[scala之七]继承

一、普通 Scala 对象

 

Person

的类定义：

// This is Scala

class Person(val firstName:String, val lastName:String, val age:Int)

{

 def toString = "[Person: firstName="+firstName+" lastName="+lastName+

                        " age="+age+"]"

}

使用

Person

类型也非常简单：

// This is Scala

object PersonApp

{

 def main(args : Array[String]) : Unit =

 {

   val bindi = new Person("Tabinda", "Khan", 38)

   System.out.println(bindi)

 }

}

 

二、Scala抽象类

// This is Scala

abstract class Person(val firstName:String, val lastName:String, val age:Int)

{

 override def toString = "[Person: firstName="+firstName+" lastName="+lastName+

                         " age="+age+"]"

 def doSomething; // note the semicolon, which is still optional

                  // but stylistically I like having it here

}

为Person创建一个子类

class Student(firstName:String, lastName:String, age:Int)

 extends Person(firstName, lastName, age)

{

 def doSomething =

 {

   System.out.println("I'm studying hard, Ma, I swear! (Pass the beer, guys!)")

 }

}

 
Scala 将函数视为值，在运行时需要切换功能的时候利用函数值。

// This is Scala   

class Person(val firstName:String, val lastName:String, val age:Int)

{

 var doSomething : (Person) => Unit =

    (p:Person) => System.out.println("I'm " + p + " and I don't do anything yet!");

  def work() =

   doSomething(this)

  override def toString = "[Person: firstName="+firstName+" lastName="+lastName+

                         " age="+age+"]"

}

object App

{

 def main(args : Array[String]) =

 {

   val bindi = new Person("Tabinda", "Khan", 38)

   System.out.println(bindi)

    bindi.work()

    bindi.doSomething =

     (p:Person) => System.out.println("I edit textbooks")

    bindi.work()

    bindi.doSomething =

     (p:Person) => System.out.println("I write HTML books")

    bindi.work()

 }

}

三、构造函数

在 Scala 中，主构造函数的参数在

class

行传递，但您也可以为这些参数使用

val

修饰符，以便在类本身上轻松引入读值器（对于

var

，则为写值器）。Scala必须实现一种语法，允许继承类调用基类，同时保留允许我们在基类上引入读值器和写值器的语法。

// This is javap

C:\Projects\scala-inheritance\code>javap -classpath classes Person

Compiled from "person.scala"

public abstract class Person extends java.lang.Object implements scala.ScalaObje

ct{

   public Person(java.lang.String, java.lang.String, int);

   public java.lang.String toString();

   public abstract void doSomething();

   public int age();

   public java.lang.String lastName();

   public java.lang.String firstName();

   public int $tag();

}

 

四、语法差异

Scala 并未像 Java 语言那样将字段与方法区分开来，允许 Scala 程序员轻而易举地向使用基类的用户 “隐藏” 字段和方法之间的差异。

// This is Scala

abstract class Person(val firstName:String, val lastName:String, val age:Int)

{

 def doSomething

  def weight : Int

  override def toString = "[Person: firstName="+firstName+" lastName="+lastName+

                         " age="+age+"]"

}

class Student(firstName:String, lastName:String, age:Int, val subject:String)

 extends Person(firstName, lastName, age)

{

 def weight : Int =

   age // students are notoriously skinny

 def doSomething =

 {

   System.out.println("I'm studying hard, Ma, I swear! (Pass the beer, guys!)")

 }

}

class Employee(firstName:String, lastName:String, age:Int)

 extends Person(firstName, lastName, age)

{

 val weight : Int = age * 4 // Employees are not skinny at all

 def doSomething =

 {

   System.out.println("I'm working hard, hon, I swear! (Pass the beer, guys!)")

 }

}

Scala 实际上允许将

weight

定义为一种方法（如

Student

中所示），也允许将其定义为字段/存取器（如

Employee

中所示）。这种句法决策使您在抽象类继承的实现方面有一定的灵活性。

五、override

Java 5 编译器引入了

@Override

注释。

@Override

验证引入继承类的方法实际上已经覆盖了基类方法。在 Scala 中，

override

已经成为语言的一部分，几乎可以忘记它会生成编译器错误。

// This is Scala

class Student(firstName:String, lastName:String, age:Int, val subject:String)

 extends Person(firstName, lastName, age)

{

 def weight : Int =

   age // students are notoriously skinny

 def doSomething =

 {

   System.out.println("I'm studying hard, Ma, I swear! (Pass the beer, guys!)")

 }

  override def toString = "[Student: firstName="+firstName+

                         " lastName="+lastName+" age="+age+

                         " subject="+subject+"]"

}

六、与java差别

Scala 的类型层次结构与 Java 语言中的对应结构略有不同；从技术上来讲，所有 Scala 类继承的基类（包括

Int

、

Float

、

Double

和其他数字类型）都是

scala.Any

类型，这定义了一组核心方法，可在 Scala 内的任意类型上使用：

==

、

!=

、

equals

、

hashCode

、

toString

、

isInstanceOf

和

asInstanceOf

，大多数方法通过名称即可轻松理解。在这里，Scala 划分为两大分支，“原语类型”继承自

scala.AnyVal

；“类类型” 继承自

scala.AnyRe

。（

scala.ScalaObject

又继承自

scala.AnyRef

。）

// This is Scala

class ScalaJavaPerson(firstName:String, lastName:String, age:Int)

 extends JavaPerson(firstName, lastName, age)

{

 val weight : Int = age * 2 // Who knows what Scala/Java people weigh?

 override def toString = "[SJPerson: firstName="+firstName+

                         " lastName="+lastName+" age="+age+"]"

}

它继承自

JavaPerson

：

// This is Java

public class JavaPerson

{

   public JavaPerson(String firstName, String lastName, int age)

   {

       this.firstName = firstName;

       this.lastName = lastName;

       this.age = age;

   }

    public String getFirstName()

   {

       return this.firstName;

   }

   public void setFirstName(String value)

   {

       this.firstName = value;

   }

    public String getLastName()

   {

       return this.lastName;

   }

   public void setLastName(String value)

   {

       this.lastName = value;

   }

    public int getAge()

   {

       return this.age;

   }

   public void setAge(int value)

   {

       this.age = value;

   }

    public String toString()

   {

       return "[Person: firstName" + firstName + " lastName:" + lastName +

           " age:" + age + " ]";

   }

    private String firstName;

   private String lastName;

   private int age;

}

在编译

ScalaJavaPerson

时，它将照常扩展

JavaPerson

，但按照 Scala 的要求，它还会实现

ScalaObject

接口。并照例支持继承自

JavaPerson

的方法，因为

ScalaJavaPerson

是一种 Scala 类型，我们可以期望它支持

Any

引用的指派，根据 Scala 的规则：

// This is Scala   

    val richard = new ScalaJavaPerson("Richard", "Campbell", 45)

   System.out.println(richard)

   val host : Any = richard

   System.out.println(host)

但在 Scala 中创建

JavaPerson

并将其指派给

Any

引用时会发生什么？

// This is Scala   

    val carl = new JavaPerson("Carl", "Franklin", 35)

   System.out.println(carl)

   val host2 : Any = carl

   System.out.println(host2)

归功于

Any

类型与

java.lang.Object

类型的相似性，几乎可以说，所有扩展

java.lang.Object

的内容都支持存储到

Any

引用之中。