04.scala对象

面向对象的概述

封装：属性、方法封装到类中

User ： int id，String name，Date birthday …

继承：

父类和子类之间的关系

User extends Person

多态（☆）

父类引用指向子类对象 精髓所在 开发框架的基石

Person person = new Person()

User user = new User()

Person person = new User()

类的定义与使用

object SimpleObjectApp { 
def main(args: Array[String]): Unit = { 
val people = new People();
people.name = "Messi" //people.age = 21 println(people.name+"...."+people.age) 
println(people.eat()) 
people.watchFootball("china")
 } 
class People{ 
var name: String =""; //get /set都有 
val age: Int =10; //只有get方法没有set方法 
def eat(): Unit = { println(name + " eating...") } 
def watchFootball(teamName: String)={ 
println(name + " is watching match of "+teamName) 
} 
}
}

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在对象中用private [this] 修饰的变量或者函数只能在class类中使用

image.png

只能在当前类里使用,所以printInfo方法能调用
而people.gender无效

spark的源码用到很多

image.png

构造器

object ConstructorApp { 
def main(args: Array[String]): Unit = { 
val person = new Person("zhangsan",30) println(person.name+ ":"+ person.age+ " : "+ person.school) 
val person2 = new Person("pk",18,"Z") println(person2.name+ ":"+ person2.age+ " : "+ person2.school+" : "+ person2.gender) 
}
}
class Person(val name: String,val age: Int){
//主构造器 
println("Person Constructor enter....") 
val school = "ustc" 
var gender: String = _ //附属构造器 
def this(name: String,age: Int,gender: String){
//附属构造器 this(name,age)//附属构造器的第一行必须要调用主构造器或者其他附属构造器 
this.gender = gender 
} 
println("Person Constructor leave....")
}

继承与重写

object ConstructorApp { 
def main(args: Array[String]): Unit = {
// val person = new Person("zhangsan",30)
// println(person.name+ ":"+// person.age+ " : "+
// person.school)// val person2 = new Person("pk",18,"Z")
// println(person2.name+ ":"+
// person2.age+ " : "+
// person2.school+" : "+
// person2.gender) 
val student = new Student("pk",19,"Math") 
println(student.name+ " " +student.age+ " " +student. major) 
}
}
class Person(val name: String,val age: Int){//主构造器 
println("Person Constructor enter....") 
val school = "ustc" 
var gender: String = _ //附属构造器 
def this(name: String,age: Int,gender: String){
//附属构造器 
this(name,age)
//附属构造器的第一行必须要调用主构造器或者其他附属构造器 
this.gender = gender 
} 
println("Person Constructor leave....")
}
class Student(name: String, age: Int,major: String) extends Person(name,age){ 
println("Student Constructor enter....") println("Student Constructor leaver....")
}

运行结果

Person Constructor enter…

Person Constructor leave…

Student Constructor enter…

Student Constructor leaver…

pk 19 null

会先调用父类构造器，再调用子类构造器，注意子类的特有参数获取不到，子类特有参数需要加var才能获取

重写：属性和函数在父类拥有时子类重写需要加overrride来修饰

override def toString: String = "override def toString"

image.png

Object类必问

抽象类

object AbstactApp {
def main(args: Array[String]): Unit = {
val student = new Student2()
println(student.name+" "+student.age+" ")
student.speak
}
}
/** * 类的一个或者多个方法没有完整的实现（只有定义，没有实现）
*/

abstract class Person2{ 
def speak 
val name: String 
val age: Int
}
class Student2 extends Person2{ 
override def speak: Unit = { 
println("say hello") 
} 
override val name: String ="pk" 
override val age: Int = 18
}

伴生类和伴生对象

object ApplyApp { 
def main(args: Array[String]): Unit = {
 }
}
//伴生类和伴生对象
//如果有一个class 还有一个class同名的object
//那么称这个object是class的伴生对象，class是object的伴生类
class ApplyTest{
}
object ApplyTest{
}

apply

object ApplyApp { 
def main(args: Array[String]): Unit = {
// for (i<- 1.to(10)){
// ApplyTest.incr
// }
// println(ApplyTest.count)
//结果为10 说明object是个单例对象 
var b = ApplyTest() 
//调用的是object方法里面的apply 加了（）默认走的是object里面的apply println("~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~") var c = new ApplyTest() println(c) c()
//类名（） ==》object.apply 
//对象（）==》Class.apply 
}
}
//伴生类和伴生对象
//如果有一个class 还有一个class同名的object
//那么称这个object是class的伴生对象，class的object的伴生类class ApplyTest{ 
def apply() ={ 
println("Class AppltTest apply...") 
new ApplyTest 
}
}
object ApplyTest{ 
println("enter....") 
var count = 0 
def incr={ 
count+=1 
} 
//最佳实战：在object的apply方法中去
new Class 
def apply() ={ 
println("Object AppltTest apply...") 
new ApplyTest 
} 
println("leaver....")
}

面试常问:
看到伴生类名字加个()
默认走的就是object的apply方法
半生对象名字加个()默认走的class的apply方法

case class

object CaseClassApp { 
//通常用在模式匹配 
def main(args: Array[String]): Unit = { println(Dog("wangcai").name) 
}
}
//case class不用newcase class Dog(name: String){
}

trait

Trait xxx extends ATrait with BTrait

例子：

12.png

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