一、Filter
package net.println.kotlin.chapter5.builtins
/**
* @author:wangdong
* @description:Kotlin常见的高阶函数
*/
fun main(args: Array<String>) {
//求（0..6）的阶乘
(0..6).map(::factorial).forEach(::println)
//阶乘过滤，只要阶乘是奇数的
//public inline fun <T> Iterable<T>.filter(predicate: (T) -> Boolean): List<T> {
// return filterTo(ArrayList<T>(), predicate)
//}
println((0..6).map(::factorial).filter { it % 2 == 1 }) //[1, 1]
//要处于奇数位上的阶乘
println((0..6).map(::factorial).filterIndexed{ index, i -> index % 2 == 1 }) //[1, 6, 120]
}
/**reduce求阶乘*/
//一个正整数的阶乘（英语：factorial）是所有小于及等于该数的正整数的积，并且有0的阶乘为1。 自然数n的阶乘写作n!。
//例如3的阶乘：1*2*3 = 6
fun factorial(n: Int): Int{
if (n == 0)return 1
//如果n >0，本次的acc*下一次的acc
return (1..n).reduce{acc,i -> acc * i}
}二、TakeWhile
package net.println.kotlin.chapter5.builtins
/**
* @author:wangdong
* @description:Kotlin常见的高阶函数
*/
fun main(args: Array<String>) {
//求（0..6）的阶乘
(0..6).map(::factorial).forEach(::println)
//1
//1
//2
//6
//24
//120
//720
//现在一个程序是，要按照顺序取数，遇到不符合要求的，程序就停止,例如遇到第一个偶数就停止
println((0..6).map(::factorial).takeWhile { it % 2 == 1 }) //[1, 1]
}
/**reduce求阶乘*/
//一个正整数的阶乘（英语：factorial）是所有小于及等于该数的正整数的积，并且有0的阶乘为1。 自然数n的阶乘写作n!。
//例如3的阶乘：1*2*3 = 6
fun factorial(n: Int): Int{
if (n == 0)return 1
//如果n >0，本次的acc*下一次的acc
return (1..n).reduce{acc,i -> acc * i}
}三、Let的使用
package net.println.kotlin.chapter5.builtins
/**
* @author:wangdong
* @description:Kotlin常见的高阶函数
*/
fun main(args: Array<String>) {
//let的使用
findPerson() ?.let { persion ->
persion.work()
}
//王栋 is working!!!
}
/**定义一个Person类*/
data class Person(var name:String, var age: Int){
fun work(){
println("$name is working!!!")
}
}
fun findPerson(): Person ?{
return Person("王栋",23)
}四、Apply
package net.println.kotlin.chapter5.builtins
/**
* @author:wangdong
* @description:Kotlin常见的高阶函数
*/
fun main(args: Array<String>) {
/**
* Calls the specified function [block] with `this` value as its receiver and returns `this` value.
*/
/*@kotlin.internal.InlineOnly
public inline fun <T> T.apply(block: T.() -> Unit): T {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
block()
return this
}*/
//apply的使用,可以直接调用work()方法，可以使用Persion中的成员变量
findPerson() ?.apply {
work()
print(age)
}
}
/**定义一个Person类*/
data class Person(var name:String, var age: Int){
fun work(){
println("$name is working!!!")
}
}
fun findPerson(): Person ?{
return Person("王栋",23)
}五、With和Apply的区别
With用来传参、Apply用来调用
/**
* Calls the specified function [block] with the given [receiver] as its receiver and returns its result.
*/
@kotlin.internal.InlineOnly
public inline fun <T, R> with(receiver: T, block: T.() -> R): R {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
return receiver.block()
}
/**
* Calls the specified function [block] with `this` value as its receiver and returns `this` value.
*/
@kotlin.internal.InlineOnly
public inline fun <T> T.apply(block: T.() -> Unit): T {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
block()
return this
}六、两种方式读文件内容
package net.println.kotlin.chapter5.builtins
import java.io.BufferedReader
import java.io.FileReader
/**
* @author:wangdong
* @description:Kotlin常见的高阶函数
*/
fun main(args: Array<String>) {
//1.推荐的输出方式
val br = BufferedReader(FileReader("hello.txt")).readText()
println(br)
/*2.同样可以输出
with(br){
var line: String?
while (true){
line = readLine() ?: break
println(line)
}
close()
}*/
//hello 王栋
}七、Use
看看Use的源码
/**
* Executes the given [block] function on this resource and then closes it down correctly whether an exception
* is thrown or not.
*
* @param block a function to process this [Closeable] resource.
* @return the result of [block] function invoked on this resource.
*/
@InlineOnly
@RequireKotlin("1.2", versionKind = RequireKotlinVersionKind.COMPILER_VERSION, message = "Requires newer compiler version to be inlined correctly.")
public inline fun <T : Closeable?, R> T.use(block: (T) -> R): R {
var exception: Throwable? = null
try {
return block(this)
} catch (e: Throwable) {
exception = e
throw e
} finally {
when {
apiVersionIsAtLeast(1, 1, 0) -> this.closeFinally(exception)
this == null -> {}
exception == null -> close()
else ->
try {
close()
} catch (closeException: Throwable) {
// cause.addSuppressed(closeException) // ignored here
}
}
}
}八、Use的小例子
package net.println.kotlin.chapter5.builtins
import java.io.BufferedReader
import java.io.FileReader
/**
* @author:wangdong
* @description:Kotlin常见的高阶函数
*/
fun main(args: Array<String>) {
//1.推荐的输出方式
val br = BufferedReader(FileReader("hello.txt")).use {
var line: String?
while (true) {
line = it.readLine() ?: break
println(line)
}
}
//hello 王栋
}好了，结束了！
