scala 面向对象之继承

scala

1.extends

Scala中，让子类继承父类，与Java一样，也是使用extends关键字

继承就代表，子类可以从父类继承父类的field和method；然后子类可以在自己内部放入父类所没有，子类特有的field和method；使用继承可以有效复用代码

class Person {
private var name = "leo"
def getName = name
}
class Student extends Person {
private var score = "A"
def getScore = score
}

子类可以覆盖父类的field和method；但是如果父类用final修饰，则该类是无法被继承的，field和method用final修饰，field和method是无法被覆盖的

final class Person { //final embellish class Person it not be extends
private var name = "leo"
def getName = name
}
class Student extends Person { //error
private var score = "A"
def getScore = score
}

eg:

scala> final class Person {
 |   private var name = "leo"
 |   def getName = name
 | }
defined class Person
scala> class Student extends Person {
 |   private var score = "A"
 |   def getScore = score
 | }
<console>:8: error: illegal inheritance from final class Person
   class Student extends Person {

override 父类final修饰的字段方法

class Person {
private var name = "leo"
final val age = 10; //final embellish field age it not be override by child class
def getName = name
}
class Student extends Person {
private var score = "A"
def getScore = score
override val age = 20 //error
override def getName = "my name is" + super.getName
}

eg:

scala> class Person {
     |   private var name = "leo"
     |   final val age = 10;   //final embellish field age it not be override by child class
     |   def getName = name
     | }
defined class Person
scala> class Student extends Person {
     |   private var score = "A"
     |   def getScore = score
     |   override val age = 20 //error
     |   override def getName = "my name is" + super.getName
     | }
<console>:11: error: overriding value age in class Person of type Int(10);
 value age cannot override final member
         override val age = 20 //error

2.isInstanceOf 和 asInstanceOf

如果我们创建了子类的对象，但是又将其赋予了父类类型的变量。则在后续的程序中，我们又需要将父类类型的变量转换为子类类型的变量，应该如何做？

首先，需要使用isInstanceOf判断对象是否是指定类的对象，如果是的话，则可以使用asInstanceOf将对象转换为指定类型
注意，如果对象是null，则isInstanceOf一定返回false，asInstanceOf一定返回null
注意，如果没有用isInstanceOf先判断对象是否为指定类的实例，就直接用asInstanceOf转换，则可能会抛出异常

class Person
class Student extends Person
val p: Person = new Student
var s: Student = null //note:isInstanceOf and asInstanceOf use[] instead of ()
if (p.isInstanceOf[Student]) s = p.asInstanceOf[Student]

eg：

scala> print(p.isInstanceOf[Student])
true
scala> print(p.isInstanceOf[Person])
true
scala>

3.getClass 和 classOf

对象.getClass可以精确获取对象的类，classOf[类]可以精确获取类，然后使用==操作符即可判断

class Person
class Student extends Person
val p: Person = new Student
p.isInstanceOf[Person]
p.getClass == classOf[Person]
p.isInstanceOf[Student]
p.getClass == classOf[Student]

eg:

scala> p.getClass == classOf[Person]
res20: Boolean = false
scala> p.getClass == classOf[Student]
res21: Boolean = true

4.使用匹配模式来进行类型判断

使用模式匹配，功能性上来说，与isInstanceOf一样，也是判断主要是该类以及该类的子类的对象即可，不是精准判断的

class Person
class Student extends Person
val p: Person = new Student
p match {
case per: Person => println("it's Person's object")
case _ => println("unknown type")
}

3.protected

还可以使用protected[this]，则只能在当前子类对象中访问父类的field和method，无法通过其他子类对象访问父类的field和method

class Person {
protected var name: String = "leo"
protected[this] var hobby: String = "game"
}
class Student extends Person {
def sayHello = println("Hello, " + name)
def makeFriends(s: Student) { //this s is not be current instance object
println("my hobby is " + hobby + ", your hobby is " + s.hobby)// error
}
}

eg1: error

scala> class Person {
     |   protected var name: String = "leo"
     |   protected[this] var hobby: String = "game"
     | }
defined class Person
scala> class Student extends Person {
     |   def sayHello = println("Hello, " + name)
     |   def makeFriends(s: Student) {
     |     println("my hobby is " + hobby + ", your hobby is " + s.hobby)
     |   }
     | }
<console>:15: error: value hobby is not a member of Student
           println("my hobby is " + hobby + ", your hobby is " + s.hobby)

eg2:

scala> class Person {
     |   protected var name: String = "leo"
     |   protected var hobby:String = "game"
     | }
defined class Person
scala> class Student extends Person {
     |   def sayHello = println("Hello, " + name)
     |   def makeFriends(s: Student) { //this s is not be current instance object
     |     println("my hobby is " + hobby + ", your hobby is " + s.hobby)
     |   }
     | }
defined class Student

4.调用父类的constructor

注意！如果是父类中接收的参数，比如name和age，子类中接收时，就不要用任何val或var来修饰了，否则会认为是子类要覆盖父类的field

class Person(val name: String, val age: Int)
class Student(name: String, age: Int, var score: Double) extends Person(name, age) {
def this(name: String) {
this(name, 0, 0)
}
def this(age: Int) {
this("leo", age, 0)
}
}

5.匿名内部类

匿名子类，也就是说，可以定义一个类的没有名称的子类，并直接创建其对象，然后将对象的引用赋予一个变量。之后甚至可以将该匿名子类的对象传递给其他函数。

class Person(protected val name: String) {
def sayHello = "Hello, I'm " + name
}
val p = new Person("leo") {//p is a anonymity child class's object
override def sayHello = "Hi, I'm " + name
}
def greeting(p: Person { def sayHello: String }) {
println(p.sayHello)
}
greeting(p)//use anonymity class's object as a method parameter

6.抽象类 abstract class and abstract field

如果在父类中，有某些方法无法立即实现，而需要依赖不同的子类来覆盖，重写实现自己不同的方法实现。此时可以将父类中的这些方法不给出具体的实现，只有方法签名，这种方法就是抽象方法。

在子类中覆盖抽象类的抽象方法时，不需要使用override关键字

abstract class Person(val name: String) {
def sayHello: Unit
}
class Student(name: String) extends Person(name) {
def sayHello: Unit = println("Hello, " + name)
}

子类必须覆盖field，以定义自己的具体field，并且覆盖抽象field，不需要使用override关键字

abstract class Person {
val name: String
}
class Student extends Person {
val name: String = "leo"
}