In Scala what is the reason that you don't need to use "new" to create a new "case class"? I tried searching for awhile now without answers.
在Scala中，您不需要使用“new”来创建新的“案例类”是什么原因？我试着寻找一段时间没有答案。
3 个解决方案
#1
37
Do you want the how or the why? As the other answer notes, the how is just the apply method on the automatically generated companion object.
你想要的方式或原因？正如另一个答案所说，如何在自动生成的伴随对象上使用apply方法。
For the why: case classes are often used to implement algebraic data types in Scala, and the new-less constructor allows code that is more elegant (creating a value looks more like deconstructing it via pattern matching, for example) and that more closely resembles ADT syntax in other languages.
原因如下：案例类经常用于在Scala中实现代数数据类型，而无新构造函数允许更优雅的代码（例如，创建值看起来更像是通过模式匹配解构它）并且更接近类似其他语言的ADT语法。
#2
14
Case class has prebuilt companion object with apply() implemented. Someone even complains about this: How to override apply in a case class companion :)
Case类具有预构建的伴随对象，并实现了apply（）。有人甚至抱怨这个：如何覆盖案例类伴侣中的应用:)
#3
2
Case classes provide you with an automatically generated apply function on their companion object that you can use like a constructor.
案例类为您的伴随对象提供了一个自动生成的应用函数，您可以像构造函数一样使用它。
In Scala decompiled byte code you will find apply function created as the following :
在Scala反编译的字节代码中，您将找到如下创建的应用函数：
object Person {
def apply(name: String, age: Integer): Person = new Person(name,age)
}
Example :
示例：
case class Person(name: String, age: Integer)
The following three all do the same thing.
以下三个都做同样的事情。
val p0 = new Person("Frank", 23) // normal constructor
val p1 = Person("Frank", 23) // this uses apply
val p2 = Person.apply("Frank", 23) // using apply manually
So if you use val p1 = Person("Frank", 23) it is not a constructor, this a method that call apply method.
因此，如果你使用val p1 = Person（“Frank”，23）它不是构造函数，这是一个调用apply方法的方法。
Please read scala-object-apply-functions for more info.
有关详细信息，请阅读scala-object-apply-functions。
#1
37
Do you want the how or the why? As the other answer notes, the how is just the apply method on the automatically generated companion object.
你想要的方式或原因？正如另一个答案所说，如何在自动生成的伴随对象上使用apply方法。
For the why: case classes are often used to implement algebraic data types in Scala, and the new-less constructor allows code that is more elegant (creating a value looks more like deconstructing it via pattern matching, for example) and that more closely resembles ADT syntax in other languages.
原因如下：案例类经常用于在Scala中实现代数数据类型，而无新构造函数允许更优雅的代码（例如，创建值看起来更像是通过模式匹配解构它）并且更接近类似其他语言的ADT语法。
#2
14
Case class has prebuilt companion object with apply() implemented. Someone even complains about this: How to override apply in a case class companion :)
Case类具有预构建的伴随对象，并实现了apply（）。有人甚至抱怨这个：如何覆盖案例类伴侣中的应用:)
#3
2
Case classes provide you with an automatically generated apply function on their companion object that you can use like a constructor.
案例类为您的伴随对象提供了一个自动生成的应用函数，您可以像构造函数一样使用它。
In Scala decompiled byte code you will find apply function created as the following :
在Scala反编译的字节代码中，您将找到如下创建的应用函数：
object Person {
def apply(name: String, age: Integer): Person = new Person(name,age)
}
Example :
示例：
case class Person(name: String, age: Integer)
The following three all do the same thing.
以下三个都做同样的事情。
val p0 = new Person("Frank", 23) // normal constructor
val p1 = Person("Frank", 23) // this uses apply
val p2 = Person.apply("Frank", 23) // using apply manually
So if you use val p1 = Person("Frank", 23) it is not a constructor, this a method that call apply method.
因此，如果你使用val p1 = Person（“Frank”，23）它不是构造函数，这是一个调用apply方法的方法。
Please read scala-object-apply-functions for more info.
有关详细信息，请阅读scala-object-apply-functions。