基础语法演练(Basic syntax walk-through

• 1 条件表达式(conditional expression)

    fun main(args: Array<String>) {
    println(max(Integer.parseInt("20"), Integer.parseInt("30"))
    }

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

30
Integer.parseInt("20")将"20"转成Int的包装类,并赋值给a ,让后比较a b,输出结果

• 2 检查空(null -checks)

  fun parseInt(str: String): Int? {
      try {
        return Integer.parseInt(str)
      } catch (e: NumberFormatException) {
        println("其中有参数不是Int类型")
      }
      return null
      }

  fun main(args: Array<String>) {
    if (args.size == 2) { //假 false
        println("No number supplied");
    } else {
        val x = parseInt("2")
        val y = parseInt("3")

        // We cannot say 'x * y' now because they may hold nulls
        if (x != null && y != null) {
            print(x * y) // Now we can
        } else {
            println("有参数为空")
      }
    }
}

6
其中要注意try catch的执行流程. Int 后的 ? 不要漏了 ,表示判断

• 3 is 关键字的使用

   fun main(args: Array<String>) {
    println(getStringLength("aaa"))
    println(getStringLength(1))
   }

   fun getStringLength(obj: Any): Int? {
    if (obj is String)
        return obj.length // no cast to String is needed
    return null
}

3
null
obj is String ? Kotlin 牛逼的地方,一看就知道意思

• 4 while 关键字的使用

    fun main(args: Array<String>) {
    var i = 0
    while (i == args.size)
        println(++i)
}

1
顺便了解下++i 和 i++ 的区别,看到var是不是联想到H5里的var,目前没看出和H5有啥区别.

• 5 for 关键字的使用

fun main(args: Array<String>) {
    for (arg in args)
        println(arg)

    // or
    println()
    for (i in args.indices)
        println(args[i])
}

• 6 in 的使用

      fun main(args: Array<String>) {
      val x = Integer.parseInt("2")
       val y = 10
       if (x in 1..y - 1)     //判断x是否在1 和 y-1 之前,其中y-1 是参与了运算的,                           
       println("OK")          //并且x的取值范围是包含了头和尾的

      //Iterate over a range: 在一定范围内迭代输出  ${a}的使用
      for (a in 1..5)
        print("${a} ")

      //Check if a number is out of range:
        println()
        val array = arrayListOf<String>()  //集合 泛型 添加元素
        array.add("aaa")
        array.add("bbb")
        array.add("ccc")

      if (x !in 0..array.size - 1)
        println("Out: array has only ${array.size} elements. x = ${x}")

      //Check if a collection contains an object:
      if ("aaa" in array) // collection.contains(obj) is called
        println("Yes: array contains aaa")

      if ("ddd" in array) // collection.contains(obj) is called
        println("Yes: array contains ddd")
      else
        println("No: array doesn't contains ddd")
     }

OK
1 2 3 4 5
Yes: array contains aaa
No: array doesn't contains ddd

• 7 when

 fun main(args: Array<String>) {
    cases("Hello")
    cases(1)
    cases(System.currentTimeMillis())
    cases(MyClass())
    cases("hello")
}

fun cases(obj: Any) {
    when (obj) {
        1 -> println("One")
        "Hello" -> println("Greeting")
        is Long -> println("Long")
        !is String -> println("Not a string")
        else -> println("Unknown")
    }
}

class MyClass() {
}

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