Implicit parameters and NullPointerException in Sc

问题:

I am trying to define a function that takes an integer and an implicit object that has the code to process that number, but i get a NullPointerException and i don know why. If I delete the first println the code works.

Is there some problem with the way i am defining the implicit objects?

Here is my code:

class A {
  def apply(n : Int) = n*2
}

def f(n: Int)(implicit o : A) = o(n)

implicit val a = new A
println(f(3))

class B extends A {
  override def apply(n: Int) = n*3
}

implicit val b = new B
println(f(3))

And this is the error:

java.lang.NullPointerException
    at Main$$anon$1.f(open.scala:5)
    at Main$$anon$1.<init>(open.scala:8)
    at Main$.main(open.scala:1)
    at Main.main(open.scala)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at scala.tools.nsc.util.ScalaClassLoader$$anonfun$run$1.apply(ScalaClassLoader.scala:78)
    at scala.tools.nsc.util.ScalaClassLoader$class.asContext(ScalaClassLoader.scala:24)
    at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:88)
    at scala.tools.nsc.util.ScalaClassLoader$class.run(ScalaClassLoader.scala:78)
    at scala.tools.nsc.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:101)
    at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:33)
    at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:40)
    at scala.tools.nsc.ScriptRunner.scala$tools$nsc$ScriptRunner$$runCompiled(ScriptRunner.scala:171)
    at scala.tools.nsc.ScriptRunner$$anonfun$runScript$1.apply(ScriptRunner.scala:188)
    at scala.tools.nsc.ScriptRunner$$anonfun$runScript$1.apply(ScriptRunner.scala:188)
    at scala.tools.nsc.ScriptRunner$$anonfun$withCompiledScript$1$$anonfun$apply$mcZ$sp$1.apply(ScriptRunner.scala:157)
    at scala.tools.nsc.ScriptRunner$$anonfun$withCompiledScript$1$$anonfun$apply$mcZ$sp$1.apply(ScriptRunner.scala:157)
    at scala.Option.exists(Option.scala:187)
    at scala.tools.nsc.ScriptRunner$$anonfun$withCompiledScript$1.apply$mcZ$sp(ScriptRunner.scala:157)
    at scala.tools.nsc.ScriptRunner$$anonfun$withCompiledScript$1.apply(ScriptRunner.scala:131)
    at scala.tools.nsc.ScriptRunner$$anonfun$withCompiledScript$1.apply(ScriptRunner.scala:131)
    at scala.tools.nsc.util.package$.waitingForThreads(package.scala:26)
    at scala.tools.nsc.ScriptRunner.withCompiledScript(ScriptRunner.scala:130)
    at scala.tools.nsc.ScriptRunner.runScript(ScriptRunner.scala:188)
    at scala.tools.nsc.ScriptRunner.runScriptAndCatch(ScriptRunner.scala:201)
    at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:58)
    at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:80)
    at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:89)
    at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)

回答1:

This isn't a problem with implicits per se. The issue is that scala is deciding to use b as the implicit parameter to f for both calls, however b isn't initialized yet when the first call happens. The scala compiler should probably complain about ambiguous implicit values here, but the use of both types A and B seems to confuse it.