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Kotlin

Type

打印对象类型

val obj: Double = 5.0

println(obj.javaClass.name)                 // double
println(obj.javaClass.kotlin)               // class kotlin.Double
println(obj.javaClass.kotlin.qualifiedName) // kotlin.Double
println(obj::clsss.simpleName)              // kotlin.String? = Double

反射class对比

// kotlin class
class Person {
    var name: String = ""
    var age: Int = 0
}

// Java class
public class Cat {

}

println("person::class=${person::class}")//得到的是KClass<Person>和Person::class一样
println("person::class.java=${person::class.java}")//得到的是Class<Person>对象
println("person::javaClass=${person::javaClass}")//未知？
println("person.javaClass.kotlin=${person.javaClass.kotlin}")//得到的是Class<Person>对象
println("cat.javaClass.kotlin=${cat.javaClass.kotlin}")//拿到的是KClass
println("Cat::class.java=${Cat::class.java}")//得到的Class<Cat>
println("cat.javaClass == Cat::class.java=${cat.javaClass == Cat::class.java}")//Class<Cat>==Class<Cat>
println("person.javaClass == Person::class.java=${person.javaClass == Person::class.java}")//Class<Person>==Class<Person>
println("person.javaClass.kotlin == Person::class=${person.javaClass.kotlin == Person::class}")//KClass<Person>==KClass<Person>
println("cat.javaClass.kotlin == Cat::class=${cat.javaClass.kotlin == Cat::class}")//KClass<Cat>==KClass<Cat>

kotlin 传参, 赋值时没有隐式类型转换, 参数接收Double, 就不能传Int 和 Float kotlin 表达式中会有上下文类型推断, 所以不需要显示转换类型 val l = 1L + 3 // Long + Int => Long JVM 中如果是nullable, 那么用Integer, Double保存; 如果是nonnull, 那么用int, double保存 Nullable的引用同样的number, 可能是不一样的对象. -128到127的数据是同一个对象, 因为JVM做的优化. 这不适用于10000

类型检查

is 和 !is // !is是is的否定, 支持智能转换(Smart casts is-checks), 支持&&和||操作, Smart casts支持when表达式和while循环

fun demo(x: Any) {
    if (x !is String || x.length == 0) return

    if (x is String) {
        print(x.length) // x is automatically cast to String
    }
}

Smart casts使用条件:

• val local variables - always, with the exception of local delegated properties.
• val properties - if the property is private or internal or if the check is performed in the same module where the property is declared. Smart casts cannot be used on open properties or properties that have custom getters.
• var local variables - if the variable is not modified between the check and the usage, is not captured in a lambda that modifies it, and is not a local delegated property.
• var properties - never, because the variable can be modified at any time by other code.

“Unsafe” cast operator, 不安全转换, 转换失败会抛异常

val x: String = y as String // y == null, 会抛异常
val x: String? = y as String? // y == null, 不会抛异常

“Safe” (nullable) cast operator, 转换失败返回null

val x: String? = y as? String

Nothing Type 标记为永远不会到达的对象, 遇到这个值后代码不会继续执行

Number

• Integer types Byte Short Int default Long

• Floating-point types (IEEE 754 standard) Float Double default

• Literal constants for numbers(不支持8进制)

  • Decimals: 123 *Longs are tagged by a capital L: 123L
  • Hexadecimals: 0x0F
  • Binaries: 0b00001011
  • Doubles by default: 123.5, 123.5e10
  • Floats are tagged by f or F: 123.5f
  • 支持下划线:

  1_000_000 0xFF_EC_DE_5E 0b11010010_01101001_10010100_10010010

显示转换类型

val b: Byte = 1
val i1: Int = b.toInt()

All number types support conversions to other types

• toByte(): Byte
• toShort(): Short
• toInt(): Int
• toLong(): Long
• toFloat(): Float
• toDouble(): Double

Boolean

Boolean Boolean?
true false null

|| && !

Char

'1'
\t – tab
\b – backspace
\n – new line (LF)
\r – carriage return (CR)
\' – single quotation mark
\" – double quotation mark
\\ – backslash
\$ – dollar sign
'\uFF00' // 支持Unicode
 digitToInt() // 如果char是一个数字, 那么可以把char转换成Int

String

val str: String = "abcd 123"

String 是不变的 String 支持下标访问字符str[1], 支持for-in迭代 String 联接使用+

String literals - Escaped strings: 使用\转义 String literals - Raw strings: 使用""", 不用\转义

支持字符串模板

val s = "abc"
println("$s.length is ${s.length}") // Prints "abc.length is 3"

Array

class Array<T> private constructor() {
    val size: Int
    operator fun get(index: Int): T
    operator fun set(index: Int, value: T): Unit

    operator fun iterator(): Iterator<T>
    // ...
}

Array提供get, set方法, 实现下标操作arr[1] arrayOf() 创建Array arrayOfNulls() 创建空Array Array(5) { i -> (i * i).toString() } 用函数创建Array Array泛型是不变的, invariant

Primitive type arrays, 没有开箱开销, ByteArray, ShortArray, IntArray, intArrayOf

Unsigned integer types

UByte: an unsigned 8-bit integer, ranges from 0 to 255
UShort: an unsigned 16-bit integer, ranges from 0 to 65535
UInt: an unsigned 32-bit integer, ranges from 0 to 2^32 - 1
ULong: an unsigned 64-bit integer, ranges from 0 to 2^64 - 1 > 无符号类型是使用`inline classes`在对应kotlin类型中实现, 将类型从无符号类型更改为有符号对应类型(反之亦然)是二进制不兼容的更改.

UByteArray: an array of unsigned bytes
UShortArray: an array of unsigned shorts
UIntArray: an array of unsigned ints
ULongArray: an array of unsigned longs

kotlin1.7, 无符号类型数组是beta版, @ExperimentalUnsignedTypes去掉非正式版API警告

UIntRange
UIntProgression
ULongRange
ULongProgression

literals

1f 1F // float
1l 1L // Long
1u 1U // UInt
1ul 1UL // ULong

Reference

Kotlin
Android 的 Kotlin 优先方法

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