ λ¬Έμ œλŠ” String의 equalsλŠ” CaseInsensitiveString의 쑴재λ₯Ό λͺ¨λ₯΄κΈ°λ•Œλ¬Έμ— falseλ₯Ό λ°˜ν™˜ν•˜λ©°, μ΄λŠ” λŒ€μΉ­μ„±μ„ μœ„λ°˜ν•œλ‹€.

equals κ·œμ•½μ„ μ–΄κΈ°λ©΄, κ·Έ 객체λ₯Ό μ‚¬μš©ν•˜λŠ” λ‹€λ₯Έ 객체듀이 μ–΄λ–»κ²Œ λ°˜μ‘ν• μ§€ μ•Œ 수 μ—†λ‹€. μ΄λŸ¬ν•œ 문제λ₯Ό ν•΄κ²°ν•˜λ €λ©΄ μ•„λž˜μ™€ 같이 String과의 연동을 ν•˜κ² λ‹€λŠ” λͺ©ν‘œλ₯Ό λ²„λ €μ•Όν•œλ‹€.

@Override
public boolean equals(Object o){
    return o instanceof CaseInsensitiveString && ((CaseInsensitiveString) o).s.equalsIgnoreCase(s);
}
CaseInsensitiveString cis = new CaseInsensitiveString("Test");
String s = "test";

System.out.println(s.equals(cis)); // false
System.out.println(cis.equals(s)); // false

좔이성

첫 번째 객체와 두 번째 객체가 κ°™κ³ , 두 번째 객체와 μ„Έ 번째 객체가 κ°™λ‹€λ©΄, 첫번째 객체와 μ„Έλ²ˆμ§Έ 객체도 κ°™μ•„μ•Όν•œλ‹€λŠ” μ˜λ―Έμ΄λ‹€.

public class Point {

    private final int x;
    private final int y;

    public Point(int x, int y){
        this.x = x;
        this.y = y;
    }

    @Override
    public boolean equals(Object o){
        if(!(o instanceof Point))
            return false;
        Point p = (Point)o;
        return p.x == x && p.y == y;
    }
}
public class ColorPoint extends Point{
    private final String color;

    public ColorPoint(int x, int y, String color){
        super(x,y);
        this.color = color;
    }
}

ColorPoint ν΄λž˜μŠ€μ—μ„œ equals λ©”μ„œλ“œλ₯Ό κ΅¬ν˜„ν•˜μ§€ μ•ŠλŠ”λ‹€λ©΄, Point equalsκ°€ μƒμ†λ˜μ–΄ 색상 μ •λ³΄λŠ” λ¬΄μ‹œλœ 채 비ꡐλ₯Ό μˆ˜ν–‰ν•˜κ²Œλœλ‹€. μ΄λ•Œ, κ·œμ•½μ€ μ–΄κΈ΄ 것은 μ•„λ‹ˆμ§€λ§Œ, μ€‘μš”ν•œ 정보λ₯Ό λ†“μΉ˜κ²Œ λ˜λ―€λ‘œ 받아듀일 수 μ—†λ‹€.

@Override
public boolean equals(Object o){
    if(!(o instanceof ColorPoint))
        return false;
    return super.equals(o) && ((ColorPoint) o).color == color;
}

λ‹€μŒκ³Ό 같이 equalsλ₯Ό κ΅¬ν˜„ν•˜λ©΄ Point와 ColorPointλ₯Ό λΉ„κ΅ν•œ 결과와 κ·Έ λ‘˜μ„ λ°”κΏ”μ„œ λΉ„κ΅ν•œ κ²°κ³Όκ°€ λ‹€λ₯Ό 수 μžˆλ‹€. ( λŒ€μΉ­μ„± μœ„λ°° )

Point p = new Point(1,2);
ColorPoint cp = new ColorPoint(1,2, Color.RED);

System.out.println(p.equals(cp)); // true
System.out.println(cp.equals(p)); // false

μš°μ„  p.equals(cp) λŠ” 색상 정보에 λŒ€ν•œ λΉ„κ΅λŠ” λ¬΄μ‹œν•˜κ³ , cp.equals(p)λŠ” λ§€κ°œλ³€μˆ˜μ˜ 클래슀 μ’…λ₯˜κ°€ λ‹€λ₯΄λ‹€λ©° 맀번 false만 λ°˜ν™˜ν•  것이닀.

    @Override
    public boolean equals(Object o){
        if(!(o instanceof Point))
            return false;
        if(!(o instanceof ColorPoint))
            return o.equals(this);
        return super.equals(o) && ((ColorPoint) o).color == color;
    }
Point p = new Point(1,2);
ColorPoint cp = new ColorPoint(1,2, Color.RED);
ColorPoint cp2 = new ColorPoint(1,2,Color.BLACK);

System.out.println(cp.equals(p)); // true
System.out.println(p.equals(cp2)); // true 
System.out.println(cp2.equals(cp)); // false

λ‹€μŒκ³Ό 같이 λ³€κ²½ν•˜λ©΄, λŒ€μΉ­μ„±μ€ λ§žμ§€λ§Œ 좔이성은 μ—¬μ „νžˆ μœ„λ°°λ˜λ©°, 이 방식은 λ¬΄ν•œ μž¬κ·€μ— 빠질 μœ„ν—˜λ„ μžˆλ‹€. ꡬ체 클래슀λ₯Ό ν™•μž₯ν•΄ μƒˆλ‘œμš΄ 값을 μΆ”κ°€ν•˜λ©΄μ„œ equals κ·œμ•½μ„ λ§Œμ‘±μ‹œν‚¬ 방법은 μ‘΄μž¬ν•˜μ§€ μ•ŠλŠ”λ‹€.

equals μ•ˆμ˜ instance 검사λ₯Ό getClass κ²€μ‚¬λ‘œ λ°”κΎΈλ©΄ κ·œμ•½λ„ μ§€ν‚€λ©΄μ„œ 상속이 κ°€λŠ₯ν•˜λ‹€κ³  생각할 수 μžˆμ§€λ§Œ, μ΄λŠ” λ¦¬μŠ€μ½”ν”„ μΉ˜ν™˜ 원칙에 μœ„λ°˜λœλ‹€. λ¦¬μŠ€μ½”ν”„ μΉ˜ν™˜ 원칙에 λ”°λ₯΄λ©΄, μ–΄λ–€ νƒ€μž…μ— μžˆμ–΄ μ€‘μš”ν•œ 속성이라면, κ·Έ ν•˜μœ„ νƒ€μž…μ—μ„œλ„ λ§ˆμ°¬κ°€μ§€λ‘œ μ€‘μš”ν•˜λ‹€. λ”°λΌμ„œ κ·Έ νƒ€μž…μ˜ λͺ¨λ“  λ©”μ„œλ“œκ°€ ν•˜μœ„ νƒ€μž…μ—μ„œλ„ λ˜‘κ°™μ΄ 잘 λ™μž‘ν•΄μ•Όν•œλ‹€.

public class ColorPoint {
    private final Color color;
    private final Point point;

    public ColorPoint(int x, int y, Color color){
        this.point = new Point(x,y);
        this.color = Objects.requireNonNull(color);
    }

    public Point asPoint(){
        return point;
    }

    @Override
    public boolean equals(Object o){
        if(!(o instanceof ColorPoint))
            return false;
        ColorPoint cp = (ColorPoint) o;
        return cp.point.equals(point) && cp.color.equals(cp);
    }

}
Point p = new Point(1,2);
ColorPoint cp = new ColorPoint(1,2, Color.RED);
ColorPoint cp2 = new ColorPoint(1,2,Color.BLACK);

System.out.println(cp.equals(p)); // false
System.out.println(p.equals(cp2)); // false 
System.out.println(cp2.equals(cp)); // false

Point λ₯Ό μƒμ†ν•˜λŠ” λŒ€μ‹  Pointλ₯Ό ColorPoint의 private ν•„λ“œλ‘œ 두고, ColorPoint와 같은 μœ„μΉ˜μ˜ 일반 Pointλ₯Ό λ°˜ν™˜ν•˜λŠ” λ·°λ₯Ό public으둜 μΆ”κ°€ν•˜λŠ” 방식이닀. μžλ°” λΌμ΄λΈŒλŸ¬λ¦¬μ—μ„œλ„ ꡬ체 클래슀λ₯Ό ν™•μž₯ν•΄ 값을 μΆ”κ°€ν•œ ν΄λž˜μŠ€κ°€ μžˆλ‹€.

public class Timestamp extends java.util.Date {

    /**
     * @serial
     */
    private int nanos;

  ...
}

TimestampλŠ” Dateλ₯Ό 상속받은 ν›„ nanos ν•„λ“œλ₯Ό μΆ”κ°€ν–ˆλ‹€. κ·Έ κ²°κ³Ό Timestamp의 equalsλŠ” λŒ€μΉ­μ„±μ„ μœ„λ°°ν•΄, Date 객체와 ν•œ μ»¬λ ‰μ…˜μ— λ„£κ±°λ‚˜ μ„œλ‘œ μ„žμ–΄ μ‚¬μš©ν•˜λ©΄, μ—‰λš±ν•˜κ²Œ λ™μž‘ν•  수 μžˆλ‹€. Timestamp의 API μ„€λͺ…에 μ£Όμ˜μ‚¬ν•­μ„ μ–ΈκΈ‰ν•˜κ³  μžˆλ‹€.

일관성

두 객체가 κ°™λ‹€λ©΄ μ–΄λŠ ν•˜λ‚˜ ν˜Ήμ€ 두 객체가 μˆ˜μ •λ˜μ§€ μ•ŠλŠ” ν•œ μ•žμœΌλ‘œλ„ μ˜μ›νžˆ κ°™μ•„μ•Ό ν•œλ‹€λŠ” μ˜λ―Έμ΄λ‹€. κ°€λ³€ κ°μ²΄λŠ” 비ꡐ μ‹œμ μ— 따라 μ„œλ‘œ λ‹€λ₯Ό μˆ˜λ„ 같을 μˆ˜λ„ μžˆλŠ” 반면, λΆˆλ³€ κ°μ²΄λŠ” ν•œλ²ˆ λ‹€λ₯΄λ©΄ λκΉŒμ§€ λ‹¬λΌμ•Όν•œλ‹€. 즉, λΆˆλ³€ 클래슀λ₯Ό λ§Œλ“€κΈ°λ‘œ ν–ˆλ‹€λ©΄ eqaulsλŠ” ν•œλ²ˆ κ°™λ‹€κ³  ν•œ 객체와 μ˜μ›νžˆ κ°™κ³ , ν•œλ²ˆ λ‹€λ₯Έ κ°μ²΄μ™€λŠ” μ˜μ›νžˆ λ‹€λ₯΄λ„둝 λ§Œλ“€μ–΄μ•Όν•œλ‹€.

  • ν΄λž˜μŠ€κ°€ λΆˆλ³€μ΄λ“  가변이든 equals의 νŒλ‹¨μ— μ‹ λ’°ν•  수 μ—†λŠ” μžμ›μ΄ λΌμ–΄λ“€κ²Œ ν•΄μ„œλŠ” μ•ˆλœλ‹€. (java.net.URL 의 equlasλŠ” 일관성을 μ–΄κΉ€)

NULL-μ•„λ‹˜

λͺ¨λ“  객체가 nullκ³Ό κ°™μ§€ μ•Šμ•„μ•Όν•œλ‹€. μ‹€μˆ˜λ‘œ NullPointException을 λ˜μ§€λŠ” μ½”λ“œλŠ” 흔할 κ²ƒμ΄μ§€λ§Œ, 이 κ·œμ•½μ€ μ΄λŸ¬ν•œ κ²½μš°λ„ ν—ˆμš©ν•˜μ§€ μ•ŠλŠ”λ‹€. λ™μΉ˜μ„± 검사λ₯Ό ν•˜λ €λ©΄ equalsλŠ” 객체λ₯Ό 적절히 ν˜•λ³€ν™˜ν•œ ν›„ ν•„μˆ˜ ν•„λ“œλ“€μ˜ 값을 μ•Œμ•„μ•Όν•˜λŠ”λ°, μ΄λ•Œ ν˜•λ³€ν™˜μ— μ•žμ„œ instanceof μ—°μ‚°μžλ‘œ μž…λ ₯ λ§€κ°œλ³€μˆ˜κ°€ μ˜¬λ°”λ₯Έ νƒ€μž…μΈμ§€ κ²€μ‚¬ν•΄μ•Όν•œλ‹€. instanceof의 첫번째 ν”Όμ—°μ‚°μžκ°€ null이면 falseλ₯Ό λ°˜ν™˜ν•˜λ―€λ‘œ λ¬΅μ‹œμ μΈ null검사λ₯Ό ν•  수 μžˆλ‹€.

    @Override
    public boolean equals(Object o){
        if(!(o instanseof MyType))
            return false;
        MyType mt = (Mytype) o;

equals κ΅¬ν˜„ 주의 사항

  1. == μ—°μ‚°μžλ₯Ό μ‚¬μš©ν•΄ μž…λ ₯이 자기 μžμ‹ μ˜ 참쑰인지 확인

  2. instanceof μ—°μ‚°μžλ‘œ μž…λ ₯이 μ˜¬λ°”λ₯Έ νƒ€μž…μΈμ§€ 확인

  3. μž…λ ₯을 μ˜¬λ°”λ₯Έ νƒ€μž…μœΌλ‘œ ν˜•λ³€ν™˜ ν•œλ‹€.

  4. μž…λ ₯ 객체와 자기 μžμ‹ μ˜ λŒ€μ‘λ˜λŠ” '핡심' ν•„λ“œλ“€μ΄ λͺ¨λ‘ μΌμΉ˜ν•˜λŠ”μ§€ ν•˜λ‚˜μ”© κ²€μ‚¬ν•œλ‹€.

μ΄λ•Œ, float, double을 μ œμ™Έν•œ κΈ°λ³Έ νƒ€μž… ν•„λ“œλŠ” ==둜 λΉ„κ΅ν•˜κ³ , μ°Έμ‘° νƒ€μž… ν•„λ“œλŠ” 각각의 equals λ©”μ„œλ“œλ‘œ, float, double은 Float.compare(float, float)와 Double.compare(double,double)둜 λΉ„κ΅ν•œλ‹€. Float.equals와 Double.equalsλŠ” μ˜€ν† λ°•μ‹±μ„ μˆ˜λ°˜ν•  수 μžˆμ–΄ μ„±λŠ₯상 μ’‹μ§€ μ•Šλ‹€.

λ•Œλ‘œλŠ” null값도 정상 κ°’μœΌλ‘œ μ·¨κΈ‰ν•˜λŠ” μ°Έμ‘° νƒ€μž… ν•„λ“œλ„ μžˆλŠ”λ°, μ΄λ•ŒλŠ” Objects.equals(Object, Object)둜 비ꡐ해 NullPointerException을 μ˜ˆλ°©ν•΄μ•Όν•œλ‹€.

λ˜ν•œ, μ–΄λ–€ ν•„λ“œλ₯Ό λ¨Όμ € λΉ„κ΅ν•˜λŠλƒκ°€ equals의 μ„±λŠ₯을 μ’Œμš°ν•˜κΈ°λ„ ν•˜λŠ”λ°, μ΅œμƒμ˜ μ„±λŠ₯을 μœ„ν•΄μ„œλŠ” λ‹€λ₯Ό κ°€λŠ₯성이 ν¬κ±°λ‚˜ λΉ„κ΅ν•˜λŠ” λΉ„μš©μ΄ μ‹Ό ν•„λ“œλ₯Ό λ¨Όμ € λΉ„κ΅ν•˜λŠ” 것이 μ’‹λ‹€. 논리적 μƒνƒœμ™€ κ΄€λ ¨ μ—†λŠ” ν•„λ“œλŠ” λΉ„κ΅ν•΄μ„œλŠ” μ•ˆλ˜λ©°, 핡심 ν•„λ“œλ‘œ 계산할 수 μžˆλŠ” νŒŒμƒ ν•„λ“œλ„ ꡳ이 비ꡐ할 ν•„μš”λŠ” μ—†λ‹€. ν•˜μ§€λ§Œ, νŒŒμƒ ν•„λ“œκ°€ 객체 μ „μ²΄μ˜ μƒνƒœλ₯Ό λŒ€ν‘œν•˜λŠ”κ²½μš° νŒŒμƒν•„λ“œλ₯Ό λΉ„κ΅ν•˜λŠ” 것이 더 λΉ λ₯Ό 수 μžˆλ‹€.

  • Object μ™Έμ˜ νƒ€μž…μ„ λ§€κ²¨λ³€μˆ˜λ‘œ λ°›λŠ” equalsλŠ” μ„ μ–Έν•˜μ§€ 말자.

@AutoValue

<dependencies>
    <dependency>
        <groupId>com.google.auto.value</groupId>
        <artifactId>auto-value</artifactId>
        <version>1.3</version>
    </dependency>
</dependencies>
import com.google.auto.value.AutoValue;

@AutoValue
public abstract class Product {

    public abstract String name();
    public abstract String price();

    @AutoValue.Builder
    public abstract static class Builder {
        public abstract Builder name(String name);
        public abstract Builder price(String price);
        public abstract Product build();
    }

    public static Product.Builder builder() {
        return new AutoValue_Product.Builder();
    }
}

ꡬ체 클래슀의 ν•˜μœ„ ν΄λž˜μŠ€μ—μ„œ 값을 μΆ”κ°€ν•  방법은 μ—†μ§€λ§Œ μš°νšŒν•  수 μžˆλŠ” 방법이 ν•˜λ‚˜μžˆλ‹€. []

equalsλ₯Ό μž¬μ •μ˜ν•  λ•ŒλŠ” hashCode도 λ°˜λ“œμ‹œ μž¬μ •μ˜ ν•΄μ•Όν•œλ‹€.()

λ₯Ό μ‚¬μš©ν•˜λ©΄ equals와 hashCodeλ₯Ό μž‘μ„±ν•΄μ€€λ‹€. ()

정적 νŒ©ν„°λ¦¬ λ©”μ†Œλ“œ - item1
μƒμ†λŒ€μ‹  μ»΄ν¬μ§€μ…˜μ„ μ‚¬μš©ν•΄λΌ - Item18
item 11
AutoValue ν”„λ ˆμž„μ›Œν¬
AutoValue 예제 보기