在 Scala 中，final val 和 val 之间的区别
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In Scala, difference between final val and val
提问by elm
In Scala, what is the difference between
在Scala中，有什么区别
val a = 1
and
和
final val fa = 1
回答by Michael Zajac
finalmembers cannot be overridden, say, in a sub-class or trait.
final成员不能被覆盖，例如，在子类或特征中。
Legal:
合法的：
class A {
val a = 1
}
class B extends A {
override val a = 2
}
Illegal:
非法的：
class A {
final val a = 1
}
class B extends A {
override val a = 2
}
You'll get an error such as this:
你会得到这样的错误：
:9: error: overriding value a in class A of type Int(1);
value a cannot override final member
:9: 错误：覆盖类型为 Int(1) 的 A 类中的值 a；
值 a 不能覆盖最终成员
回答by wingedsubmariner
In Scala, finaldeclares that a member may not be overridden in subclasses. For example:
在 Scala 中，final声明一个成员不能在子类中被覆盖。例如：
class Parent {
val a = 1
final val b = 2
}
class Subclass extends Parent {
override val a = 3 // this line will compile
override val b = 4 // this line will not compile
}
Also, as discussed in Why are `private val` and `private final val` different?, if a final valfield is holding a "constant value", a constant primitive type, access to it will be replaced with the bytecode to load that value directly.
另外，正如为什么`private val`和`private final val`不同？, 如果一个final val字段持有一个“常量值”，一个常量原始类型，对它的访问将被替换为字节码以直接加载该值。
回答by Jin
You also cannot use non-finalvals in (Java) annotations.
您也不能finalval在 (Java) 注释中使用 non- s。
For example, this:
例如，这个：
@GameRegistry.ObjectHolder(Reference.MOD_ID)
object ModItems{
}
will only compile if MOD_IDis declared as final.
仅当MOD_ID声明为final.
