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KonstantinKhane5LA
KonstantinKhan
and
e5LA
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feat: Kotlin Data classes features (#48)
Co-authored-by: e5LA <[email protected]>
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‎src/main/kotlin/codecollection/kotlinfeatures/DataClass.kt‎

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

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

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# 🛠️ Kotlin Features
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This package contains examples of **Kotlin features**.
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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.
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Whether you're learning, referencing, or contributing - this collection is for you.
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---
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## 🗂️ Available Features
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| Feature Name | Description | File |
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|-----------------------|----------------------------|--------------------------------|
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| Data classes | Kotlin data class features | [`DataClass.kt`](DataClass.kt) |
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| _...more coming soon_ |
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---
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## 🙌 Want to Help?
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- 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)
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- 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
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import kotlin.test.Test
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import kotlin.test.assertEquals
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import kotlin.test.assertNotEquals
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import kotlin.test.assertNotSame
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import kotlin.test.assertNull
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import kotlin.test.assertSame
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import kotlin.test.assertTrue
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class DataClassTest {
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val person1 = Person(firstName = "Alice", age = 30, email = "[email protected]")
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val person2 = Person(firstName = "Bob", age = 32, email = "[email protected]")
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val person3 = Person(firstName = "Alice", age = 30, email = "[email protected]")
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val mutablePerson = MutablePerson("Charlie", 35, "[email protected]")
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val address = Address("New York", "Washington Street")
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val employee1 = Employee("Developer", person1, address)
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val employeeDefault = Employee(person = person1, address = address)
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@Test
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fun `should create person with correct properties`() {
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assertEquals("Alice", person1.firstName)
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assertEquals(30, person1.age)
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assertEquals("[email protected]", person1.email)
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}
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@Test
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fun `toString() should return formatted string`() {
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assertEquals(
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"Person(firstName=Alice, age=30, [email protected])",
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person1.toString(),
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)
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}
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@Test
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fun `equals should work correctly for same content`() {
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assertEquals(person1, person3)
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assertNotEquals(person1, person2)
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}
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@Test
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fun `hashCode should be same for equal objects`() {
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assertEquals(person1.hashCode(), person3.hashCode())
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assertNotEquals(person1.hashCode(), person2.hashCode())
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}
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@Test
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fun `should work correctly in hashset`() {
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val peopleSet = hashSetOf(person1, person2)
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assertTrue(peopleSet.contains(person1))
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assertTrue(peopleSet.contains(person2))
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assertTrue(peopleSet.contains(person3))
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assertEquals(2, peopleSet.size)
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}
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@Test
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fun `should work correctly in hashmap`() {
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val salary =
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hashMapOf(
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person1 to 30000,
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person2 to 27000,
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)
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assertEquals(30000, salary[person1])
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assertEquals(27000, salary[person2])
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assertEquals(30000, salary[person3])
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assertEquals(2, salary.size)
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}
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@Test
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fun `mutation var properties should break hashmap key`() {
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val testMap = hashMapOf(mutablePerson to "Developer")
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assertEquals("Developer", testMap[mutablePerson])
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mutablePerson.age = 36
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assertNull(testMap[mutablePerson])
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}
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@Test
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fun `mutation var properties should change hashCode()`() {
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val originalHashCode = mutablePerson.hashCode()
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mutablePerson.age = 36
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val newHashCode = mutablePerson.hashCode()
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assertNotEquals(originalHashCode, newHashCode)
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}
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@Test
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fun `copy() should create new instance with identical properties`() {
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val copy = person1.copy()
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assertEquals(person1, copy)
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assertNotSame(person1, copy)
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}
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@Test
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fun `copy() should create new instance with updated properties`() {
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val updated = person1.copy(email = "[email protected]")
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assertEquals(person1.firstName, updated.firstName)
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assertEquals(person1.age, updated.age)
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assertEquals("[email protected]", updated.email)
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assertEquals("[email protected]", person1.email)
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}
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@Test
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fun `copy() should perform shallow copy of reference types`() {
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val employee2 = employee1.copy()
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assertNotSame(employee1, employee2)
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assertSame(employee1.person, employee2.person)
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assertSame(employee1.address, employee2.address)
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}
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@Test
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fun `mutation of referenced objects should affect the original and its copy`() {
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val employee2 = employee1.copy()
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employee2.address.street = "Park Avenue"
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assertEquals("Park Avenue", employee1.address.street)
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assertEquals("Park Avenue", employee2.address.street)
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}
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@Test
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fun `should use default value when not specified`() {
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assertEquals("Intern", employeeDefault.position)
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}
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@Test
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fun `copy() use default value`() {
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val copiedEmployee = employeeDefault.copy()
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assertEquals("Intern", copiedEmployee.position)
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}
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@Test
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fun `destructuring should work correctly`() {
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val (name, age, email) = person1
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assertEquals("Alice", name)
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assertEquals(30, age)
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assertEquals("[email protected]", email)
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}
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@Test
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fun `componentN() should work correctly`() {
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assertEquals("Alice", person1.component1())
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assertEquals(30, person1.component2())
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assertEquals("[email protected]", person1.component3())
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}
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}

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