用字段覆盖无参数方法时的空指针异常 [英] null pointer exception when overriding a parameterless method with a field

问题描述

我正在研究在Scala中编程"，它说您可以用字段覆盖无参数方法，广告给出了以下示例:

I am working through "Programming in Scala" and it says that you can override a parameterless method with a field, ad gives the following example:

abstract class Element {
    def contents: Array[String]
    val height: Int = contents.length
    val width: Int = if (height == 0) 0 else contents(0).length
}

class ArrayElement(conts: Array[String]) extends Element {
    val contents:Array[String] = conts
}

val a = Array("Hello", "mom")
println(a(0))
val x = new ArrayElement(a)
println(x.contents)
println("Hello, " + x.height)

但是此代码为我生成了一个空指针异常.用"def内容:"替换"val内容:"可以正常工作.如果内容数组确实正确传递，我真的无法理解空指针异常的来源.它似乎来自"val height = contents.length"行，因为用"def height"替换它们也可以正常运行.我对这个示例不了解什么?

However this code produces a null pointer exception for me. Replacing "val contents:" with "def contents:" works fine. I can't really understand where the null pointer exception is coming from, if the contents array is indeed being correctly passed. It seems to be coming from the "val height = contents.length" line, because replacing those with "def height" also runs correctly. What am I not understanding about this example?

推荐答案

这与抽象类中的评估顺序有关.

This is about evaluation order in your abstract class.

val在初始化时评估一次，而def在每次访问时评估. NullPointerException发生在初始化期间，因为height在尚未初始化时访问contents.

vals are evaluated once at initialization time, whereas defs are evaluated every time they are accessed. The NullPointerException happens during initialization time, because height accesses contents at a time when it hasn't been initialized yet.

正如您建议的那样，将height和width转换为def s是防止该问题的一种方法.

As you suggest, turning height and width into defs is one way to prevent the problem.

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