[Android] 【Kotlin】初识Kotlin（二）

• if表达式当作三元运算符
  
• if块
  

fun main() {
  val flag = 1 == 2
  println(if (flag) "true" else "false")
}

fun main() {
  val a = -114
  val b = -1919
  val max = if (a > b) {
    println("$a is larger than $b.")
    println("max variable holds value of a.")
    a
  } else {
    println("$b is larger than $a.")
    println("max variable holds value of b.")
    b
  }
  println("max = $max")
}
/*
  -114 is larger than -1919.
  max variable holds value of a.
  max = -114
  进程已结束,退出代码0
*/

fun main() {
  val a = -114
  val b = -1919
  val max = if (a > b) shutA(a,b) else shutB(a,b)
  println("max = $max")
}
fun shutA(a: Int, b: Int): Int {
  println("$a is larger than $b.")
  println("max variable holds value of a.")
  return a
}
fun shutB(a: Int, b: Int): Int {
  println("$b is larger than $a.")
  println("max variable holds value of b.")
  return b
}

fun main() {
  println("输入符号")
  val result = when (val operator = readLine()) {
    "+" -> 0
    "-" -> 1
    "*" -> 2
    "/" -> 3
    else -> -1
  }
  println("selected index = $result")
}

fun main() {
  val a = 12
  val b = 5
  println("输入符号 +, -, * , /")
  when (val operator = readLine()) {
    "+" -> println("$a + $b = ${a + b}")
    "-" -> println("$a - $b = ${a - b}")
    "*" -> println("$a * $b = ${a * b}")
    "/" -> println("$a / $b = ${a / b}")
    else -> println("$operator 不是有效符号")
  }
}

fun main() {
  val n = -1
  when (n) {
    1, 2, 3 -> println("n is a positive integer less than 4.")
    0 -> println("n is zero")
    -1, -2 -> println("n is a negative integer greater than 3.")
  }
}

fun main() {
  val mutableListOf = mutableListOf("String", false, 1, 'c')
  mutableListOf.forEach() {
    when(it) {
      is String -> println("String")
      is Boolean -> println("Boolean")
      is Char -> println("Char")
      else -> println("Int")
    }
  }
}

fun main() {
  println("请输入你的分数")
  when(readLine()?.toInt()) {
    in 0 until 60 -> println("不及格")
    in 60 until 80 -> println("良好")
    in 80 .. 100 -> println("优秀")
    else -> println("请输入正确的成绩")
  }
}

fun main() {
  val tmp = "1"
  val tmp2 = "2"
  when {
    tmp == "1" -> println(tmp)
    tmp2 == "2" -> println(tmp2)
  }
}

fun main(args: Array<String>) {
  first@ for (i in 1..4) {
    second@ for (j in 1..2) {
      println("i = $i; j = $j")
      if (i == 2)
        break@first
    }
  }
}
/*
  i = 1; j = 1
  i = 1; j = 2
  i = 2; j = 1
*/

fun main(args: Array<String>) {
  here@ for (i in 1..5) {
    for (j in 1..4) {
      if (i == 3 || j == 2)
      continue@here
      println("i = $i; j = $j")
    }
  }
}
/*
  i = 1; j = 1
  i = 2; j = 1
  i = 4; j = 1
  i = 5; j = 1
*/

fun add(a:Int,b:Int) : Int {
  return a+b
}

fun add(a:Int,b:Int) : Int = a+b

fun add(a:Int,b:Int) = a+b

• shl - 符号左移
  
• shr - 符号右移
  
• ushr - 无符号右移
  
• and - 按位和
  
• or - 按位或
  
• xor - 按位异或
  
• inv - 按位反转
  

• 只有一个参数
  
• 在方法前必须加infix关键字
  
• 必须是成员方法或者扩展方法
  

class Student() {
  infix fun callNumber(number: String) = println("学生的学号为$number")
}
fun main() {
  val student = Student()
  student callNumber "20203199"
}
/*
  学生的学号为20203199
*/

infix fun Int.add(tmp:Int) = this+tmp
fun main() {
  println(1 add 2)
}
/*
3
*/

fun add(a:Int = 114, b:Int = 1919) :Int {
  println("first == $a second == $b")
  return a+b
}
fun main() {
  println(add())
}
/*
  first == 114 second == 1919
  2033
*/

fun add(a:Int = 114, b:Int = 1919) :Int {
  println("first == $a second == $b")
  return a+b
}
fun main() {
  println(add(810))
}
/*
  first == 810 second == 1919
  2729
*/

fun add(a:Int = 114, b:Int = 1919) :Int {
  println("first == $a second == $b")
  return a+b
}
fun main() {
  println(add(b=810))
}
/*
  first == 114 second == 810
  924
*/

fun add(a:Int = 114, b:Int = 1919, myMethod : ()->Unit ) :Int {
  myMethod()
  println("first == $a second == $b")
  return a+b
}
fun main() {
  val res = add(myMethod = { println("this is test method") })
  println(res)
}
/*
  this is test method
  first == 114 second == 1919
  2033
*/

fun add(a:Int = 114, b:Int = 1919, myMethod : ()->Unit ) :Int {
  myMethod()
  println("first == $a second == $b")
  return a+b
}
fun main() {
  val res = add(myMethod = { testMethod() })
  println(res)
}
fun testMethod() = println("this is test method")

fun add(a:Int = 114, b:Int = 1919, myMethod : ()->Unit ) :Int {
  myMethod()
  println("first == $a second == $b")
  return a+b
}
fun main() {
  val res = add() {
    println("测试方法")
    println("你当然可以在这个方法中写很多内容！")
  }
  println(res)
}

fun add(a:Int = 114, b:Int = 1919, myMethod : ()->Unit, myMethod2:()->Unit ) :Int {
  myMethod()
  myMethod2()
  println("first == $a second == $b")
  return a+b
}
fun main() {
  val res = add(myMethod = {println("测试1")}, myMethod2 = { println("测试2") })
  println(res)
}

fun add(a:Int = 114, b:Int = 1919, myMethod : ()->Unit, myMethod2:()->Unit ) :Int {
  myMethod()
  myMethod2()
  println("first == $a second == $b")
  return a+b
}
fun main() {
  val res = add(myMethod = {println("测试1")}) {
    println("测试2")
  }
  println(res)
}

fun myAppend(vararg ch:Char): String {
  val res = StringBuffer()
  ch.forEach {
    res.append(it)
  }
  return res.toString()
}
fun main() {
  val myAppend = myAppend('w', 'o', 'o', 'd', 'w', 'h', 'a', 'l', 'e')
  println(myAppend)
}
/*
  woodwhale
*/