Kotlin的Lambda表达式

最近在学《Kotlin实战》，顺便记录一下Lambda表达式那一章相关知识点

声明对象

class Person (val name:String ,val age:Int)
val people= listOf(Person("ds",12), Person("afdsdf",54))
//println(people.maxBy { it.age }) 这里it也可以使用
println(people.maxBy (Person :: name))

这样打印结果：main.chapter5.Person@2c7b84de

增加申明对象时data
data class Person (val name:String ,val age:Int)
结果：Person(name=ds, age=12)

Lambda也可以这样写： println(people.maxBy { p:Person ->p.age })
或者：
println(people.maxBy({ p: Person -> p.age }))

输出全部名字：
val name=people.joinToString (separator = " ",transform = {p: Person -> p.name})
println(name)
或者：
val name=people.joinToString (separator = " ") { p: Person -> p.name}

Person ::age   在Kotlin中，有个名字：【成员引用】。

关于filter和map

一个简单的例子

val list = listOf(3,5,7,13233)
print(list.filter { it % 3==0 })
println(people.map(Person::name).filter{it.startsWith("d")})

对象filter：

 val people= listOf(Person("dfds",12), Person("fd",53))
 print(people.filter { it.age>50 }) 
 map 输出所有名字
 println(people.map{it.name})
 maxBy：按年龄
 println(people.filter { it.age== people.maxBy (Person :: age)?.age ?: null }) 

all，any，count，find,groupBy和java8的都差不多

any：任意一个
val judgeBy20={p : Person -> p.age <= 27}
println(people.any(judgeBy20))
结果：true
换成all之后，结果为false
换成count
结果：1
size写法：
println(people.filter(judgeBy20).size)
find：
println(people.find(judgeBy20))
groupby:
println(people.groupBy{it.age})

字符串分组

val list1= listOf("a","abc","ababa","bdaaandadbb");
println(list1.groupBy (String::first ))
结果：
{a=[a, abc, ababa], b=[bdaaandadbb]}

flatMap和flatten：处理嵌套集合中的元素

   data class Book (val name:String,val authors :List<String>)
   flatMap：
    val books= listOf(Book("a1", listOf("a","aa","aaa")),
        Book("b1", listOf("b","bb","bbb")),
        Book("c1", listOf("c","cc","ccc"))
  )
  println(books.flatMap { it.authors })

list集合

//集合转序列
 var lists=people.asSequence()
        .map(Person::name)
        .filter { it.startsWith("d") }
        .toList()

for (i in 0 until lists.size){
    println(lists[i])
}
  
序列：0-100求和
val num= generateSequence(0) { it+1 }
val num0=num.takeWhile { it<=100 }
println(num0.sum())

一个demo，打印字母表

fun alphabet():String {
val result=StringBuilder()
for (letter in 'A'..'Z'){
    result.append(letter)
}
result.append("fdddfsadasf");
return result.toString()
}

改进1：
fun alphabet1():String {
val result=StringBuilder()
return with(result){
    for (letter in 'A'..'Z'){
        result.append(letter)
    }
    result.append("fdddfsadasf");
    this.toString()
}
}

改进2：
fun alphabet2()=with(StringBuilder()){
    for (letter in 'A'..'Z'){
        append(letter)
    }
    append("fdddfsadasf")
    toString()
}

改进3：
fun alphabet3()= buildString{
for (letter in 'A'..'Z'){
    append(letter)
}
append("fdddfsadasf")
toString()
}

 apply改进：
fun alphabetApply()= StringBuilder().apply{
for (letter in 'A'..'Z'){
    append(letter)
}
append("fdddfsadasf")
}.toString()

apply有点类似java中builder。apply时钟返回作为实参传递给的对象。接受者就是对象。

filter函数demo

fun String.filter(predicate: (Char) -> Boolean):String {
val sb=StringBuilder()
for (index in 0 until length) {
    val element=get(index)
    if (predicate(element)) sb.append(element)
}
return sb.toString()
}

 println("ddfjhhk345576878dsfsdf".filter { it in 'a'.. 'z' })

函数类

fun processTheAnswer(f:(Int) -> Int){
println(f(42))
}

java中可以直接调用