在Scala中处理java.lang.NullPointerException

Question

Say I have a model class as shown below:

class User () {
  var id: Option[java.lang.Long] = _
  var name: Option[String] = _
  var date: Option[java.util.Date] = _

  def this(id: java.lang.Long,name: String, date: java.util.Date){
    this()
    this.id=Option(id)
    this.name=Option(name)
    this.date=Option(date)
  }
}

I tried to create an object by doing

var obj = new User
obj.id=12345
obj.name=user

Obviously obj.date is null , and I kept getting a java.lang.NullPointerException at this point. How should I handle this exception?

Answer 1

I would advise you to initialize the values directly through the constructor. This looks like a POSO (Plain Old Scala Object), so maybe you would be better off with an immutable case class:

case class User(id: Option[Long], name: Option[String], date: Option[Date])

Now if you want to initialize only some of the properties, you can either pass None or create an auxiliary constructor via the companion object:

case class User(id: Option[Long], name: Option[String], date: Option[Date])
object User {
  def apply(id: Option[Long], name: Option[String]) = {
    User(id, name, None)
  }
}

Edit:

If Java interop is important, you can define multiple constructors inside the case class which will be visible in Java:

case class User(id: Option[Long], name: Option[String], date: Option[Date]) {
  def this(id: Long, name: String) = {
    this(Some(id), Some(name), None)
  }
}

Answer 2

One nice thing about Option[T] is that it has a safe default value None . So initialise your vars to None instead of null .

You should also think about making your model class immutable.

Answer 3

More easy way to do this is

case class User(id: Option[Long]=None, name: Option[String]=None, date: Option[Date]=None)

In this case if you give a value it takes the value or else it would take None as a default value.