Scala 上下文界定

上下文界定的类型参数形式为T：M的形式，其中M是一个泛型，这种形式要求存在一个M[T]类型的隐式值：

  /**
    * 上下文界定
    */
  @Test
  def testOrdering_Class_Context() = {
    class User(val userName: String, val age: Int)

    class UserOrderingObject extends Ordering[User] {
      override def compare(x: User, y: User): Int = x.age - y.age
    }

    class Pair[T: Ordering](val first: T, val second: T) {
      def smaller(implicit ord: Ordering[T]) = {
        if (ord.compare(first, second) > 0)
          second
        else
          first
      }
    }

    val u1 = new User("u1", 12)
    val u2 = new User("u2", 13)
    //提供隐式值，隐式值需要提供在创建Pair之前
    implicit val uo = new UserOrderingObject
    val p = new Pair(u1, u2)

    println(p.smaller.age)

  }

  /**
    * 上下文界定
    */
  @Test
  def testOrdering_Object_Context_1() = {
    class User(val userName: String, val age: Int)

    //TODO 隐式object伴生对象
    implicit object UserOrderingObject extends Ordering[User] {
      override def compare(x: User, y: User): Int = x.age - y.age
    }

    class Pair[T: Ordering](val first: T, val second: T) {
      def smaller() = {
        //TODO 获取隐式对象
        val ord = implicitly[Ordering[T]]
        if (ord.compare(first, second) > 0)
          second
        else
          first
      }
    }

    val u1 = new User("u1", 12)
    val u2 = new User("u2", 13)
    // 有隐式伴生对象不再需要提供隐式值
    val p = new Pair(u1, u2)

    println(p.smaller.age)

  }

  /**
    * 上下文界定
    */
  @Test
  def testOrdering_Object_Context_2() = {
    class User(val userName: String, val age: Int)

     object UserOrderingObject extends Ordering[User] {
      override def compare(x: User, y: User): Int = x.age - y.age
    }

    class Pair[T: Ordering](val first: T, val second: T) {
      def smaller() = {

        val ord = implicitly[Ordering[T]]
        if (ord.compare(first, second) > 0)
          second
        else
          first
      }
    }

    val u1 = new User("u1", 12)
    val u2 = new User("u2", 13)
    implicit val uo = UserOrderingObject
    val p = new Pair(u1, u2)

    println(p.smaller.age)

  }