1、概述

在Scala中，trait是一种特殊的概念，它类似于Java中的接口。trait可以定义方法和字段，但是不能实例化。类可以扩展trait，从而获得trait中定义的方法和字段。

在Scala中，extend关键字用于扩展类或特质。当一个类扩展另一个类时，它继承了父类的所有字段和方法。当一个类扩展一个特质时，它必须实现特质中定义的所有抽象方法。

2、实战代码

trait ScoreQuery {
  def queryScore(subject: String): Option[Double]
}

class A extends ScoreQuery {
  override def queryScore(subject: String): Option[Double] = subject match {
    case "语文" => Some(80)
    case "数学" => Some(90)
    case "英语" => Some(85)
    case _ => None
  }
}

class B extends ScoreQuery {
  override def queryScore(subject: String): Option[Double] = subject match {
    case "美术" => Some(95)
    case "体育" => Some(87)
    case "音乐" => Some(92)
    case _ => None
  }
}

class C extends ScoreQuery {
  private val a = new A()
  private val b = new B()

  override def queryScore(subject: String): Option[Double] =
    a.queryScore(subject).orElse(b.queryScore(subject)) match {
      case s: Some[Double] => s
      case None => subject match {
        case "物理" => Some(88)
        case "化学" => Some(93)
        case "生物" => Some(89)
        case _ => None
      }
    }
}

class D extends C {
  private val a = new A()
  private val b = new B()

  override def queryScore(subject: String): Option[Double] =
    super.queryScore(subject).orElse(a.queryScore(subject)).orElse(b.queryScore(subject)) match {
      case s: Some[Double] => s
      case None =>
        subject match {
          case "政治" => Some(74)
          case "历史" => Some(81)
          case _ => None
        }
    }
}

object BindTest {

  def main(args: Array[String]): Unit = {
    val a = new A()
    val b = new B()
    val c = new C()
    val d = new D()

    println(a.queryScore("语文"))
    println(b.queryScore("体育"))
    println(c.queryScore("化学"))
    println(d.queryScore("政治"))
    println("*" * 100)
    println(c.queryScore("语文"))
    println(c.queryScore("体育"))
    println(d.queryScore("语文"))
    println(d.queryScore("体育"))

  }

}

3、控制台输出

Some(80.0)
Some(87.0)
Some(93.0)
Some(74.0)
****************************************************************************************************
Some(80.0)
Some(87.0)
Some(80.0)
Some(87.0)