Variables and data types (04) end

Unit type 、Null The type and Nothing type （ a key ）

Basic explanation

data type	describe
Unit	It means no value , And other languages void equivalent . The result type used as a method that does not return any results .Unit Just an instance value , It's written in （）
Null	null,Null Type has only one instance value null
Nothing	Nothing Type in the Scala Type level lowest end of ; It's a subtype of any other type . When a function , We are sure that there is no normal return value , It can be used Nothing To specify the return type , This has an advantage , That is, we can return the value （ abnormal ） Assign to other functions or variables （ Compatibility ）

Case practice

Run code

package chapter02

import chapter01.Student

object Test07_DataType {

    //5 Empty type 
    //5.1 Null value Unit
    Unit  Type is used to identify the process , That is, a function without an explicit return value .
     thus it can be seen ,Unit  Be similar to  Java  Inside  void.Unit  There is only one example ——( ), This example has no real 
     Qualitative meaning 

    def m1():Unit = {
      println("m1 Called execution ")

    }
    val a = m1()
    println("a:"+a)

    //5.2 Null reference Null
    Null  Class has only one instance object ,Null  Be similar to  Java  Medium  null  quote .Null  Can be assigned to any 
     Meaning reference type （AnyRef）, But cannot assign to value type （AnyVal）

    //val n:Int = null      // FALSE ; Value types cannot receive empty references 
    var student : Student = new Student("alice", 20)
    student = null
    println(student)

    //5.3 Nothing
    Nothing, Can be the return type of a method without a normal return value , Very intuitive to tell you this side 
     The method will not return normally , And because of  Nothing  Is a subclass of any other type , It can also be used with methods that require a return value 
     Rong .

    def m2(n:Int):Int ={
      if (n == 0)
        throw new NullPointerException
      else
        return n
    }

    val b= m2(2)
    println("b:"+b)



  }
}

Running results