Case Class（十九）

简介

  Case Class是样例类，能够被优化以用于模式匹配。

• 构造器中的参数如果不被声明为var的话，默认是val类型
• 自动创建伴生对象，实现apply方法，可以不直接显示地new对象
• 伴生对象实现unapply方法，从而可以将case class应用于模式匹配
• 实现toString、hashCode、copy、equals方法

简单实例

//抽象类Person
abstract class Person

//case class Student
case class Student(name:String,age:Int,studentNo:Int) extends Person
//case class Teacher
case class Teacher(name:String,age:Int,teacherNo:Int) extends Person
//case class Nobody
case class Nobody(name:String) extends Person

object CaseClassDemo{
  def main(args: Array[String]): Unit = {
    //case class 会自动生成apply方法，从而省去new操作
    val p:Person=Student("john",18,1024)  
    //match case 匹配语法  
    p  match {
      case Student(name,age,studentNo)=>println(name+":"+age+":"+studentNo)
      case Teacher(name,age,teacherNo)=>println(name+":"+age+":"+teacherNo)
      case Nobody(name)=>println(name)
    }
  }
}

常用方法

//toString方法演示
scala> val s=Teacher("john",38,1024)
s: Teacher = Teacher(john,38,1024)

//无参copy方法演示
scala> val s1=s.copy()
s1: Teacher = Teacher(john,38,1024)

//copy方法是深度拷贝
scala> println(s eq s1)
false

//equal方法根据对象内容进行比较
scala> println(s equals s1)
true

scala> println(s == s1)
true

//hashcode方法
scala> s1.hashCode
res45: Int = 567742485

//toString方法
scala> s1.toString
res46: String = Teacher(john,38,1024)

//带一个参数的copy方法
scala> s1.copy(name="stephen")
res47: Teacher = Teacher(stephen,38,1024)
//带二个参数的copy方法
scala> s1.copy(name="stephen",age=58)
res49: Teacher = Teacher(stephen,58,1024)
//带三个参数的copy方法
scala> s1.copy(name="stephen",age=58,teacherNo=2015)
res50: Teacher = Teacher(stephen,58,2015)

多参数的Case Class

abstract class Person

case class Student( name:String, age:Int, studentNo:Int) extends Person

case class Teacher( name:String, age:Int, teacherNo:Int) extends Person

case class Nobody( name:String) extends Person

//SchoolClass为接受多个Person类型参数的类
case class SchoolClass(classDescription:String,persons:Person*)

//下列代码给出的是其模式匹配应用示例
object CaseClassDemo{
  def main(args: Array[String]): Unit = {
     val sc=SchoolClass("学途无忧网Scala培训班",Teacher("摇摆少年梦",27,2015),Student("摇摆少年梦",27,2015))
     sc match{
       case SchoolClass(_,_,Student(name,age,studetNo))=>println(name)
       case _ => println("Nobody")
     }
  }
}

sealed case class

sealed关键字作用

• 其修饰的trait，class只能在当前文件里面被继承
• 用sealed修饰的目的是告诉scala编译器在检查模式匹配时，让scala能在编译时检查代码是否有漏掉什么没case到的代码，减少编程的错误。

简单实例

//Person最前面加了个关键字sealed
sealed abstract class Person

case class Student( name:String, age:Int, studentNo:Int) extends Person

case class Teacher( name:String, age:Int, teacherNo:Int) extends Person

case class Nobody( name:String) extends Person

case class SchoolClass(classDescription:String,persons:Person*)

object CaseClassDemo{
  def main(args: Array[String]): Unit = {
     val s:Person=Student("john",18,1024)
     //这边仅仅给出了匹配Student的情况，在编译时
     //编译器会提示
     //match may not be exhaustive. It would fail on the following inputs: Nobody(_), Teacher(_, _, _)
     s match{
       case Student(name,age,studentNo)=>println("Student")
     }
  }
}

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