代码在这里：
abstract class Element {
def contents: Array[String]
val height = contents.length
val width = if(height ==0 ) 0 else contents(0).length
override def toString = contents(0)
}
class ArrayElement(override val contents: Array[String]) extends Element
class LineElementT(s: String) extends Element {
override def contents = Array(s)
}
class LineElementF(s: String) extends Element {
override val contents = Array(s)
}
除了LineElementF之外,这三个子类都可以,LineElementF在使用val lef = new LineElementF(“Wrong”)创建实例时抛出NullPointerException
解决方法
这个NPE发生在高度初始化时.在高度初始化时,LineElementF的val内容未初始化.
您可以通过将高度和宽度都声明为惰性来避免此NPE：
lazy val height = contents.length lazy val width = if(height ==0 ) 0 else contents(0).length
