scala 半生对象和半生类

object：半生对象
object中的构造器在第一次调用执行一次，以后调用的话不会多次执行。
object会有自己的构造方法，默认是没有参数的构造方法


object放一些静态的成员或方法
如SparkContext同时有类和object

半生类和半生对象放在同一个文件夹中
半生对象中有一个apply方法
构造类的时候一般不用new Person（）而是不用new


class Person是object Person的半生类
object Person是class Person的半生对象
scala> /////////////////////

scala> object Person{
     |   println("scala")
     |   var salary = 0.0
     |   def getSalary = salary
     | }
defined module Person
warning: previously defined class Person is not a companion to object Person.
Companions must be defined together; you may wish to use :paste mode for this.

scala> Person.getSalary  --object名直接调用
scala      --只打印一次，后面执行不打印
res131: Double = 0.0

scala> Person.getSalary
res132: Double = 0.0


scala> /////////////////////


scala> class Person{
     |   println("scala in class...")
     |   var salary = 0.1
     |   def getSalary = salary
     | }
defined class Person
warning: previously defined object Person is not a companion to class Person.
Companions must be defined together; you may wish to use :paste mode for this.

scala> Person.getSalary  --还是上面object的调用
res133: Double = 0.0

scala> val p = new Person   --class只能new出来使用
scala in class...
p: Person = Person@2059a675

scala> p.getSalary
res134: Double = 0.1