kotlin入门(二)

kotlin入门(一)
kotlin入门(二)
kotlin入门(三)

注意: 部分代码来自 https://try.kotlinlang.org/#/Examples/

类

不需要new关键字就能新建对象:

fun main(args: Array<String>) {
    Greeter("world").greet();
}
class Greeter(val name: String) {
    fun greet() {
        println("Hello, ${name}");
    }
}

构造函数

fun main(args: Array<String>) {
    Greeter("world").greet();
}
class Greeter () {
    var mName: String = ""
    constructor(name: String) : this() {
        println("constructor")
        this.mName=name
    }
    init{
        println("init")  //init代码块会在构造函数之前执行
    }
    fun greet() {
        println("Hello, ${mName}");
    }
}

拷贝函数

data class User(val name: String, val id: Int)

fun main(args: Array<String>) {
    val user = User("Alex", 1)
    println(user) // toString()

    val secondUser = User("Alex", 1)
    val thirdUser = User("Max", 2)

    println("user == secondUser: ${user == secondUser}")
    println("user == thirdUser: ${user == thirdUser}")

    // copy() function
    println(user.copy())
    println(user.copy("Max"))
    println(user.copy(id = 2))
    println(user.copy("Max", 2))
}

泛型

此处的例子是Pair类:

fun main(args: Array<String>) {
    val pair = Pair(1, "one")

    println("${pair.first} , ${pair.second}")

    val (num, name) = pair

    println("num = $num, name = $name")
}

class Pair<K, V>(val first: K, val second: V) {
    operator fun component1(): K {
        return first
    }
    operator fun component2(): V {
        return second
    }
}

HashMap

fun main(args: Array<String>) {
    val map = hashMapOf<String, Int>()
    map.put("two", 2)
    map.put("one", 1)
    for ((key, value) in map) {
        println("key = $key, value = $value")
    }
}

ArrayList

    val test = ArrayList<String>()
    test.add("111");
    test.add("222");
    
    for (i in test.indices){
        println(test.get(i))
    }

List

挑出所有的奇数:

fun main(args: Array<String>) {
    val numbers = listOf(1, 2, 3)
    println(numbers.filter(::isOdd))
}
fun isOdd(x: Int) = x % 2 != 0

挑出所有长度为奇数的字符串:

fun main(args: Array<String>) {
    val oddLength = compose(::isOdd, ::length)
    val strings = listOf("a", "ab", "abc")
    println(strings.filter(oddLength))
}
fun isOdd(x: Int) = x % 2 != 0
fun length(s: String) = s.length

fun <A, B, C> compose(f: (B) -> C, g: (A) -> B): (A) -> C {
    return { x -> f(g(x)) }
}

将java代码移植到kotlin

java代码, 作用是将数字集合转化成json字符串:

public String toJSON(Collection<Integer> collection) {
        StringBuilder sb = new StringBuilder();
        sb.append("[");
        Iterator<Integer> iterator = collection.iterator();
        while (iterator.hasNext()) {
            Integer element = iterator.next();
            sb.append(element);
            if (iterator.hasNext()) {
                sb.append(", ");
            }
        }
        sb.append("]");
        return sb.toString();
    }

kotlin代码:

fun toJSON(collection: Collection<Int>): String {
    val sb = StringBuilder()
    sb.append("[")
    val iterator = collection.iterator()
    while (iterator.hasNext())
    {
        val element = iterator.next()
        sb.append(element)
        if (iterator.hasNext())
        {
            sb.append(", ")
        }
    }
    sb.append("]")
    return sb.toString()
}

奇怪的kotlin语法(把 IF 表达式的结果赋值给一个变量)

fun main(args: Array<String>) {
    val a = 1
    val b = 2
    val max = if (a > b) {
        print("Choose a")
        a
    } else {
        print("Choose b")
        b
    }
    print(max);
}

输出结果是:

Choose b2

读取文件, 放到一个字符串里输出

    var text = File("C:\\Users\\Administrator\\Desktop\\1.txt").readText()
    println(text)

读取文件的每一行并输出: 方法一

    val inputStream: InputStream = File("C:\\Users\\Administrator\\Desktop\\1.txt").inputStream()
    val lineList = mutableListOf<String>()

    inputStream.bufferedReader().useLines { lines -> lines.forEach { lineList.add(it)} }
    lineList.forEach{println(it)}

读取文件的每一行并输出: 方法二

    File("C:\\Users\\Administrator\\Desktop\\1.txt").useLines { lines -> lines.forEach { println(it) } }

发送网络请求

    val html = URL("http://www.jianshu.com").readText()
    println(html)

利用gson解析json

在gradle中先引入gson:

compile group: 'com.google.code.gson', name: 'gson', version: '2.8.2'

fun main(args: Array<String>) {


    val gson = Gson()
    var user= User(100L, "jim", "....");
    val json = gson.toJson(user);

    println(json)

    var user2 = gson.fromJson(json, User::class.java )

    println("id="+user2.id+", name="+user2.name)
}
data class User(
        @SerializedName("id") val id: Long,
        @SerializedName("name") val name: String,
        @SerializedName("description") val description: String
)