第2章 Kotlin 语法基础第2章 Kotlin 语法基础

>>> val a:Int = 1
>>> a
1

>>> val a = 1 // 根据值 1 编译器能够自动推断出 `Int` 类型
>>> a
1

>>> val a = 1
>>> a++
error: val cannot be reassigned
a++
^

>>> var b = 1
>>> b = b + 1
>>> b
2

>>> val _x = 1
>>> val y = 2
>>> val ip_addr = "127.0.0.1"
>>> _x
1
>>> y
2
>>> ip_addr
127.0.0.1

inline fun f(crossinline body: () -> Unit) {
    val f = object: Runnable {
        override fun run() = body()
    }
}

package com.easy.kotlin

fun main(args: Array<String>) {
    println(max(1, 2))
}

fun max(a: Int, b: Int): Int {
    // 表达式返回值
    val max = if (a > b) a else b
    return max
}

fun max3(a: Int, b: Int): Int {
    val max = if (a > b) {
        print("Max is a")
        a // 最后的表达式作为该代码块的值
    } else {
        print("Max is b")
        b // 同上
    }
    return max
}

if(true) 1 else 0

>>> if("a") 1
error: type mismatch: inferred type is String but Boolean was expected
if("a") 1
   ^

>>> if(1) println(1)
error: the integer literal does not conform to the expected type Boolean
if(1)
   ^

>>> if(true) println(1) else println(0)
1
>>> if(true) { println(1)}  else{ println(0)}
1

fun isLeapYear(year: Int): Boolean {
    var isLeapYear: Boolean
    if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)) {
        isLeapYear = true
    } else {
        isLeapYear = false
    }
    return isLeapYear
}

fun main(args: Array<String>) {
    println(isLeapYear(2017)) // false
    println(isLeapYear(2020)) // true
}

fun casesWhen(obj: Any?) {
    when (obj) {
        0,1,2,3,4,5,6,7,8,9 -> println("${obj} ===> 这是一个0-9之间的数字")
        "hello" -> println("${obj} ===> 这个是字符串hello")
        is Char -> println("${obj} ===> 这是一个 Char 类型数据")
        else -> println("${obj} ===> else类似于Java中的 case-switch 中的 default")
    }
}

fun main(args: Array<String>) {
    casesWhen(1)
    casesWhen("hello")
    casesWhen('X')
    casesWhen(null)
}

1 ===> 这是一个0-9之间的数字
hello ===> 这个是字符串hello
X ===> 这是一个 Char 类型数据
null ===> else类似于Java中的 case-switch 中的 default

0,1,2,3,4,5,6,7,8,9 -> println("${obj} ===> 这是一个0-9之间的数字")

fun switch(x: Int) {
    val s = "123"
    when (x) {
        -1, 0 -> print("x == -1 or x == 0")
        1 -> print("x == 1")
        2 -> print("x == 2")
        8 -> print("x is 8")
        parseInt(s) -> println("x is 123")
        else -> { // 注意这个块
            print("x is neither 1 nor 2")
        }
    }
}

    val x = 1
    val validNumbers = arrayOf(1, 2, 3)
    when (x) {
        in 1..10 -> print("x is in the range")
        in validNumbers -> print("x is valid")
        !in 10..20 -> print("x is outside the range")
        else -> print("none of the above")
    }

fun fact(n: Int): Int {
    var result = 1
    when (n) {
        0, 1 -> result = 1
        else -> result = n * fact(n - 1)
    }
    return result
}

fact(10) // 3628800

for (item in collection) {
    print(item)
}

for (i in array.indices) {
    print(array[i])
}

for ((index, value) in array.withIndex()) {
    println("the element at $index is $value")
}

if (i in 1..10) { // 等同于1 <= i && i <= 10
    println(i) 
}

(1..10).forEach { print(it) }

fun sumFact(n: Int): Int {
    var sum = 0
    for (i in 1..n) {
        sum += fact(i)
    }
    return sum
}

sumFact(10) // 4037913

package com.easy.kotlin

fun main(args: Array<String>) {
    var x = 10
    while (x > 0) {
        x--
        println(x)
    }

    var y = 10
    do {
        y = y + 1
        println(y)
    } while (y < 20) // y的作用域包含此处
}

    for (i in 1..10) {
        println(i)
        if (i % 2 == 0) {
            break
        }
    } // break to here

1
2

    for (i in 1..10) {
        if (i % 2 == 0) {
            continue
        }
        println(i)
    }

1
3
5
7
9

fun sum(a: Int,b: Int): Int{
    return a+b // 这里的return不能省略
}

fun max(a: Int, b: Int): Int {
 if (a > b){
 return a // return不能省略
} else{
 return b // return不能省略
}

>>> fun sum(a: Int,b: Int) = a + b
>>> fun max(a: Int, b: Int) = if (a > b) a else b

>>> sum(1,10)
11

>>> max(1,2)
2

>>> val sum=fun(a:Int, b:Int) = a+b
>>> sum
(kotlin.Int, kotlin.Int) -> kotlin.Int
>>> sum(1,1)
2

>>> val sumf = fun(a:Int, b:Int) = {a+b}
>>> sumf
(kotlin.Int, kotlin.Int) -> () -> kotlin.Int
>>> sumf(1,1)
() -> kotlin.Int
>>> sumf(1,1).invoke()
2

>>> fun sumf(a:Int,b:Int) = {a+b}
>>> sumf(1,1)
() -> kotlin.Int
>>> sumf(1,1).invoke()
2
>>> fun maxf(a:Int, b:Int) = {if(a>b) a else b}
>>> maxf(1,2)
() -> kotlin.Int
>>> maxf(1,2).invoke()
2

() -> kotlin.Int
() -> kotlin.Int

scala> def maxf(x:Int, y:Int) = { if(x>y) x else y }
maxf: (x: Int, y: Int)Int

scala> def maxv(x:Int, y:Int) = if(x>y) x else y
maxv: (x: Int, y: Int)Int

scala> maxf(1,2)
res4: Int = 2

scala> maxv(1,2)
res6: Int = 2

    val intArray = intArrayOf(1, 2, 3, 4, 5)
    intArray.forEach {
        if (it == 3) return // 在Lambda表达式中的return 直接返回最近的外层函数
        println(it)
    }

1
2

    val intArray = intArrayOf(1, 2, 3, 4, 5)
    intArray.forEach(fun(a: Int) { 
        if (a == 3) return // 从最近的函数中返回
        println(a)
    })

1
2
4
5

    val intArray = intArrayOf(1, 2, 3, 4, 5)
    intArray.forEach here@ {
        if (it == 3) return@here // 指令跳转到 lambda 表达式标签 here@ 处。继续下一个it=4的遍历循环
        println(it)
    }

1
2
4
5

    val intArray = intArrayOf(1, 2, 3, 4, 5)
    intArray.forEach {
        if (it == 3) return@forEach // 返回到 @forEach 处继续下一个循环
        println(it)
    }

1
2
4
5

>>> Nothing::class
class java.lang.Void

>>> fun fail(msg:String):Nothing{ throw IllegalArgumentException(msg) }
>>> fail("XXXX")
java.lang.IllegalArgumentException: XXXX
    at Line57.fail(Unknown Source)

>>> val ex = throw Exception("YYYYYYYY")
error: 'Nothing' property type needs to be specified explicitly
val ex = throw Exception("YYYYYYYY")
    ^

>>> val ex:Nothing = throw Exception("YYYYYYYY")
java.lang.Exception: YYYYYYYY

    KtSingleValueToken LBRACKET    = new KtSingleValueToken("LBRACKET", "[");
    KtSingleValueToken RBRACKET    = new KtSingleValueToken("RBRACKET", "]");
    KtSingleValueToken LBRACE      = new KtSingleValueToken("LBRACE", "{");
    KtSingleValueToken RBRACE      = new KtSingleValueToken("RBRACE", "}");
    KtSingleValueToken LPAR        = new KtSingleValueToken("LPAR", "(");
    KtSingleValueToken RPAR        = new KtSingleValueToken("RPAR", ")");
    KtSingleValueToken DOT         = new KtSingleValueToken("DOT", ".");
    KtSingleValueToken PLUSPLUS    = new KtSingleValueToken("PLUSPLUS", "++");
    KtSingleValueToken MINUSMINUS  = new KtSingleValueToken("MINUSMINUS", "--");
    KtSingleValueToken MUL         = new KtSingleValueToken("MUL", "*");
    KtSingleValueToken PLUS        = new KtSingleValueToken("PLUS", "+");
    KtSingleValueToken MINUS       = new KtSingleValueToken("MINUS", "-");
    KtSingleValueToken EXCL        = new KtSingleValueToken("EXCL", "!");
    KtSingleValueToken DIV         = new KtSingleValueToken("DIV", "/");
    KtSingleValueToken PERC        = new KtSingleValueToken("PERC", "%");
    KtSingleValueToken LT          = new KtSingleValueToken("LT", "<");
    KtSingleValueToken GT          = new KtSingleValueToken("GT", ">");
    KtSingleValueToken LTEQ        = new KtSingleValueToken("LTEQ", "<=");
    KtSingleValueToken GTEQ        = new KtSingleValueToken("GTEQ", ">=");
    KtSingleValueToken EQEQEQ      = new KtSingleValueToken("EQEQEQ", "===");
    KtSingleValueToken ARROW       = new KtSingleValueToken("ARROW", "->");
    KtSingleValueToken DOUBLE_ARROW       = new KtSingleValueToken("DOUBLE_ARROW", "=>");
    KtSingleValueToken EXCLEQEQEQ  = new KtSingleValueToken("EXCLEQEQEQ", "!==");
    KtSingleValueToken EQEQ        = new KtSingleValueToken("EQEQ", "==");
    KtSingleValueToken EXCLEQ      = new KtSingleValueToken("EXCLEQ", "!=");
    KtSingleValueToken EXCLEXCL    = new KtSingleValueToken("EXCLEXCL", "!!");
    KtSingleValueToken ANDAND      = new KtSingleValueToken("ANDAND", "&&");
    KtSingleValueToken OROR        = new KtSingleValueToken("OROR", "||");
    KtSingleValueToken SAFE_ACCESS = new KtSingleValueToken("SAFE_ACCESS", "?.");
    KtSingleValueToken ELVIS       = new KtSingleValueToken("ELVIS", "?:");
    KtSingleValueToken QUEST       = new KtSingleValueToken("QUEST", "?");
    KtSingleValueToken COLONCOLON  = new KtSingleValueToken("COLONCOLON", "::");
    KtSingleValueToken COLON       = new KtSingleValueToken("COLON", ":");
    KtSingleValueToken SEMICOLON   = new KtSingleValueToken("SEMICOLON", ";");
    KtSingleValueToken DOUBLE_SEMICOLON   = new KtSingleValueToken("DOUBLE_SEMICOLON", ";;");
    KtSingleValueToken RANGE       = new KtSingleValueToken("RANGE", "..");
    KtSingleValueToken EQ          = new KtSingleValueToken("EQ", "=");
    KtSingleValueToken MULTEQ      = new KtSingleValueToken("MULTEQ", "*=");
    KtSingleValueToken DIVEQ       = new KtSingleValueToken("DIVEQ", "/=");
    KtSingleValueToken PERCEQ      = new KtSingleValueToken("PERCEQ", "%=");
    KtSingleValueToken PLUSEQ      = new KtSingleValueToken("PLUSEQ", "+=");
    KtSingleValueToken MINUSEQ     = new KtSingleValueToken("MINUSEQ", "-=");
    KtKeywordToken NOT_IN      = KtKeywordToken.keyword("NOT_IN", "!in");
    KtKeywordToken NOT_IS      = KtKeywordToken.keyword("NOT_IS", "!is");
    KtSingleValueToken HASH        = new KtSingleValueToken("HASH", "#");
    KtSingleValueToken AT          = new KtSingleValueToken("AT", "@");

    KtSingleValueToken COMMA       = new KtSingleValueToken("COMMA", ",");

package com.easy.kotlin

data class Point(val x: Int, val y: Int)
operator fun Point.unaryMinus() = Point(-x, -y)

package com.easy.kotlin

import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4

@RunWith(JUnit4::class)
class OperatorDemoTest {

    @Test
    fun testPointUnaryMinus() {
        val p = Point(1, 1)
        val np = -p
        println(np) //Point(x=-1, y=-1)
    }
}

>>> val a=10
>>> val b=3
>>> a+b
13
>>> a-b
7
>>> a/b
3
>>> a%b
1
>>> a..b
10..3
>>> b..a
3..10

>>> ""+1
1
>>> 1+""
error: none of the following functions can be called with the arguments supplied: 
public final operator fun plus(other: Byte): Int defined in kotlin.Int
public final operator fun plus(other: Double): Double defined in kotlin.Int
public final operator fun plus(other: Float): Float defined in kotlin.Int
public final operator fun plus(other: Int): Int defined in kotlin.Int
public final operator fun plus(other: Long): Long defined in kotlin.Int
public final operator fun plus(other: Short): Int defined in kotlin.Int
1+""
 ^

>>> 1.toString()+""
1

data class Counter(var index: Int)

operator fun Counter.plus(increment: Int): Counter {
    return Counter(index + increment)
}

package com.easy.kotlin

import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4

@RunWith(JUnit4::class)
class OperatorDemoTest 
    @Test
    fun testCounterIndexPlus() {
        val c = Counter(1)
        val cplus = c + 10
        println(cplus) //Counter(index=11)
    }
}

y = x?:0

val y = if(x!==null) x else 0

String name = "Elvis Presley";
String displayName = (name != null) ? name : "Unknown";

String name = "Elvis Presley";
String displayName = name?:"Unknown"

>>> val x = null
>>> val y = x?:0
>>> y
0
>>> val x = false
>>> val y = x?:0
>>> y
false
>>> val x = ""
>>> val y = x?:0
>>> y

>>> val x = "abc"
>>> val y = x?:0
>>> y
abc

data class Person(val name: String, val age: Int)

infix fun Person.grow(years: Int): Person {
    return Person(name, age + years)
}

package com.easy.kotlin

import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4

@RunWith(JUnit4::class)
class InfixFunctionDemoTest {

    @Test fun testInfixFuntion() {
        val person = Person("Jack", 20)
        println(person.grow(2))
        println(person grow 2)
    }
}

Person(name=Jack, age=22)
Person(name=Jack, age=22)

package com.easy.kotlin

fun what(){ // 包级函数
    println("This is WHAT ?")
}

fun main(args:Array<String>){ // 一个包下面只能有一个main函数
    println("Hello,World!")
}

class Motorbike{ // 包里面的类
    fun drive(){
        println("Drive The Motorbike ...")
    }
}

package com.easy.kotlin

import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4

@RunWith(JUnit4::class)
class PackageDemoTest {

    @Test
    fun testWhat() {
        what()
    }

    @Test
    fun testDriveMotorbike(){
        val motorbike = Motorbike()
        motorbike.drive()
    }
}

package com.easy.kotlin.test

import com.easy.kotlin.Motorbike // 导入类Motorbike
import com.easy.kotlin.what // 导入包级函数what
import org.junit.Test
import org.junit.runner.RunWith
import org.junit.runners.JUnit4

@RunWith(JUnit4::class)
class PackageDemoTest {

    @Test
    fun testWhat() {
        what()
    }

    @Test
    fun testDriveMotorbike() {
        val motorbike = Motorbike()
        motorbike.drive()
    }

}

kotlin.*
kotlin.annotation.*
kotlin.collections.*
kotlin.comparisons.* （自 1.1 起）
kotlin.io.*
kotlin.ranges.*
kotlin.sequences.*
kotlin.text.*

java.lang.*
kotlin.jvm.*

kotlin.js.*