主方法中的Scala App val初始化 [英] Scala App val initialization in main method

问题描述

我有一些代码:

object Main extends App
{
    val NameTemplate = """^([A-Za-z]+)_(\d+)\.png""".r

    override def main (args:Array[String])
    {
        // Why is NameTemplate null here?
    }
}

为什么在主方法中未初始化NameTemplate?

Why is NameTemplate not initialized within the main method?

推荐答案

如果使用的是App特征，则无需覆盖main方法-只需将代码编写在:

If you are using App trait, then you don't need to override main method - just write your code in the body of the object:

object Main extends App {
    val NameTemplate = """^([A-Za-z]+)_(\d+)\.png""".r

    println(NameTemplate)

    val NameTemplate(name, version) = args(0)

    println(name + " v" + version)

}

之所以有效，是因为App特性扩展了 DelayedInit 特征具有非常特殊的初始化过程.如示例所示，您甚至可以使用args访问参数.

It works because App trait extends DelayedInit trait which has very special initialization procedure. You can even access arguments with args, as shown in the example.

如果您不想扩展App，您仍然需要编写main方法，但是在这种情况下，它可以按预期工作:

You still need to write main method if you don't want to extend App, but in this case it will work as expected:

object Main {
    val NameTemplate = """^([A-Za-z]+)_(\d+)\.png""".r

    def main(args: Array[String]) {
        println(NameTemplate)

        val NameTemplate(name, version) = args(0)

        println(name + " v" + version)
    }

}

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