问题
UPDATE - 2014/Sep/17
It turns out that even the solution in the prior update (from 2013/Feb/19) fails to work if one places println(Value.Player2) as the first command; i.e. the ordinals are still assigned incorrectly.
I have since created a verifiable working solution as a Gist. The implementation waits to assign the ordinals until after all JVM class/object initialization completes. It also facilitates extending/decorating each enumeration member with additional data while still being very efficient for (de)serialization.
I have also created a StackOverflow answer which elaborates on all the different enumeration patterns being used in Scala (including the solution in the Gist I mention above).
I am working with a fresh install of the TypeSafe IDE (Eclipse with ScalaIDE pre-installed). I'm on Windows 7-64bit. And I have had mixed success with the Scala Worksheet. It has already hard crashed my machine (to a full reset or once to the blue screen of death) three times in less than an hour. So, this may be a bug in the Scala Worksheet. I'm not sure yet and don't have time to chase down that issue. However, this enum issue is stopping me from testing.
I am using the following code in the Scala Worksheet:
package test
import com.stack_overflow.Enum
object WsTempA {
object Value extends Enum {
sealed abstract class Val extends EnumVal
case object Empty extends Val; Empty()
case object Player1 extends Val; Player1()
case object Player2 extends Val; Player2()
}
println(Value.values)
println(Value.Empty)
}
The above works fine. However, if you comment out the first println, the second line throws an exception: java.lang.ExceptionInInitializerError. And I am just enough of a Scala newbie to not understand why it's occurring. Any help would be deeply appreciated.
Here's the stack trace from the right side of the Scala Worksheet (left side stripped to display nicely here):
java.lang.ExceptionInInitializerError
at test.WsTempA$Value$Val.<init>(test.WsTempA.scala:7)
at test.WsTempA$Value$Empty$.<init>(test.WsTempA.scala:8)
at test.WsTempA$Value$Empty$.<clinit>(test.WsTempA.scala)
at test.WsTempA$$anonfun$main$1.apply$mcV$sp(test.WsTempA.scala:14)
at org.scalaide.worksheet.runtime.library.WorksheetSupport$$anonfun$$exe
cute$1.apply$mcV$sp(WorksheetSupport.scala:76)
at org.scalaide.worksheet.runtime.library.WorksheetSupport$.redirected(W
orksheetSupport.scala:65)
at org.scalaide.worksheet.runtime.library.WorksheetSupport$.$execute(Wor
ksheetSupport.scala:75)
at test.WsTempA$.main(test.WsTempA.scala:11)
at test.WsTempA.main(test.WsTempA.scala)
Caused by: java.lang.NullPointerException
at test.WsTempA$Value$.<init>(test.WsTempA.scala:8)
at test.WsTempA$Value$.<clinit>(test.WsTempA.scala)
... 9 more
The class com.stack_overflow.Enum comes from this StackOverflow thread. I have pasted in my version here for simplicity (in case I missed something critical during the copy/paste operation):
package com.stack_overflow
//Copied from https://stackoverflow.com/a/8620085/501113
abstract class Enum {
type Val <: EnumVal
protected var nextId: Int = 0
private var values_ = List[Val]()
private var valuesById_ = Map[Int ,Val]()
private var valuesByName_ = Map[String,Val]()
def values = values_
def valuesById = valuesById_
def valuesByName = valuesByName_
def apply( id : Int ) = valuesById .get(id ) // Some|None
def apply( name: String ) = valuesByName.get(name) // Some|None
// Base class for enum values; it registers the value with the Enum.
protected abstract class EnumVal extends Ordered[Val] {
val theVal = this.asInstanceOf[Val] // only extend EnumVal to Val
val id = nextId
def bumpId { nextId += 1 }
def compare( that:Val ) = this.id - that.id
def apply() {
if ( valuesById_.get(id) != None )
throw new Exception( "cannot init " + this + " enum value twice" )
bumpId
values_ ++= List(theVal)
valuesById_ += ( id -> theVal )
valuesByName_ += ( toString -> theVal )
}
}
}
Any sort of guidance would be greatly appreciated.
UPDATE - 2013/Feb/19
After several cycles with Rex Kerr, here is the updated versions of the code that now works:
package test
import com.stack_overflow.Enum
object WsTempA {
object Value extends Enum {
sealed abstract class Val extends EnumVal
case object Empty extends Val {Empty.init} // <---changed from ...Val; Empty()
case object Player1 extends Val {Player1.init} // <---changed from ...Val; Player1()
case object Player2 extends Val {Player2.init} // <---changed from ...Val; Player2()
private val init: List[Value.Val] = List(Empty, Player1, Player2) // <---added
}
println(Value.values)
println(Value.Empty)
println(Value.values)
println(Value.Player1)
println(Value.values)
println(Value.Player2)
println(Value.values)
package com.stack_overflow
//Copied from https://stackoverflow.com/a/8620085/501113
abstract class Enum {
type Val <: EnumVal
protected var nextId: Int = 0
private var values_ = List[Val]()
private var valuesById_ = Map[Int ,Val]()
private var valuesByName_ = Map[String,Val]()
def values = values_
def valuesById = valuesById_
def valuesByName = valuesByName_
def apply( id : Int ) = valuesById .get(id ) // Some|None
def apply( name: String ) = valuesByName.get(name) // Some|None
// Base class for enum values; it registers the value with the Enum.
protected abstract class EnumVal extends Ordered[Val] {
val theVal = this.asInstanceOf[Val] // only extend EnumVal to Val
val id = nextId
def bumpId { nextId += 1 }
def compare(that: Val ) = this.id - that.id
def init() { // <--------------------------changed name from apply
if ( valuesById_.get(id) != None )
throw new Exception( "cannot init " + this + " enum value twice" )
bumpId
values_ ++= List(theVal)
valuesById_ += ( id -> theVal )
valuesByName_ += ( toString -> theVal )
}
}
}
回答1:
There are two problems here: one is that the code fails to work due to initialization issues, and the other is that you are having IDE problems. I'm going to address only the code failure.
The problem is that you need Empty() run before you actually access Empty, but accessing inner case objects doesn't run the initializer on the outer object since they're only pretending to be members of the inner object. (There is no variable inside Value that holds Empty.)
You can bypass this problem by running the apply() method as part of the initializer for Empty:
object Value extends Enum {
sealed abstract class Val extends EnumVal
case object Empty extends Val { apply() }
case object Player1 extends Val { apply() }
case object Player2 extends Val { apply() }
}
Now your initialization error should go away. (Because you have to do it this way, I suggest that apply() is actually a bad choice of name; maybe set or somesuch would be better (shorter).
If you stick the println into a main method, here's what the bytecode looks like for printing Value.values:
public void main(java.lang.String[]);
Code:
0: getstatic #19; //Field scala/Predef$.MODULE$:Lscala/Predef$;
3: getstatic #24; //Field WsTempA$Value$.MODULE$:LWsTempA$Value$;
6: invokevirtual #30; //Method Enum.values:()Lscala/collection/immutable/List;
9: invokevirtual #34; //Method scala/Predef$.println:(Ljava/lang/Object;)V
12: return
Note line 3, where you get a static field (which means that the JVM ensures that the field is initialized) for Values itself. But if you go for Empty you get
public void main(java.lang.String[]);
Code:
0: getstatic #19; //Field scala/Predef$.MODULE$:Lscala/Predef$;
3: getstatic #24; //Field WsTempA$Value$Empty$.MODULE$:LWsTempA$Value$Empty$;
6: invokevirtual #28; //Method scala/Predef$.println:(Ljava/lang/Object;)V
9: return
Now line 3 refers not to Values but to the inner object, which means the inner object's initializer gets called first, which then calls the outer object's initializer, which then sees that the inner object's initializer is supposed to be done (but it's actually not done)...and it calls a method on it and...boom.
If you put the apply inside the Empty initializer, you are saved because the Value initializer, even though it's called out of order, doesn't call methods on Empty any longer. So it miraculously works out. (Just make sure you don't introduce any other method calls!)
来源：https://stackoverflow.com/questions/14947179/using-a-custom-enum-in-the-scala-worksheet-i-am-receiving-an-error-java-lang-ex