scala基础备忘

---

声明一个变量

声明一个常量

显式指定类型

定义一个main函数

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println("hello scala")
  }
}

定义一个普通函数

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println(hello("scala"))
  }

  def hello(name : String) : String = {
    "hello " + name
  }
}

函数不带参数时可以省略括号

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println(hello())
  }

  def hello() : String = {
    "hello scala"
  }
}

定义一个匿名函数

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println(add(1,3))
  }

  def add = (x : Int,y : Int) => x + y
}

把函数赋给一个常量

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println(add(1,3))
  }

  val add = (x : Int,y : Int) => x + y
}

柯里化

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println(add(1)(2))
  }

  def add(x : Int)(y : Int) = x + y
}

可变参数

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println(variable("hello","admln"))
  }

  def variable(s : String*) = {
    s.foreach(x => println(x))
  }
}

参数默认值

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
     println(theDefault())
    println(theDefault("scala"))
  }

  def theDefault(s : String="admln") : String= {
    "hello " + s
  }
}

判断表达式

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
    val num = 5;
    val result = if(num > 0) 1 else 0
    println(result)
  }
}

while循环表达式

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
    var (n,r) = (10,0)
    while(n>0) {
      r = r + n
      n = n - 1
    }
    println(r)
  }
}

for循环表达式

package org.admln.scala

class HelloScala {

}
object HelloScala {
  def main (args: Array[String]) {
    //1<=10
    for(i <- 1 to 10) {
      println(i)
    }
    //1<10
    for(i <- 1 until 10) {
      println(i)
    }
　　 //只输出偶数
    for(i <- 1 to 10 if i % 2 == 0 ) {
      println(i)
    }
  }
}

创建一个类

package org.admln.scala

class HelloScala {

}
class Person {
  var name : String =  _
  val age = 24
}
object MyPerson {
  def main(args: Array[String]) {
    val p = new Person
    p.name = "admln"
    println(p.name + " " + p.age)
  }
}

使用私有属性（外部不能访问）

package org.admln.scala

class HelloScala {

}
class Person {
  var name : String =  _
  val age = 24
  private[this] val gender = "male"
}
object MyPerson {
  def main(args: Array[String]) {
    val p = new Person
    p.name = "admln"
    println(p.name + " " + p.age)
  }
}

构造器

package org.admln.scala

class HelloScala {

}
class Person(var name : String,val age : Int){
  println("main constructor")
  var gender : String = _
  def this(name : String,age : Int,gender : String) {
    this(name,age)
    this.gender = gender
  }
}
object MyPerson {
  def main(args: Array[String]) {
    val p = new Person("admln",24,"male")
    println(p.name + " " + p.age)
  }
}

继承

package org.admln.scala

class HelloScala {

}
class Person(var name : String,val age : Int){
  println("main constructor")
  var gender : String = _
  def this(name : String,age : Int,gender : String) {
    this(name,age)
    this.gender = gender
  }
}
class Student(name : String,age : Int,val major : String) extends Person(name,age) {
  println("subclass is person,major is" + major)
}
object MyPerson {
  def main(args: Array[String]) {
    val s = new Student("admln",24,"spark")
  }
}

覆写父类的方法和字段

package org.admln.scala

class HelloScala {

}
class Person(var name : String,val age : Int){
  println("main constructor")
  var gender : String = "mail"
  def this(name : String,age : Int,gender : String) {
    this(name,age)
    this.gender = gender
  }
  val school = "usa"
}
class Student(name : String,age : Int,val major : String) extends Person(name,age) {
  println("subclass is person,major is" + major)
  override def toString = "override method"
  override val school = "china"
}
object MyPerson {
  def main(args: Array[String]) {
    val s = new Student("admln",24,"spark")
    println(s.toString)
    println(s.gender)
  }
}

抽象类

package org.admln.scala

class HelloScala {

}
abstract class Person {
  def speak
  var name : String
  val age : Int
}
class Worker extends Person {
  def speak {
    println("speak")
  }
  var name = "admln"
  val age = 24
}
object HelloScala extends App {
  val worker = new Worker
  worker.speak
  println(worker.name + " " + worker.age)
}

App是trait的子类，内部实现了main方法并管理我们写的代码

trait

trait中可以带有实现的方法，也可以带有抽象方法，使用trait的方式是使用with混入类中

package org.admln.scala

class UseTrait {

}
trait Logger {
  def log(msg : String) {
    println("log: " + msg)
  }
}

class ConcreteLogger extends Logger {
  def concreteLog {
    log("It's admln")
  }
}

object UseTrait {
  def main(args: Array[String]) {
    val logger = new ConcreteLogger
    logger.concreteLog
  }
}

覆写父类的抽象方法（不用使用override关键字）

package org.admln.scala

class UseTrait {

}
trait Logger {
  def log(msg : String)
}

trait ConcreteLogger extends Logger {
  def log(msg : String) {
    println("log: " + msg)
  }
}

class Test extends ConcreteLogger {
  def test {
    log("It's admln")
  }
}

object UseTrait {
  def main(args: Array[String]) {
    val test = new Test
    test.test
  }
}

如果子trait覆写父trait已经实现的方法，就必须使用override关键字

package org.admln.scala

class UseTrait {

}
trait Logger {
  def log(msg : String) {
    println("log: " + msg)
  }
}

trait ConcreteLogger extends Logger {
  override def log(msg : String) {
    println(" child log: " + msg)
  }
}

class Test extends ConcreteLogger {
  def test {
    log("It's admln")
  }
}

object UseTrait {
  def main(args: Array[String]) {
    val test = new Test
    test.test
  }
}

在对象中混入trait

package org.admln.scala

class UseTrait {

}
trait Logger {
  def log(msg : String) {
    println("log: " + msg)
  }
}

trait ConcreteLogger extends Logger {
  override def log(msg : String) {
    println(" child log: " + msg)
  }
}

class Test extends Logger{
  def test {
    log("It's admln")
  }
}

object UseTrait {
  def main(args: Array[String]) {
    val test = new Test with ConcreteLogger
    test.test
  }
}

apply方法和静态方法

package org.admln.scala

class UsageOfApply {

}

class ApplyTest {
  def test {
    println("test")
  }
}

object ApplyTest {
  def apply() = new ApplyTest
  def staticTest {
    println("It's static method")
  }
}

object UsageOfApply {
  def main(args: Array[String]) {
    ApplyTest.staticTest
    val at = ApplyTest()
    at.test
  }
}

object 中的方法和属性都是静态的，所以是单例对象的理想载体
object本身就是一个单例对象

---

函数式编程，把运算过程尽量写成一系列嵌套的函数调用

函数的定义

def 函数名称(变量名称 ： 变量类型) : 函数返回值类型 = { 函数体 }

如果函数仅包含一条语句，那么花括号可以选择不写

值函数

匿名函数

(参数名称 ： 参数类型) => 表达式

把匿名函数赋值给一个常量

---

闭包

代码与用到的非局部变量的混合

闭包 = 代码 + 非局部变量

---

高阶函数map函数

下划线代表数组中的每一个元素

高阶函数filter函数

高阶函数reduce函数

---

scala中的集合：List、Set、Tuple、Map

它们内部都是以apply方式来完成实例化的

List和Set

和java中的Set一样，里面不会存在相同元素

Tuple

下标从1开始

Map

Option

Option代表一个可有可无的值，有两个子类：Some和None

zip操作

partition操作

flatten操作

flatmap操作：map和flatten的结合

---