一、继承
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 子类可以覆盖父类的field和method；但是如果父类用final修饰，field和method用final修饰，则该类是无法被继承的， field和method是无法被覆盖的； ### scala> :paste // Entering paste mode (ctrl-D to finish) class Person { private var name = "leo" def getName = name } class Student extends Person { private var score = "A" def getscore = score } // Exiting paste mode, Now interpreting. defined class Person defined class Student warning: prevIoUsly defined object Person is not a companion to class Person. Companions must be defined together; you may wish to use :paste mode for this. scala> val s = new Student s: Student = Student@985696 scala> s.getName res11: String = leo scala> s.getscore res12: String = A scala> s.getName res13: String = leo scala> s.name <console>:14: error: value name is not a member of Student s.name ^
2、override和super
//Scala中，如果子类要覆盖一个父类中的非抽象方法，则必须使用override关键字; //override关键字可以帮助我们尽早地发现代码里的错误，比如：override修饰的父类方法的方法名我们拼写错了；比如要覆盖的父类方法的参数我们写错了；等等 //此外，在子类覆盖父类方法之后，如果我们在子类中就是要调用父类的被覆盖的方法呢？ 那就可以使用super关键字，显式地指定要调用父类的方法 ### scala> :paste // Entering paste mode (ctrl-D to finish) class Person { private var name = "leo" def getName = name } class Student extends Person { private var score = "A" def getscore = score override def getName = "Hi, I'm a student, my name is " + super.getName } // Exiting paste mode, Now interpreting. defined class Person defined class Student warning: prevIoUsly defined object Person is not a companion to class Person. Companions must be defined together; you may wish to use :paste mode for this. scala> val s = new Student s: Student = Student@618ad2aa scala> s.getName res15: String = Hi, I'm a student, my name is leo
3、override field
// Scala中，子类可以覆盖父类的val field，而且子类的val field还可以覆盖父类的val field的getter方法；只要在子类中使用override关键字即可 ### scala> :paste // Entering paste mode (ctrl-D to finish) class Person { val name: String = "Person" def age: Int = 0 } class Student extends Person { override val name: String = "leo" override val age: Int = 30 } // Exiting paste mode, Now interpreting. defined class Person defined class Student warning: prevIoUsly defined object Person is not a companion to class Person. Companions must be defined together; you may wish to use :paste mode for this. scala> val p = new Person p: Person = Person@71d9cb05 scala> p.name res16: String = Person scala> p.age res17: Int = 0 scala> val s = new Student s: Student = Student@1ac4ccad scala> s.name res18: String = leo scala> s.age res19: Int = 30
4、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 ### scala> class Person defined class Person warning: prevIoUsly defined object Person is not a companion to class Person. Companions must be defined together; you may wish to use :paste mode for this. scala> class Student extends Person defined class Student scala> val p: Person = new Student p: Person = Student@f324455 scala> var s: Student = null s: Student = null scala> if (p.isinstanceOf[Student]) s = p.asInstanceOf[Student]
5、getClass和classOf
// isinstanceOf只能判断出对象是否是指定类以及其子类的对象，而不能精确判断出，对象就是指定类的对象 // 如果要求精确地判断对象就是指定类的对象，那么就只能使用getClass和classOf了 // 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 ### scala> class Person defined class Person scala> class Student extends Person defined class Student scala> val p: Person = new Student p: Person = Student@77468bd9 scala> p.isinstanceOf[Person] res0: Boolean = true scala> p.getClass == classOf[Person] res1: Boolean = false scala> p.getClass == classOf[Student] res2: Boolean = true
6、使用模式匹配进行类型判断
// 但是在实际开发中，比如spark的源码中，大量的地方都是使用了模式匹配的方式来进行类型的判断，这种方式更加地简洁明了， 而且代码得可维护性和可扩展性也非常的高 // 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 ### scala> class Person defined class Person scala> class Student extends Person defined class Student scala> val p: Person = new Student p: Person = Student@192d3247 scala> p match { | case per: Person => println("it's Person's object") | case _ => println("unkNown type") | } it's Person's object
7、protected
// 跟java一样，scala中同样可以使用protected关键字来修饰field和method，这样在子类中就不需要super关键字，直接就可以访问field和method // 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 ### scala> :paste // Entering paste mode (ctrl-D to finish) class Person { protected var name: String = "leo" } class Student extends Person { def makeFriends(s: Student) { println("Hi, my name is " + name + ", your name is " + s.name) } } // Exiting paste mode, Now interpreting. defined class Person defined class Student scala> val s1 = new Student s1: Student = Student@63355449 scala> val s2 = new Student s2: Student = Student@1ab3a8c8 scala> s1.makeFriends(s2) Hi, my name is leo, your name is leo scala> :paste // Entering paste mode (ctrl-D to finish) class Person { protected[this] val name: String = "leo" } class Student extends Person { def makeFriends(s: Student) { println("my name is " + name + ", your name is " + s.name) } } // Exiting paste mode, Now interpreting. <console>:22: error: value name is not a member of Student println("my name is " + name + ", your name is " + s.name) ^
// Scala中，每个类可以有一个主constructor和任意多个辅助constructor，而每个辅助constructor的第一行都必须是调用其他辅助constructor或者是主constructor； 因此子类的辅助constructor是一定不可能直接调用父类的constructor的 // 只能在子类的主constructor中调用父类的constructor，以下这种语法，就是通过子类的主构造函数来调用父类的构造函数 // 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 ### scala> :paste // Entering paste mode (ctrl-D to finish) 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, 0.0) } def this(age: Int) { this("leo", age, 0) } } // Exiting paste mode, Now interpreting. defined class Person defined class Student scala> val s = new Student("leo", 30, 100) s: Student = Student@783a467b scala> s.name res6: String = leo scala> s.age res7: Int = 30 scala> s.score res8: Double = 100.0 scala> val s2 = new Student("leo") s2: Student = Student@6f204a1a scala> s2.name res9: String = leo scala> s2.age rES10: Int = 0 scala> val s3 = new Student(30) s3: Student = Student@3224a577 scala> s3.name res11: String = leo scala> s3.age res12: Int = 30 scala> s3.score res13: Double = 0.0
9、匿名子类
// 在Scala中，匿名子类是非常常见，而且非常强大的。Spark的源码中也大量使用了这种匿名子类。 // 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。 ### scala> :paste // Entering paste mode (ctrl-D to finish) class Person(protected val name: String) { def sayHello = "Hello, I'm " + name } val p = new Person("leo") { override def sayHello = "Hi, I'm " + name } def greeting(p: Person {def sayHello: String}) { println(p.sayHello) } // Exiting paste mode, Now interpreting. defined class Person p: Person = $anon$1@3f270e0a greeting: (p: Person{def sayHello: String})Unit scala> greeting(p) Hi, I'm leo
10、抽象类
// 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来来覆盖，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。 // 而一个类中如果有一个抽象方法，那么类就必须用abstract来声明为抽象类，此时抽象类是不可以实例化的 // 在子类中覆盖抽象类的抽象方法时，不需要使用override关键字 ### scala> :paste // Entering paste mode (ctrl-D to finish) abstract class Person(val name: String) { def sayHello: Unit } class Student(name: String) extends Person(name) { def sayHello: Unit = println("Hello, " + name) } // Exiting paste mode, Now interpreting. defined class Person defined class Student scala> val p = new Person <console>:11: error: class Person is abstract; cannot be instantiated val p = new Person ^ scala> val s = new Student("leo") s: Student = Student@368239c8 scala> s.sayHello Hello, leo
11、抽象field
// 如果在父类中，定义了field，但是没有给出初始值，则此field为抽象field // 抽象field意味着，scala会根据自己的规则，为var或val类型的field生成对应的getter和setter方法，但是父类中是没有该field的 // 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 ### scala> :paste // Entering paste mode (ctrl-D to finish) abstract class Person { val name: String } class Student extends Person { val name: String = "leo" } // Exiting paste mode, Now interpreting. defined class Person defined class Student scala> val s = new Student s: Student = Student@1794d431 scala> s.name res1: String = leo
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