Kotlin基础知识

1. 改进点/基础

//安全判空
val length = text?.length;
		  
//类型转换
if (object is Car) {
	var car = object as Ca
}

//操作符变化
&   and
| 	or
^	xor
>> 	shr
<<	shl

//打印日志
print("123123")
println("asdassda")

2. 字符串

//多行字符串
val xx = """asdasdasd
		  asdasdsd """;
			
//字符串模版
var a = 1; val s1 = "a is $a"

4. 语法

var name = "123"; 	//可变变量
val name = "asdasd";	//不可变量，相当于java的final
var name: String = ...;

4.1 function

//定义function
fun sum(a: Int, b: Int): Int {
	return a + b;
}

//默认参数
fun read(b: Array<Byte>, off: Int = 0, len: Int = b.size) {
}

//可变参数
fun <T> asList(vararg ts: T): List<T> {
    val result = ArrayList<T>()
    for (t in ts) // ts is an Array
        result.add(t)
    return result
}
val a = arrayOf(1, 2, 3)

//扩展对象上的function(替换代理模式)， 待扩展对象+方法名+返回值
fun Any?.funExt(): String {
	if (this == null) return "null"
	return toString()+"11111";
}
val str = "SSSS";     println (str.funExt());

//若返回值可能为null， 必须显示定义，否则运行时报错
fun parseInt(str: String): Int? {
}

4.2 属性

//扩展属性
val <T> List<T>.lastIndex: Int{
	get() = size - 1
}

4.3 Lambdas

//lambdas表达式定义
//单个入参时，默认it
val sum = { x: Int, y: Int -> x + y };	

//高阶函数 函数作为方法入参
//示例1--
fun <T, R> List<T>.map(transform: (T) -> R): List<R> {
	val result = arrayListOf<R>()
	for (item in this)
		result.add(transform(item))
	return result
}
//调用方式： val doubled = ints.map { value -> value * 2 }
	或是  	ints.map { it * 2 }

//示例2--内联函数
fun <T> max(collection: Collection<T>, less: (T, T) -> Boolean): T? {
	var max: T? = null
	for (it in collection)
		if (max == null || less(max, it))
			max = it;
	return max
}
//调用： max(strings, { a, b -> a.length < b.length })

//未使用的参数可使用_代替
map.forEach { _, value -> println("$value!") }

5. Class

5.1 Class类和继承

//定义
class Invoice { }
或 class Person(val firstName: String, val lastName: String, var age: Int) {
		// ...
	}

//构建函数，一个主，多个次构造函数
//可自带默认参数
class Customer(val customerName: String = "")	

//主构造函数是class声明的一部分, 构造函数默认public
class Customer(name: String) {
	init {
		logger.info("Customer initialized with value ${name}")
	}
}
class DontCreateMe private constructor () { }	//构造方法置为private

//次构造函数,可多个
class Person(val name: String) {
	constructor(name: String, parent: Person) : this(name) {
		parent.children.add(this)
	}
}

//继承
class MyView : View {
	constructor(ctx: Context) : super(ctx)
	constructor(ctx: Context, attrs: AttributeSet) : super(ctx, attrs)
}

//覆写方法--overide 
//kotlin中重写必须明确指定，使用open声明(否则编译报错)可被重写，open的fun必须包含在open的class中
open class Base {
	open fun v() {}
	fun nv() {}
}
class Derived() : Base() {
	override fun v() {}
}

//覆写规则
open class A {
	open fun f() { print("A") }
	fun a() { print("a") }
}

interface B {
	fun f() { print("B") } // interface members are 'open' by default
	fun b() { print("b") }
}

class C() : A(), B {
	// The compiler requires f() to be overridden:
	override fun f() {
		super<A>.f() // call to A.f()
		super<B>.f() // call to B.f()
	}
}	

//覆写property

//Abstract

5.2 property和Fields

class Address { var name: String = ... 	};

//改写getter和setter方法
val isEmpty: Boolean get() = this.size == 0;
var stringRepresentation: String
	get() = this.toString()
	set(value) {
		setDataFromString(value) // parses the string and assigns values to other properties
	}

//private setter
var setterVisibility: String = "abc"
	private set

5.3 interface接口

interface MyInterface {
	fun bar()
	fun foo() { }
}
class Child : MyInterface {
	override fun bar() {
		// body
	}
}

5.4 Data Classes 只包含数据的Class

//针对此种模型，kotlin额外定义了一些扩展方法
-- equals()/hashCode() pair,
-- toString() of the form "User(name=John, age=42)",
data class User(val name: String = "", val age: Int = 0);	//示例1

//copy方法
val jack = User(name = "Jack", age = 1)
val olderJack = jack.copy(age = 2)

5.5 泛型Generics

6. 逻辑操作

6.1 for循环

for (item in items) {
	println(item)
}

6.2 退出循环-break label

//退出多重for循环
lxx@ for (i in 1..10) {
    for (j in 1..10) {
        println("x=$i;y=$j");
        if (i == 5) break@lxx;
    }
}

//forEach中退出当次循环
mutableListOf(1, 2, 3,4).forEach {
    if (it == 2) return@forEach
    print(it);
}

6.3 when

//替换switch Case
var grade = when (score) {
	9, 10 -> "Excellent" 
	in 6..8 -> "Good"
	4, 5 -> "Ok"
	in 1..3 -> "Fail"
	else -> "Fail"
}

when {
	"orange" in items -> println("juicy")
	"apple" in items -> println("apple is fine too")
}

6.4替换3目运算符

val text = if (x > 5)
			  "x > 5"
		   else "x <= 5"
fun maxOf(a: Int, b: Int) = if (a > b) a else b	

15. 其它

15.1 main函数

class KotlinTe { 	}
fun main(args: Array<String>) {
	var output: String? = "123123";
	println(ou);
}

15.2 与java互相调用

//java中调kotlin
	1).调用相同

//kotlin中调java
	1).kotlin中无new关键字，其它属性-方法调用相同

15.3 默认导入

kotlin.*; 							kotlin.annotation.*; 	kotlin.collections.*;
kotlin.comparisons.* (since 1.1); 	kotlin.io.*; 			kotlin.ranges.*;
kotlin.sequences.*; 				kotlin.text.*;	

java.lang.*; 	kotlin.jvm.*;

15.4 kotlin的maven依赖

IDEA maven工程中创建kt文件时会提示增加kotlin依赖，默认即可。(包括dependency和build两类标签)