Scala 多继承问题

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") }

 

转载于:https://www.cnblogs.com/AK47Sonic/p/8016449.html