Kotlin OOP, Null Safety और Lambdas

class Student(val name: String, var marks: Int) {
    fun passed() = marks >= 33
    fun describe() = "$name scored $marks (${if (passed()) "pass" else "fail"})"
}

fun main() {
    val s = Student("Riya", 82)
    println(s.describe())
    s.marks = 25
    println(s.describe())
}

data class Note(val id: Int, val title: String, val isDone: Boolean = false)

fun main() {
    val a = Note(1, "Maths homework")
    val b = a.copy(isDone = true)        // copy with one change
    println(a)
    println(b)
    println(a == Note(1, "Maths homework"))   // compares values
    val (id, title) = a                   // destructuring
    println("$id → $title")
}

open class Shape(val name: String) {
    open fun area(): Double = 0.0
    fun info() = "$name area = ${"%.2f".format(area())}"
}

class Circle(private val r: Double) : Shape("Circle") {
    override fun area() = Math.PI * r * r
}

class Rect(private val w: Double, private val h: Double) : Shape("Rectangle") {
    override fun area() = w * h
}

interface Clickable { fun onClick(): String }

class SaveButton : Clickable {
    override fun onClick() = "Saving note…"
}

fun main() {
    val shapes: List<Shape> = listOf(Circle(2.0), Rect(3.0, 4.0))
    shapes.forEach { println(it.info()) }
    println(SaveButton().onClick())
}

object AppConfig {                     // singleton – exactly one instance
    const val BASE_URL = "https://codingeasily.in/"
    var darkMode = false
}

class Counter {
    companion object {                 // like Java "static"
        const val MAX = 3
        fun create() = Counter()
    }
    var value = 0
    fun inc() { if (value < MAX) value++ }
}

enum class Priority(val label: String) { LOW("Low"), MEDIUM("Medium"), HIGH("High") }

fun main() {
    println(AppConfig.BASE_URL)
    val c = Counter.create()
    repeat(5) { c.inc() }
    println(c.value)
    println(Priority.HIGH.label)
    println(Priority.entries.joinToString { it.name })
}

fun main() {
    var subject: String? = null           // ? = may be null
    println(subject?.length)              // safe call → null, no crash
    println(subject ?: "General")         // Elvis: default value

    subject = "Physics"
    println(subject?.length)
    subject?.let { println("Subject is $it") }   // runs only if not null

    val input = "abc"
    val number = input.toIntOrNull() ?: 0
    println(number)
}

data class Note(val title: String, val priority: Int, val isDone: Boolean)

fun main() {
    val notes = listOf(
        Note("Maths homework", 2, false),
        Note("Buy notebook", 0, true),
        Note("Physics revision", 1, false)
    )
    val pending = notes.filter { !it.isDone }
    val titles = pending.map { it.title.uppercase() }
    val sorted = notes.sortedByDescending { it.priority }
    println(titles)
    println(sorted.first().title)
    println(notes.count { it.isDone })

    val onClick: (String) -> Unit = { title -> println("Clicked $title") }
    onClick("Maths homework")

    val note = Note("Draft", 1, false)
    // scope function "with": use the object's properties directly
    println(with(note) { "$title has priority $priority" })
}

class MainActivity : AppCompatActivity() {          // inheritance
    override fun onCreate(savedInstanceState: Bundle?) {   // override + nullable Bundle
        super.onCreate(savedInstanceState)
        binding.btnSave.setOnClickListener { save() }       // lambda
        val name = intent.getStringExtra("name") ?: "Guest" // Elvis
    }
}

@Entity data class Note(@PrimaryKey val id: Long, val title: String)  // data class
object TipsApi { const val BASE_URL = "https://codingeasily.in/" }   // object