对恒定值使用def和val有什么影响 [英] What are the implications of using def vs. val for constant values

问题描述

在Scala中使用 def 与 val 来定义一个常量，不可变的值有什么影响？我明显可以写出如下内容：

  val x = 3; 
 def y = 4; 
 var a = x + y; // 7 
  

这两种说法有什么区别？哪一个表现更好/是推荐方式还是更习惯？

解决方案

假设这些是类级声明：

编译器会产生一个 val final ，这可以导致更好的优化代码VM

def 不会将值存储在对象实例中，因此会节省内存，但要求每次评估该方法。

为了两全其美，创建伴随对象并声明常量为 val s there。

即而不是

  class Foo {
 val MyConstant = 42 
} 
  
 
 $ b $ p $  class Foo { } 
 
 object Foo {
 val MyConstant = 42 
} 
  




What are the implications of using def vs. val in Scala to define a constant, immutable value? I obviously can write the following:

val x = 3;
def y = 4;
var a = x + y; // 7

What's the difference between those two statements? Which one performs better / is the recommended way / more idiomatic? When would I use one over the other?

解决方案

Assuming these are class-level declarations:

The compiler will make a val final, which can lead to better-optimised code by the VM.

A def won't store the value in the object instance, so will save memory, but requires the method to be evaluated each time.

For the best of both worlds, make a companion object and declare constants as vals there.

i.e. instead of

class Foo {
  val MyConstant = 42
}

this:

class Foo {}

object Foo {
  val MyConstant = 42
}

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