Scala-Lazy

Posted 2020-08-01

在Scala中，提供了lazy值的特性，也就是说，如果将一个变量声明为lazy，则只有在第一次使用该变量时，变量对应的表达式才会发生计算。这种特性对于特别耗时的计算操作特别有用，比如打开文件进行IO，进行网络IO等。

scala> import scala.io.Source._
import scala.io.Source._

scala> lazy val lines = fromFile("C:\\Users\\Administrator\\Desktop\\test.txt").mkString
lines: String = <lazy>

scala> println(lines)
def sayHello(name:String,age:Int)={
if(age >18){
printf("Hi , %s , you are big boy !!!",name)
age
}else{
printf("Hi , %s , you are little boy !!!",name)
age
}
}


scala> lazy val lines = fromFile("C:\\Users\\Administrator\\Desktop\\test2222.txt").mkString
lines: String = <lazy>

scala> println(lines)
java.io.FileNotFoundException: C:\Users\Administrator\Desktop\test2222.txt (系统找不到指定的文件。)
  at java.io.FileInputStream.open0(Native Method)
  at java.io.FileInputStream.open(FileInputStream.java:195)
  at java.io.FileInputStream.<init>(FileInputStream.java:138)
  at scala.io.Source$.fromFile(Source.scala:91)
  at scala.io.Source$.fromFile(Source.scala:76)
  at scala.io.Source$.fromFile(Source.scala:54)
  at .lines$lzycompute(<console>:16)
  at .lines(<console>:16)
  ... 33 elided

scala> val lines = fromFile("C:\\Users\\Administrator\\Desktop\\test2222.txt").mkString
java.io.FileNotFoundException: C:\Users\Administrator\Desktop\test2222.txt (系统找不到指定的文件。)
  at java.io.FileInputStream.open0(Native Method)
  at java.io.FileInputStream.open(FileInputStream.java:195)
  at java.io.FileInputStream.<init>(FileInputStream.java:138)
  at scala.io.Source$.fromFile(Source.scala:91)
  at scala.io.Source$.fromFile(Source.scala:76)
  at scala.io.Source$.fromFile(Source.scala:54)
  ... 33 elided

scala>