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映射

映射（Map）是一种可迭代的键值对结构（也称映射或关联）。Scala的Predef类提供了隐式转换，允许使用另一种语法：key -> value，来代替(key, value)。如：Map(“x” -> 24, “y” -> 25, “z” -> 26)等同于Map((“x”, 24), (“y”, 25), (“z”, 26))，却更易于阅读。

collections

colon-equals

It’s very likely you := in a SBT build file. See github.com/harrah/xsbt/wiki/Settings for all of SBT’s assignmenty operators.

IMHO : In My Humble Opinion 恕我直言
What is the difference between = and := in Scala?

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scala def 定义函数的区别

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def foo = {1}

A function which should return a non-Unit value must be declared with the = notation (although of course the compiler can infer the return-type from the expression’s type).

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def foo {1}

返回类型为Unit
In Scala if a method declaration does not have an equal sign before its body, the compiler infers that the result type will be Unit

参考
scala: ‘def foo = {1}’ vs ‘def foo {1}’

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scala apply方法

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class ApplyOperation{

}

class ApplyTest{
	def apply() = println("I am into spark so much!")
	def haveTry: Unit ={
		println("have a try on apply")
	}
}

object ApplyTest{
	def apply() ={
		println("I am into Scala so much")

		new ApplyTest
	}
}

object ApplyOperation{
	def main(args : Array[String]){
		val array = Array(1,2,3,4)

		val a = ApplyTest()// 调用object 的apply

		a.haveTry

		a() // class apply use
	}
}

输出
I am into Scala so much
have a try on apply
I am into Spark so much!!!

参考
scala apply方法 笔记
动手实战Scala中的apply方法和单例对象

scala case class apply

Scala会给case类自动添加一个单例对象

参考探索Scala（4）– Case Classes

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with 关键字

with关键字可以用来实现包装器的功能,新建withSample.scala文件示例如下：

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trait Friendly {
  def greet() = "Hi"
}

class Dog extends Friendly {
  override def greet() = "Woof"
}

class HungryDog extends Dog {
  override def greet() = "I'd like to eat my own dog food"
}

trait ExclamatoryGreeter extends Friendly {
  override def greet() = super.greet() + "!"
}

var pet: Friendly = new Dog
println(pet.greet())

pet = new HungryDog
println(pet.greet())

pet = new Dog with ExclamatoryGreeter
println(pet.greet())

pet = new HungryDog with ExclamatoryGreeter
println(pet.greet())

用scala命令调用(scala withSample.scala)，输出如下

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woof
I'd like to eat my own dog food
woof!
I'd like to eat my own dog food!

张逸的Scala中的Partial Function一文中提到PartialFunction的定义如下:

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trait PartialFunction[-A, +B] extends (A => B) { self =>
  import PartialFunction._
  def isDefinedAt(x: A): Boolean
  def applyOrElse[A1 <: A, B1 >: B](x: A1, default: A1 => B1): B1 =
    if (isDefinedAt(x)) apply(x) else default(x)
}

追本溯源，是因为这里对偏函数值的调用，实则是调用了AbstractPartialFunction的apply()方法(case语句相当于是继承AbstractPartialFunction的子类)：

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	abstract class AbstractPartialFunction[@specialized(scala.Int, scala.Long, scala.Float, scala.Double, scala.AnyRef) -T1, @specialized(scala.Unit, scala.Boolean, scala.Int, scala.Float, scala.Long, scala.Double, scala.AnyRef) +R] extends Function1[T1, R] with PartialFunction[T1, R] { self =>
    def apply(x: T1): R = applyOrElse(x, PartialFunction.empty)
}

看到里面用到了with PartialFunction[T1,R]。

Scala 学习笔记（一）

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java和scala调用函数的区别

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object.method();

在scala中，这样调用,don’t need the semi-colon:

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object.method()

甚至都不用括号(and then you can omit the parentheses):

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object.method

如果方法只有一个参数,

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object.method(param)

你可以把点换成空格，而且可以把括号去掉(you can change the Scala syntax to use a space instead of a dot, while also dropping the parentheses):

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object method param

Scala methods that take a single parameter can be invoked without dots or parentheses.

Scala methods: dots, spaces, and parentheses

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