forEach
遍历所有Element执行传入的action匿名函数，没有返回值
源码如下所示：
@kotlin.internal.HidesMembers
public inline fun <T> Iterable<T>.forEach(action: (T) -> Unit): Unit {
for (element in this) action(element)
}
代码示例如下所示：
val list = listOf(1, 2, 3, 4, 5)
list.forEach(::println)
map
遍历所有Element执行传入的transform匿名函数，返回转换后的类型实例的集合
源码如下所示：
public inline fun <T, R> Iterable<T>.map(transform: (T)-> R): List<R> {
return mapTo(ArrayList<R>(collectionSizeOrDefault(10)), transform)
}
public inline fun <T, R, C : MutableCollection<in R>> Iterable<T>.mapTo(destination: C, transform: (T) -> R): C {
for (item in this)
destination.add(transform(item))
return destination
}
代码示例如下所示：
val list = listOf(1, 2, 3, 4, 5)
val newList = list.map {
it * 2
}
newList.forEach(::println)
flatMap
遍历集合的集合，返回一个转换后的集合
源码如下所示：
public inline fun <T, R> Iterable<T>.flatMap(transform: (T) -> Iterable<R>): List<R> {
return flatMapTo(ArrayList<R>(), transform)
}
public inline fun <T, R, C : MutableCollection<in R>> Iterable<T>.flatMapTo(destination: C, transform: (T) -> Iterable<R>): C {
for (element in this) {
val list = transform(element)
destination.addAll(list)
}
return destination
}
代码示例如下所示：
val list = listOf(
1..10,
12..24)
list.flatMap { intRange ->
intRange.map { intElement ->
"$intElement"
}
}.forEach(::println)
reduce
求和或求阶乘等，返回结果类型有要求
源码如下所示：
public inline fun <S, T : S> Iterable<T>.reduce(operation: (acc: S, T) -> S): S {
val iterator = this.iterator()
if (!iterator.hasNext()) throw UnsupportedOperationException("Empty collection can't be reduced.")
var accumulator: S = iterator.next()
while (iterator.hasNext()) {
accumulator = operation(accumulator, iterator.next())
}
return accumulator
}
代码示例如下所示：
val list = listOf(1 ,2, 3, 4, 5)
val sum = list.reduce { acc, i ->
acc + i
}
println(sum)
val factorialNum = (1..5).reduce { acc, i -> acc * i }
println(factorialNum)
}
fold
可以是加强版的带初始值的，不限制返回结果类型的reduce，也可以做变换
源码如下所示：
public inline fun <T, R> Iterable<T>.fold(initial: R, operation: (acc: R, T) -> R): R {
var accumulator = initial
for (element in this) accumulator = operation(accumulator, element)
return accumulator
}
代码示例如下所示：
fun factorial(n: Int): Int {
if (n == 0) return 1
return (1..n).reduce { acc, i -> acc * i }
}
fun main(args: Array<String>) {
val sum = (1..3).map(::factorial).fold(5) { acc, i ->
acc + i
}
println(sum)
// 嫌弃字符串最后还有`,`可使用foldIndexed处理
val str = (1..6).map(::factorial).fold(StringBuilder()) { acc, i ->
acc.append(i).append(",")
}
println(str)
println((1..6).map(::factorial).joinToString(","))
}
filter
过滤一些数据，为true则保留，为false则被过滤掉
源码如下所示：
public inline fun <T> Iterable<T>.filter(predicate: (T) -> Boolean): List<T> {
return filterTo(ArrayList<T>(), predicate)
}
public inline fun <T, C : MutableCollection<in T>> Iterable<T>.filterTo(destination: C, predicate: (T) -> Boolean): C {
for (element in this) if (predicate(element)) destination.add(element)
return destination
}
代码示例如下所示：
val list = listOf("a", "b", "", "c")
list.filter(String::isNotEmpty).forEach(::println)
takeWhile
当遇到第一个不符合条件的立马结束迭代，把前面的符合条件的数据存入集合并返回
源码如下所示：
public inline fun <T> Iterable<T>.takeWhile(predicate: (T) -> Boolean): List<T> {
val list = ArrayList<T>()
for (item in this) {
if (!predicate(item))
break
list.add(item)
}
return list
}
代码示例如下所示：
val list = listOf(2, 4, 6, 8, 1, 2, 3, 4)
list.takeWhile {
it % 2 == 0
}.forEach(::println)
let
实际上是一个作用域函数，如定义一个变量在一个特定的作用域范围内。let函数另一个作用就是可以避免写一些判断null的操作。
源码如下所示：
@kotlin.internal.InlineOnly
public inline fun <T, R> T.let(block: (T) -> R): R {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
return block(this)
}
代码示例如下所示：
fun main(args: Array<String>) {
val city = getCityById(110101)
println(city?.id)
println(city?.name)
println(city?.country)
city?.let {
println(city.id)
println(city.name)
println(city.country)
}
}
data class City(val id: Long, val name: String, val country: String = "China")
fun getCityById(id: Long): City? {
return City(id, "Beijing")
}
apply
传入的是扩展的lambda表达式，在调用对象的作用域内直接调用属性或方法，不同于let函数
源码如下所示：
@kotlin.internal.InlineOnly
public inline fun <T> T.apply(block: T.() -> Unit): T {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
block()
return this
}
代码示例如下所示：
fun main(args: Array<String>) {
val city = getCityById(110101)
city?.apply {
println(id)
println(name)
println(country)
}
}
data class City(val id: Long, val name: String, val country: String = "China")
fun getCityById(id: Long): City? {
return City(id, "Beijing")
}
with
不同于apply，主要区别在于receiver是传入进来的
源码如下所示：
@kotlin.internal.InlineOnly
public inline fun <T, R> with(receiver: T, block: T.() -> R): R {
contract {
callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
return receiver.block()
}
代码示例如下所示：
fun main(args: Array<String>) {
val br = BufferedReader(FileReader("build.gradle"))
with(br) {
var line: String?
while (true) {
line = readLine()?:break
println(line)
}
close()
}
}
use
传入closable接口，封装关闭的功能
源码如下所示：
@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
}
}
}
}
代码示例如下所示：
fun main(args: Array<String>) {
val br = BufferedReader(FileReader("build.gradle"))
br.use {
var line: String?
while (true) {
line = readLine()?:break
println(line)
}
}
}
