Scala syntax in Kafka

Question

I am reading the source code of the class kafka.core.log.LogSegment. Where the syntax of scala gives me huge confusion. I know I could make it clear if I can learn scala in a systematic way But I just don't have that much time since my project awaits.

Definition of the methods:

@volatile private var _maxTimestampSoFar: Option[Long] = None//**#pos 0 constructor??**
  def maxTimestampSoFar_=(timestamp: Long): Unit = _maxTimestampSoFar = Some(timestamp)//**definition 1**
  def maxTimestampSoFar: Long = {//**definition2**
    if (_maxTimestampSoFar.isEmpty)
      _maxTimestampSoFar = Some(timeIndex.lastEntry.timestamp)
    _maxTimestampSoFar.get
  }

Where they are called:

  if (largestTimestamp > maxTimestampSoFar) {//**#pos 3.getter**
        maxTimestampSoFar = largestTimestamp//**#pos4 set the value?**
        offsetOfMaxTimestampSoFar = shallowOffsetOfMaxTimestamp
      }

What confuses me can be concluded into the following:

What is usage of this kind of method with an extra "_" after the identifier of the method like this one here: the maxTimestampSoFar_. When I checked the usage of definition 1 and definition 2, there occurrence overlaps, from which can I conclude they are regarded as the same method like overloaded twins?But since they have different parameters, why we need a difference in the identifier?
As for the place the method is called, is my understanding correct? Is pos 4 the place where definition 1 of the method is called? Then the argument of the parameter is passed just by using the "=" ?
If the second assumption is correct, then the at above pos 0, is it the call of Option's constructor? This is like calling the default constructor?

Hope anyone can help me. Appreciate that.

Harald Gliebe · Accepted Answer · 2020-04-27 06:40:33Z

2

The method name contains also the equals sign, so is maxTimestampSoFar_=. That is how setters are defined in Scala (see Scala getters/setters - best practice?)
Yes, what looks like an assignment in pos 4 will invoke the method defined in 1
Option[Long] can either contain None or Some(<long value>), pos 0 in the code initializes the variable with value None

answered Apr 27, 2020 at 6:40

Harald Gliebe

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3 Comments

Boyu Zhang Over a year ago

"The method name contains also the equals sign". What does the equal sign do with getter or setter? I noticed that the setter has an extra equal sign is, is this a coding convention or due to some reason else?

Boyu Zhang Over a year ago

I am still not confident about the inner logic. ` def maxTimestampSoFar_=(timestamp: Long): Unit = _maxTimestampSoFar = Some(timestamp)`. So can I think it the way that when the setter is called at pos4, it goes one equal sign down to wait the argument and when it get a long, it go further down to the assignment of the field?

Harald Gliebe Over a year ago

The assingment maxTimestampSoFar = value is translated to an invocation maxTimestampSoFar_=(value) That's a Scala naming convention: docs.scala-lang.org/style/…

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