scala.MatchError: <SomeStringvalue> (class java.lang.String)
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scala.MatchError: <SomeStringvalue> (of class java.lang.String)
提问by user 923227
I was getting the error:
我收到错误：
Exception in thread "main" scala.MatchError: SomeStringValue (of class java.lang.String)
I found that it was causing because of "SomeStringValue" was not present in any of the cases:
我发现这是因为在任何情况下都不存在“SomeStringValue”：
val test = "SomeStringValue"
test match {
case "Name" => println("Name")
case "Age" => println("Age")
case "Sex" => println("Sex")
}
When I added the else case: _ it ran correctly as below.
当我添加 else 案例时： _ 它运行正确，如下所示。
val test = "SomeStringValue"
test match {
case "Name" => println("Name")
case "Age" => println("Age")
case "Sex" => println("Sex")
case _ => println("Nothing Matched!!")
}
Question: What is the reason that there should always be a matching value in the case construct in Scala?
问题：在 Scala 中，case 构造中总是应该有一个匹配值的原因是什么？
回答by Cyrille Corpet
The matchconstruct is an expression in itself.
该match构建体是在本身的表达式。
Suppose, instead of having printlnstatements, you had integers, then the whole block would be a value of type Integer:
假设，而不是println语句，你有整数，那么整个块将是一个类型的值Integer：
val test = "SomeStringValue"
val count: Int = test match {
case "Name" => 1
case "Age" => 2
case "Sex" => 3
}
Now, what value should countbe ? That's why the matchstatement must handle all possible cases.
现在，应该count是什么值？这就是为什么该match语句必须处理所有可能的情况。
In some cases (such as when pattern matching against a sealed traitor sealed abstract class), the compiler will be able to give you a warning, but most of the time, the error will be thrown at runtime, so you really need to be careful about it.
在某些情况下（例如当模式匹配 asealed trait或 时sealed abstract class），编译器将能够给你一个警告，但大多数时候，错误会在运行时抛出，所以你真的需要小心。
