Scala-Object object, Apply method, higher-order function

Object

Object content is static

If it is the same as the class name, it becomes a companion object

There is no static keyword in Scala

Inherit App, you can omit the main method

Apply method

Apply method can only be created in object

class Student(var name : String)

object Student {
	def apply(name : String) = {
		println("调用了apply方法")
		new Student(name)
	}
	
	def main(args : Array[String]) : Unit = {
		var student = Student("Freedom")
		println(student.name)
	}
}

Higher order function

（1）map

Equivalent to a loop, operate on each element in a collection (receive a function), return a new collection

var list = List(1,2,3)
list.map((i : Int) => i * 2)
list.map(_*2)

（2）foreach

foreach has no return value

（3）filter

Filter, select the data that meets, the parameter requires a function that returns a boolean value, filter out all the data that is true

var list = List(1,2,3)
list.filter((i : Int) => i % 2 == 0)
list.filter(_%2 == 0)

（4）zip

Merge two sets

List(1,2,3,).zip(List(4,5,6))
List(1,2,3).zip(List(4,5))

（5）partition

Partition based on the result of an assertion (that is, a certain condition, anonymous function)

var list = List(1,2,3)
list.partition((i : Int) => i % 2 == 0)
list.partition(_%2 == 0)

（6）find

Find the first element that meets the condition

var list = List(1,2,3)
list.find((i : Int) => i % 2 == 0)
list.find(_%2 == 0)

（7）flatten

Find the first element that meets the condition

List(List(1,2,3),List(4,5,6)).flatten

（8）flatmap

Equivalent to a map + flatten

var list = List(List(1,2,3),List(4,5,6))
list.flatmap(i => i.map(_*2))

Closures (nesting of functions)

In a function, contains the definition of another function, you can access the variables of the external function in the internal function

def out(factor: Double) = {
	(x : Double) => x * factor
}
var triple = out(3)
triple(10)
out(3)(10)

Currying

Convert a function with multiple parameters into a function chain, each node is a single function

def add(x: Int,y : Int) = x + y   //原始
def add(x : Int) = (y : Int) => x + y  //闭包
def add(x : Int)(y : Int) = x + y