How come I cannot assign variables in Scala in the following manner?

开发者_运维知识库I have stripped down the method so it doesn't make logical sense but I am just trying to resolve the compile error

def getVWAP(date: Date, maxEvents: Int): Double = {

    var startDateTime = null;

    if (maxEvents > 0) {
        startDateTime = date; // error
    }

    0.0
}

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Scala has used type inference to deduce the type of the variable startDateTime, which you did not specify a type for. So, Scala emits the following error:

error: type mismatch;
 found   : Date
 required: Null
           startDateTime = date; // error

This says, it thinks startDateTime should be of type Null, but you are giving it a Date.

The fix is to explicitly type startDateTime as follows:

var startDateTime : Date = /* some sort of default date */

If your startDateTime is truly optional, consider using Scala's Option instead of using null. This would make your function look like this:

def getVWAP(date: Date, maxEvents: Int): Double = {
    var startDateTime: Option[Date] = None;
    if (maxEvents > 0) {
        startDateTime = Some(date);
    }
    0.0
}

You can read more about the philosophy of Option versus null here. Overly summarized, null is about run-time checking, resulting in a NullPointerException if some variable is null, and Option is about compile-time checking, resulting in a compiler error indicating a potential non-value must be handled. Using Option says you'd rather know at compile time.

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Because startDateTime is declared with type Null. Can't assign a date to that. Declare a type for startDateTime in your declaration.

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It's probably because they type of startDateTime is not inferred as a Date. Try replacing that line with

var startDateTime:Date = //(some initialized Date value)

One big difference between Scala and Java that you'll find is the avoidance of using null. If you really want a variable that might not be set, you should look into Options

In that case, you would define your startDateTime as

var startDateTime:Option[Date] = None
if(maxEvents > 0){
  startDateTime = Some(date)
}