Scala class members and constructor parameters nam

问题:

Consider the following class written in Java:

class NonNegativeDouble {
    private final double value;
    public NonNegativeDouble(double value) {
        this.value = Math.abs(value);
    }
    public double getValue() { return value; }
}

It defines a final field called value that is initialized in the constructor, by taking its parameter called alike and applying a function to it.

I want to write something similar to it in Scala. At first, I tried:

class NonNegativeDouble(value: Double) {
  def value = Math.abs(value)
}

But the compiler complains: error: overloaded method value needs result type

Obviously the compiler thinks that the expression value inside the expression Math.abs(value) refers to the method being defined. Therefore, the method being defined is recursive, so I need to state its return type. So, the code I wrote does not do what I expected it to do: I wanted value inside Math.abs(value) to refer to the constructor parameter value, and not to the method being defined. It is as if the compiler implicitly added a this. to Math.abs(this.value).

Adding val or var (or private ... variants) to the constructor parameter doesn't seem to help.

So, my question is: can I define a property with the same name as a constructor parameter, but maybe a different value? If so, how? If not, why?

Thanks!

回答1:

No, you can't. In Scala, constructor parameters are properties, so it makes no sense to redefine them.

The solution, naturally, is to use another name:

class NonNegativeDouble(initValue: Double) {
  val value = Math.abs(initValue)
}

Used like this, initValue won't be part of the instances created. However, if you use it in a def or a pattern matching declaration, then it becomes a part of every instance of the class.

回答2:

@Daniel C. Sobral

class NonNegativeDouble(initValue: Double) {
  val value = Math.abs(initValue)
}

your code is right, but "constructor parameters are properties",this is not true.

A post from the official site said,

A parameter such as class Foo(x : Int) is turned into a field if it is referenced in one or more methods

And Martin's reply confirms its truth:

That's all true, but it should be treated as an implementation technique. That's why the spec is silent about it.

So normally, we can still treat primary constructor parameters as normal method parameter, but when the parameters is referenced by any of the methods, the compiler will cleverly turn it into a private field.

If any formal parameter preceded by the val, the compiler generates an getter definition automatically.if var, generates a setter additionally. see the language speification section 5.3.

That's all about primary constructor parameters.

回答3:

You can consider parametric field

class NonNegativeDouble(val value: Double, private val name: String ){
  if (value < 0) throw new IllegalArgumentException("value cannot be negative")
  override def toString = 
    "NonNegativeDouble(value = %s, name = %s)" format (value, name)
}

val tom = "Tom"
val k = -2.3

val a = new NonNegativeDouble(k.abs, tom)
a: NonNegativeDouble = NonNegativeDouble(value = 2.3, name = Tom)

a.value
res13: Double = 2.3

a.name
<console>:12: error: value name in class NonNegativeDouble cannot be accessed in NonNegativeDouble
     a.name

val b = new NonNegativeDouble(k, tom)
java.lang.IllegalArgumentException: value cannot be negative
...

It's defines fields and parameters with the same names "value", "name". You can add modifiers such as private ...

回答4:

In the case of case classes it should be:

case class NonNegativeDouble(private val initValue: Double) {
  val value = Math.abs(initValue)
  def copy(value: Double = this.value) = NonNegativeDouble(value)
}

The implementation of copy is required to prevent the sintesized version of the compiler that will bind the initValue argument.

I expect that the compiler is smart enough to not retain the «extra space» for the initValue. I haven't verified this behaviour.