Design Patterns MCP Server

{ "id": "companion-objects", "name": "Companion Objects Pattern", "category": "Creational", "description": "Static-like functionality within classes for factory methods, constants, and utilities", "when_to_use": "Factory methods, constants, utility functions, singleton behavior", "benefits": "Encapsulation, type safety, inheritance support, access to private members", "drawbacks": "Singleton behavior, testing complexity, potential memory leaks", "use_cases": "Factory patterns, constants, caching, serialization", "complexity": "Low", "tags": ["kotlin", "factory", "singleton", "static", "encapsulation"], "examples": { "kotlin": { "language": "kotlin", "code": "class DatabaseConnection private constructor(val url: String) {\n \n companion object {\n private val connections = mutableMapOf<String, DatabaseConnection>()\n \n fun getConnection(url: String): DatabaseConnection {\n return connections.getOrPut(url) {\n DatabaseConnection(url).apply { connect() }\n }\n }\n \n const val DEFAULT_TIMEOUT = 5000\n \n fun createInMemory(): DatabaseConnection {\n return DatabaseConnection(\"jdbc:sqlite::memory:\").apply { connect() }\n }\n }\n \n private fun connect() { /* connection logic */ }\n \n fun executeQuery(query: String): ResultSet { /* implementation */ }\n}\n\n// Usage\nval conn1 = DatabaseConnection.getConnection(\"jdbc:sqlite:db.db\")\nval conn2 = DatabaseConnection.getConnection(\"jdbc:sqlite:db.db\") // Same instance\nval memoryDb = DatabaseConnection.createInMemory()\n\n// Named companion object\nclass JsonParser {\n companion object Factory {\n fun fromJson(json: String): JsonParser {\n return JsonParser().apply { parse(json) }\n }\n \n fun fromFile(file: File): JsonParser {\n return fromJson(file.readText())\n }\n }\n \n private fun parse(json: String) { /* parsing logic */ }\n}\n\n// Usage\nval parser1 = JsonParser.fromJson(\"{\"key\": \"value\"}\")\nval parser2 = JsonParser.fromFile(File(\"config.json\"))\n\n// Companion object implementing interface\ninterface ConfigFactory<T> {\n fun create(): T\n fun createFromFile(path: String): T\n}\n\nclass AppConfig private constructor(val settings: Map<String, Any>) {\n \n companion object : ConfigFactory<AppConfig> {\n override fun create(): AppConfig {\n return AppConfig(mapOf(\"debug\" to false, \"timeout\" to 30))\n }\n \n override fun createFromFile(path: String): AppConfig {\n val file = File(path)\n val settings = /* parse file */\n return AppConfig(settings)\n }\n }\n}\n\n// Constants and utilities\nclass MathUtils {\n companion object {\n const val PI = 3.14159\n const val E = 2.71828\n \n fun factorial(n: Int): Long {\n return if (n <= 1) 1 else n * factorial(n - 1)\n }\n \n fun gcd(a: Int, b: Int): Int {\n return if (b == 0) a else gcd(b, a % b)\n }\n }\n}\n\n// Usage\nval area = MathUtils.PI * radius * radius\nval fact = MathUtils.factorial(5)\nval divisor = MathUtils.gcd(48, 18)" } } }