Kotlin学习(一)

1.定义函数

    /**
     * 带有两个 Int 参数、返回 Int 的函数
     */
    fun sum(a: Int, b: Int): Int {
        return a + b
    }

a:Int 其中a表示传入参数,Int表示参数类型,括号外:Int表示返回值类型

    /**
     * 将表达式作为函数体 返回值类型自动推断的函数
     */
    fun sum1(a: Int, b: Int) = a + b

    /**
     *函数返回无意义的值
     */
    fun printSum(a: Int, b: Int, c: Int): Unit {
        println("sum of $a and $b and $c is ${a + b + c}")
    }

没有返回值时,用:Unit表示默认可以不写,$符号表示转义

    fun printSum(a: Int, b: Int) {
        println("sum of $a and $b is ${a + b}")
    }

2.赋值语句,val相当于final只能赋值一次,var可以多次赋值

    fun test2() {
        val a: Int = 1//立即赋值
        val b = 2 //自动推断出其数据类型
        val c: Int//如果没有初始值数据类型不能忽略
        c = 3 //明确赋值

        //可变变量
        var x = 5//自动推断出Int类型
        x += 1
    }

3.使用字符串模板

    fun test3() {
        var a = 3
        val str1 = "a is $a"
        a = 6
        val str2 = "${str1.replace("is", "am")} but now is $a"
        println(str2)
    }

4.使用条件表达式

    private fun test4() {
        val max = getMax(3, 3)
        println(max)

        println(maxof(100, 100))
    }

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

    fun getMax(a: Int, b: Int): Int {
        if (a > b) {
            return a
        } else if (a < b) {
            return b
        } else {
            return a
        }
    }

5.使用可空值及null的检测(返回值后面加?)

    fun getStringLength(obj: Any): Int? {
        if (obj is String) {
            return obj.length
        }
        return null

    }

6.使用类型检测及自动类型转换(通过is)

 fun getStringLength(obj: Any): Int? {
        if (obj is String) {
            return obj.length
        }
        return null

    }

    fun getStringLength1(obj: Any): Int? {
        if (obj !is String) {
            return null
        }
        return obj.length
    }

    fun getStringLength2(obj: Any): Int? {
        if (obj is String && obj.length > 0) {
            return obj.length
        }
        return null
    }

7.for循环

    private fun test7() {
        val items = listOf("1", "2", "3", "4", "5", "5897")
        for (item in items) {
            println(item)
        }
    }

8. while循环

    private fun test8() {
        var items = listOf<String>("wade", "kobe", "james", "paul", "Irving", "rose", "jordan")
        var index = 0
        while (index < items.size) {
            println("$index is ${items[index]}")
            index++
        }
    }

9.使用when表达式

    private fun describe(obj: Any?) {
        when (obj) {
            is Int -> println("obj is int")
            is String -> println("obj is String")
            is Long -> println("obj is Long")
            else -> println("unknow")
        }
    }

10.使用区间, 使用 in 运算符来检测某个数字是否在指定区间内

 private fun testRange(a: Int) {
        val list = listOf<String>("a", "b", "c")
        println(list.lastIndex)
        if (0 in 0..list.lastIndex) {
            println("0 is in Range")
        } else {
            println("0 is Out of Range")
        }
        if (list.size - 1 !in list.indices) {
            println("list size is out of valid list indices range too")
        }
        //step后面的数字表示每隔 该数字-1打印一次
        for (x in 1..10 step 5) {
            println(x)
        }
        // 9 down to 0 表示从9到0
        for(x in 9 downTo 0 step 2){
            println(x)
        }
    }

11.使用集合 进行迭代

 private fun test10() {
        val items = listOf<String>("wa","orange","noy","hui","cdf")
        for(item in items){
            println(item)
        }

        when{
            "orange" in items -> println("juicy")
            "apple" in items  -> println("apple")
            else              -> println("unKnow")
        }

        //使用lambda表达式来过滤与yin'she
        val fruits = listOf<String>("apple","banana","orange","pear","melon")
        fruits
                .filter { it.startsWith("b") }
                .sortedBy { it }
                .map { it.toLowerCase() }
                .forEach { println(it) }

    }