Kotlin实战读书笔记（二 基础）

fun max(a: Int,b: Int): Int {  // if是表达式（有值的是表达式）
  return if(a>b) a else b
}

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

• 尽量使用 val

val message: String     // 不可变引用可根据条件赋值
if (canPerformOperation()){
     message = "Success"
    // do something
}else{ message = "Failed" }

"Hello,$name"  = ”Hello,${name}"
"Hello,${obj.name}"
"Hello, ${ if (array.size > 0)  array[0] else "Guest" }" //在花括号类使用引号

public class Person {  // java
  private final String name;
  public Person(String name) {
      this.name = name;
  }
  public getName(){ return name; }
}
              ==
class Person(val name: String)  //kotlin

class Person( 
    val name: String, //只读，有getter方法
    var isMarried: Boolean //可变，有getter、setter方法
)

class Rectangle(val height: Int, var width: Int){
    val isSquare: Boolean
        get(){ return height == width }
}

enum class Color { RED,ORANGE,BLUE} //  enum 在class前面才是关键字，所以可做变量名
enum class Color( val r: Int, val g: Int , val b: Int){
    RED(255,0,0),ORANGE(255,165,0); //kotlin唯一必须使用分好的地方，在枚举类中将属性和方法分隔开
    fun rgb() = (r * 265 +g)* 265 + b
}

fun getWarmth(color: Color) = when(color){
    Color.RED,Color.ORANGE -> "warm"
    Color.BLUE -> "cold"
}
import package.Color.* //导入Color的常量
fun getWarmth(color: Color) = when(color){
      RED,ORANGE -> "warm" 
      BLUE -> "cold"
}

fun mix(c1: Color,c2: Color) = when (setOf(c1,c2)){
      setOf(RED,YELLOW) -> ORANGE
      setOf(BLUE,YELLOW) -> GREEN
      else -> throw Exception("Dirty color")
}
                ==
fun mixOptimized(c1: Color,c2: Color) = when{ //when没传参
    c1 == RED && c2 ==YELLOW || c1 == YELLOW && c2 ==RED -> ORANGE
    c1 == BLUE && c2 ==YELLOW || c1 == YELLOW && c2 ==BLUE -> ORANGE
    else -> throw Exception("Dirty color")
}

(1+2)+4 = 7 ：

interface Expr
class Num(val value: Int): Expr
class Sum(val left: Expr,val right: Expr):Expr
fun eval(e: Expr) : Int = when(e){
    is Num -> { 
        // do something
        e.value  //不用像java一样判断了类型还要强转
    } 
    is Sum -> eval(e.left) + eval(e.right)
    else -> throw IllegalArgumentException("Unknown expression")
 }
eval(Sum(Sum(Num(1),Num(2)),Num(4))) // = 7

1..10  // 1-10,闭合的区间
100 downTo 1 step 2  // 100-1的偶数
0 until size == 0...size-1
'A'..'Z' // A到Z的字符区间

for((key,value) in map) {
    value == map[key] // map[key] == map.get(key)
    map[key] = newVal // == map.put(key,newVal)
}

for( item in list)
for((index,item) in list.withIndex()) //带下标迭代数组

'c' in 'a'..'z' // true 底层实现：'a' <= 'c' && 'c' <= 'z'
'c' !in '0'..'9' // false
param in comparable1..comparable2 // in表达式对实现了comparable的所有对象有效