第21章：Scalaの型パラメータ ref: http://qiita.com/f81@github/items/7a664df8f4b87d86e5d8

file0.java

public class ArrayList<E> {...}

file1.java

final List<String> arrays = new ArrayList<String>()

file10.scala

scala> new School[Animal]
<console>:10: error: type arguments [Animal] do not conform to class School's type parameter bounds [A <: Person]
       val res2 =
           ^
<console>:11: error: type arguments [Animal] do not conform to class School's type parameter bounds [A <: Person]
              new School[Animal]
                  ^

file11.scala

scala> class School[A >: Person]
defined class School

file12.scala

scala> new School[Animal]
res0: School[Animal] = School@4d832421

file13.scala

scala> new School[Person]
res1: School[Person] = School@4d032cfc

file14.scala

scala> new School[Student]
<console>:11: error: type arguments [Student] do not conform to class School's type parameter bounds [A >: Person]
       val res2 =
           ^
<console>:12: error: type arguments [Student] do not conform to class School's type parameter bounds [A >: Person]
              new School[Student]
                  ^

file15.scala

scala> class School[A >: Student <: Animal]
defined class School

file16.java

final List<String> list = new ArrayList<String>();
final List<Object> objList = list;

file17.scala

sealed abstract class List[+A] extends AbstractSeq[A] with LinearSeq[A] with Product with GenericTraversableTemplate[A, List] with LinearSeqOptimized[A, List[A]]

file18.scala

class List[+A]

file19.scala

scala> class School[+T]
defined class School

scala> def receive(args: School[Person]) = println("success!!!")
receive: (args: School[Person])Unit

file2.scala

object TypeParamSample {

  class TypeParam[T](val t: T) {
    def get: T = this.t
  }

  def main(args: Array[String]): Unit = {
    val stringTypeParam = new TypeParam[String]("test")
    println(stringTypeParam.get)

    val intTypeParam = new TypeParam[Int](1)
    println(intTypeParam.get)
  }

}

file20.scala

scala> receive(new School[Person])
success!!!

file21.scala

scala> receive(new School[Student])
success!!!

file22.scala

scala> receive(new School[Animal])
<console>:12: error: type mismatch;
 found   : School[Animal]
 required: School[Person]
              receive(new School[Animal])
                      ^

file23.scala

scala> class School[-T]
defined class School

scala> def receive(args: School[Person]) = println("success!!!")
receive: (args: School[Person])Unit

scala> receive(new School[Person])
success!!!

scala> receive(new School[Animal])
success!!!

scala> receive(new School[Student])
<console>:13: error: type mismatch;
 found   : School[Student]
 required: School[Person]
              receive(new School[Student])
                      ^

file24.scala

scala> class School[T]
defined class School

scala> def receive(args: School[Person]) = println("success!!!")
receive: (args: School[Person])Unit

scala> receive(new School[Person])
success!!!

scala> receive(new School[Animal])
<console>:12: error: type mismatch;
 found   : School[Animal]
 required: School[Person]
Note: Animal >: Person, but class School is invariant in type T.
You may wish to define T as -T instead. (SLS 4.5)
              receive(new School[Animal])
                      ^

scala> receive(new School[Student])
<console>:13: error: type mismatch;
 found   : School[Student]
 required: School[Person]
Note: Student <: Person, but class School is invariant in type T.
You may wish to define T as +T instead. (SLS 4.5)
              receive(new School[Student])
                      ^

file3.scala

  class TypeParam[T](val t: T) {
    def get: T = this.t
  }

file4.scala

  def main(args: Array[String]): Unit = {
    val stringTypeParam = new TypeParam[String]("test")
    println(stringTypeParam.get)

    val intTypeParam = new TypeParam[Int](1)
    println(intTypeParam.get)
  }

file5.txt

$ scalac TypeParamSample.scala
$ scala TypeParamSample
test
1

file6.scala

scala> class Animal
defined class Animal

scala> class Person extends Animal
defined class Person

scala> class Student extends Person
defined class Student

file7.scala

scala> class School[A <: Person]
defined class School

file8.scala

scala> new School[Student]
res0: School[Student] = School@7de3ec13

file9.scala

scala> new School[Person]
res1: School[Person] = School@35203079