create examples Kotlin Data classes features

src/main/kotlin/codecollection/kotlinfeatures/DataClass.kt

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+// Data classes can't be abstract, open, sealed, or inner.
+// The primary constructor must have at least one parameter.
+
+package codecollection.kotlinfeatures
+
+data class Person(
+    val firstName: String,
+    val age: Int,
+    val email: String,
+)
+
+// Attention! Using 'var' is not recommended in the data classes. See the example.
+data class MutablePerson(
+    var firstName: String,
+    var age: Int,
+    var email: String,
+)
+
+data class Address(
+    var city: String,
+    var street: String,
+)
+
+data class Employee(
+    val position: String = "Intern", // default property
+    val person: Person,
+    val address: Address,
+)
+
+fun main() {
+    println("=== BASIC DATA CLASS FEATURES ===")
+
+    // Create instances
+    val person1 = Person(firstName = "Alice", age = 30, email = "[email protected]")
+    val person2 = Person(firstName = "Bob", age = 32, email = "[email protected]")
+    val person3 = Person(firstName = "Alice", age = 30, email = "[email protected]")
+
+    demonstrateToString(person1, person2)
+    demonstrateToEquals(person1, person2, person3)
+    demonstrateToHashCode(person1, person2, person3)
+    demonstrateToCopy(person1)
+    demonstrateToDestructuring(person1)
+}
+
+fun demonstrateToString(person1: Person, person2: Person) {
+    // toString() - automatically generated
+    println("\n=== toString() ===")
+    println("Person 1: $person1")
+    println("Person 2: $person2")
+    // Output:
+    // Person 1: Person(firstName=Alice, age=30, [email protected])
+    // Person 2: Person(firstName=Bob, age=32, [email protected])
+}
+
+fun demonstrateToEquals(person1: Person, person2: Person, person3: Person) {
+    // equals() - automatically generated
+    println("\n=== equals() ===")
+    println("person1 == person3: ${person1 == person3}") // true
+    println("person1 == person2: ${person1 == person2}") // false
+}
+
+fun demonstrateToHashCode(person1: Person, person2: Person, person3: Person) {
+    // hashCode() - automatically generated
+    println("\n=== hashCode() ===")
+    println("person1.hashCode() equals person3.hashCode(): ${person1.hashCode() == person3.hashCode()}") // true
+
+    // Checking in HashSet
+    println("\n=== Checking in HashSet ===")
+    val peopleSet = hashSetOf(person1, person2)
+    println("set contains person1: ${peopleSet.contains(person1)}") // true
+    println("set contains person3: ${peopleSet.contains(person3)}") // true, because hashCode() and equals() are the same
+    // Output:
+    // set contains person1: true
+    // set contains person3: true
+
+    // Checking in HashMap
+    println("\n=== Checking in HashMap ===")
+
+    val salary =
+        hashMapOf(
+            person1 to 30000,
+            person2 to 27000,
+        )
+
+    println("person1 salary: ${salary[person1]}") // 30000
+    println("person3 salary: ${salary[person3]}") // 30000, because person3 equals person1 (same hashCode and equals)
+
+    // Important! The use "var" is not recommended in the data classes.
+    println("\n=== Mutation properties: ===")
+    val mutablePerson = MutablePerson("Charlie", 35, "[email protected]")
+    val roleMap = hashMapOf(mutablePerson to "Developer")
+    println("before mutation: ${mutablePerson.hashCode()}")
+    mutablePerson.age = 36
+    println("after mutation: ${mutablePerson.hashCode()}") // Different!
+    println("Map size: ${roleMap.size}") // Map still contains the entry but can't find it.
+}
+
+fun demonstrateToCopy(person1: Person) {
+    // copy() - automatically generated.
+    // Attention! It is shallow copy. See example
+    println("\n=== copy() ===")
+
+    val person1Updated = person1.copy(age = 31, email = "[email protected]")
+
+    println("Updated person 1: $person1Updated")
+    // Output: Updated person 1: Person(firstName=Alice, age=31, [email protected])
+
+    println("\n=== Shallow copy: ===")
+    val originalAddress = Address("New York", "Washington Street")
+    val employee1 = Employee("Developer", person1, originalAddress)
+    val employee2 = employee1.copy()
+    println("before mutation:")
+    println("employee1: $employee1")
+    println("employee2: $employee2")
+    // employee1: Employee(position=Developer, person=Person(firstName=Alice, age=30, [email protected]), address=Address(city=New York, street=Washington Street))
+    // employee2: Employee(position=Developer, person=Person(firstName=Alice, age=30, [email protected]), address=Address(city=New York, street=Washington Street))
+
+    employee2.address.street = "Park Avenue"
+    println("after mutation:")
+    println("employee1: $employee1") // The street has changed too!
+    println("employee2: $employee2")
+    // employee1: Employee(position=Developer, person=Person(firstName=Alice, age=30, [email protected]), address=Address(city=New York, street=Park Avenue))
+    // employee2: Employee(position=Developer, person=Person(firstName=Alice, age=30, [email protected]), address=Address(city=New York, street=Park Avenue))
+}
+
+fun demonstrateToDestructuring(person1: Person) {
+    // Destructuring - automatically generated componentN() functions corresponding to the properties in their order of declaration.
+    println("\n=== Destructuring: ===")
+    val (name, age, email) = person1
+    println("Destructured: name=$name, age=$age, email=$email")
+    // Output: Destructured: name=Alice, age=30, [email protected]
+
+    // You can also use componentN() functions directly
+    println("name: ${person1.component1()}")
+    println("age: ${person1.component2()}")
+    println("email: ${person1.component3()}")
+    // Output:
+    // name: Alice
+    // age: 30
+    // email: [email protected]
+}

src/main/kotlin/codecollection/kotlinfeatures/README.md

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+# 🛠️ Kotlin Features
+
+This package contains examples of **Kotlin features**.
+
+Each feature is implemented in its own file and is tested with a corresponding unit test - keeping the code clean, minimal, and easy to understand.
+
+Whether you're learning, referencing, or contributing - this collection is for you.
+
+---
+
+## 🗂️ Available Features
+
+| Feature Name          | Description                | File                           |
+|-----------------------|----------------------------|--------------------------------|
+| Data classes          | Kotlin data class features | [`DataClass.kt`](DataClass.kt) |
+| _...more coming soon_ |
+
+---
+
+## 🙌 Want to Help?
+
+- Check out the [issues labeled `type: kotlinfeature`](https://github.com/e5LA/kotlin-code-collection/issues?q=is%3Aissue%20state%3Aopen%20label%3A%22type%3A%20kotlinfeature%22)
+- Or submit your own idea as new [Issue](https://github.com/e5LA/kotlin-code-collection/issues/new)!

src/test/kotlin/codecollection/kotlinfeatures/DataClassTest.kt

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+package codecollection.kotlinfeatures
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertNotEquals
+import kotlin.test.assertNotSame
+import kotlin.test.assertNull
+import kotlin.test.assertSame
+import kotlin.test.assertTrue
+
+class DataClassTest {
+    val person1 = Person(firstName = "Alice", age = 30, email = "[email protected]")
+    val person2 = Person(firstName = "Bob", age = 32, email = "[email protected]")
+    val person3 = Person(firstName = "Alice", age = 30, email = "[email protected]")
+
+    val mutablePerson = MutablePerson("Charlie", 35, "[email protected]")
+
+    val address = Address("New York", "Washington Street")
+
+    val employee1 = Employee("Developer", person1, address)
+    val employeeDefault = Employee(person = person1, address = address)
+
+    @Test
+    fun `should create person with correct properties`() {
+        assertEquals("Alice", person1.firstName)
+        assertEquals(30, person1.age)
+        assertEquals("[email protected]", person1.email)
+    }
+
+    @Test
+    fun `toString() should return formatted string`() {
+        assertEquals(
+            "Person(firstName=Alice, age=30, [email protected])",
+            person1.toString(),
+        )
+    }
+
+    @Test
+    fun `equals should work correctly for same content`() {
+        assertEquals(person1, person3)
+        assertNotEquals(person1, person2)
+    }
+
+    @Test
+    fun `hashCode should be same for equal objects`() {
+        assertEquals(person1.hashCode(), person3.hashCode())
+        assertNotEquals(person1.hashCode(), person2.hashCode())
+    }
+
+    @Test
+    fun `should work correctly in hashset`() {
+        val peopleSet = hashSetOf(person1, person2)
+        assertTrue(peopleSet.contains(person1))
+        assertTrue(peopleSet.contains(person2))
+        assertTrue(peopleSet.contains(person3))
+        assertEquals(2, peopleSet.size)
+    }
+
+    @Test
+    fun `should work correctly in hashmap`() {
+        val salary =
+            hashMapOf(
+                person1 to 30000,
+                person2 to 27000,
+            )
+        assertEquals(30000, salary[person1])
+        assertEquals(27000, salary[person2])
+        assertEquals(30000, salary[person3])
+        assertEquals(2, salary.size)
+    }
+
+    @Test
+    fun `mutation var properties should break hashmap key`() {
+        val testMap = hashMapOf(mutablePerson to "Developer")
+        assertEquals("Developer", testMap[mutablePerson])
+        mutablePerson.age = 36
+        assertNull(testMap[mutablePerson])
+    }
+
+    @Test
+    fun `mutation var properties should change hashCode()`() {
+        val originalHashCode = mutablePerson.hashCode()
+        mutablePerson.age = 36
+        val newHashCode = mutablePerson.hashCode()
+        assertNotEquals(originalHashCode, newHashCode)
+    }
+
+    @Test
+    fun `copy() should create new instance with identical properties`() {
+        val copy = person1.copy()
+        assertEquals(person1, copy)
+        assertNotSame(person1, copy)
+    }
+
+    @Test
+    fun `copy() should create new instance with updated properties`() {
+        val updated = person1.copy(email = "[email protected]")
+        assertEquals(person1.firstName, updated.firstName)
+        assertEquals(person1.age, updated.age)
+        assertEquals("[email protected]", updated.email)
+        assertEquals("[email protected]", person1.email)
+    }
+
+    @Test
+    fun `copy() should perform shallow copy of reference types`() {
+        val employee2 = employee1.copy()
+
+        assertNotSame(employee1, employee2)
+        assertSame(employee1.person, employee2.person)
+        assertSame(employee1.address, employee2.address)
+    }
+
+    @Test
+    fun `mutation of referenced objects should affect the original and its copy`() {
+        val employee2 = employee1.copy()
+        employee2.address.street = "Park Avenue"
+        assertEquals("Park Avenue", employee1.address.street)
+        assertEquals("Park Avenue", employee2.address.street)
+    }
+
+    @Test
+    fun `should use default value when not specified`() {
+        assertEquals("Intern", employeeDefault.position)
+    }
+
+    @Test
+    fun `copy() use default value`() {
+        val copiedEmployee = employeeDefault.copy()
+        assertEquals("Intern", copiedEmployee.position)
+    }
+
+    @Test
+    fun `destructuring should work correctly`() {
+        val (name, age, email) = person1
+        assertEquals("Alice", name)
+        assertEquals(30, age)
+        assertEquals("[email protected]", email)
+    }
+
+    @Test
+    fun `componentN() should work correctly`() {
+        assertEquals("Alice", person1.component1())
+        assertEquals(30, person1.component2())
+        assertEquals("[email protected]", person1.component3())
+    }
+}