Scala 多继承问题
Posted AK47Sonic
tags:
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多继承问题：
object LoadIssueDemo extends App {
import java.io.PrintWriter
trait Logger {
def log(msg: String): Unit
}
trait FileLogger extends Logger {
val fileName: String
val fileOutput = new PrintWriter(fileName: String)
fileOutput.println("#")
def log(msg: String): Unit = {
fileOutput.print(msg)
fileOutput.flush()
}
}
class Person
class Student(var name: String) extends Person with FileLogger {
override val fileName: String = "file.log"
}
new Student("Win").log("trait demo")
}
运行结果：
原因：fileName还未初始化，就被方法调用。
解决方法：
1. 提前定义 （代码不够优雅）
object PreDefineDemo extends App{
import java.io.PrintWriter
trait Logger{
def log(msg: String): Unit
}
trait FileLogger extends Logger {
val fileName: String
val fileOutput = new PrintWriter(fileName: String)
fileOutput.println("#")
def log(msg: String): Unit ={
fileOutput.print(msg)
fileOutput.flush()
}
}
class Person
class Student(var name: String) extends Person with FileLogger{
override val fileName: String = "file.log"
}
new { override val fileName: String = "file.log"} with Student("Win").log("trait demo")
}
2. 懒加载（推荐）
object LazyLoadDemo extends App{
import java.io.PrintWriter
trait Logger{
def log(msg: String): Unit
}
trait FileLogger extends Logger {
val fileName: String
lazy val fileOutput = new PrintWriter(fileName: String)
def log(msg: String): Unit ={
fileOutput.print(msg)
fileOutput.flush()
}
}
class Person
class Student(var name: String) extends Person with FileLogger{
override val fileName: String = "file.log"
}
val s = new Student("Win")
s.log("#")
s.log("Lazy demo")
}
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