变量与类型

// 只读变量
val name: String = "Kotlin"
val age = 25  // 类型推断

// 可变变量
var count: Int = 0
var message = "Hello"

val byte: Byte = 127
val short: Short = 32767
val int: Int = 2147483647
val long: Long = 9223372036854775807L

val float: Float = 3.14f
val double: Double = 3.14159265359

val i: Int = 100
val l: Long = i.toLong()
val d: Double = i.toDouble()
val s: String = i.toString()

val char: Char = 'A'
val string: String = "Hello"

// 多行字符串
val text = """
    Line 1
    Line 2
    Line 3
""".trimIndent()

// 字符串模板
val name = "Kotlin"
val greeting = "Hello, $name!"
val length = "Length is ${name.length}"

val isTrue: Boolean = true
val isFalse = false

// 逻辑运算
val result = isTrue && !isFalse

val numbers = arrayOf(1, 2, 3, 4, 5)
val strings = arrayOf("a", "b", "c")

// 指定类型
val ints: IntArray = intArrayOf(1, 2, 3)

// 访问元素
val first = numbers[0]
numbers[1] = 10

// 遍历
for (num in numbers) {
    println(num)
}

var name: String = "Kotlin"  // 不可为空
// name = null  // 编译错误

var nullableName: String? = "Kotlin"  // 可为空
nullableName = null  // OK

// 安全调用
val length = nullableName?.length

// Elvis 操作符
val len = nullableName?.length ?: 0

// 非空断言
val l = nullableName!!.length  // 可能抛出异常

val obj: Any = "Hello"

// 类型检查
if (obj is String) {
    println(obj.length)  // 自动转换
}

// 安全转换
val str: String? = obj as? String

// 不安全转换
val str2: String = obj as String