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Scala example source code file (instanceOfAndTypeMatching.scala)

This example Scala source code file (instanceOfAndTypeMatching.scala) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Scala tags/keywords

any, any, array, i, inner, inner, inners, list, methodinner, methodinners, outer, outer, these, these

The Scala instanceOfAndTypeMatching.scala source code

// Summary of incorrect or questionable behavior.
// Full code and successful parts follow.

object Summary {
  class Outer {
    class Inner { }
    def f() = { class MethodInner ; new MethodInner }
  }
  
  // 1 static issue:
  // 
  //   Given method in MethodInner: def g(other: MethodInner) = ()
  //   method1.g(method1) fails to compile with type error.
  //
  //   Note that this cannot be worked around by widening the return type
  //   of f() because MethodInner is declared inside of f.  So there is no way
  //   I see for a class declared inside a method to receive members of its
  //   own declared type -- not only the narrow type of those from this
  //   instance, but ANY members, because there is no Foo#Bar syntax which will
  //   traverse a method.
  //
  // 4 runtime issues:
  // 
  //   From the outside:     inner1.isInstanceOf[outer2.Inner] is true, should (maybe) be false
  //   From inside inner1:   inner2.isInstanceOf[Outer.this.Inner] is true, should (maybe) be false
  //   From the outside:     inner1 match { case _: outer2.Inner => true ... } is true, should definitely be false
  //   From inside method1:  method2 match { case _: MethodInner => true ... } is true, should definitely be false
  //
  //   Note that the fact that every test returns true on instances of MethodInner means
  //   that it is impossible to draw any type distinction between instances.  As far as one
  //   can tell, they are all of the same type regardless not only of whether they were
  //   created on the same method invocation but whether they are contained in the same
  //   instance of Outer.
  //
  //   WRT "same method invocation", see Iterator.duplicate for an example of this.
}

// Tests

class Outer {
  class Inner {
    def passOuter(other: Outer) = ()                  // pass any Outer
    def passThisType(other: Outer.this.type) = ()     // pass only this Outer instance
    def passInner(other: Inner) = ()                  // pass only Inners from this Outer instance
    def passInner2(other: Outer.this.Inner) = ()      // same as above
    def passInnerSharp(other: Outer#Inner) = ()       // pass any Inner
    
    def compareSimpleWithTypeMatch(other: Any) = other match {
      case _: Inner => true
      case _        => false
    }
    def compareSimpleWithInstanceOf(other: Any) = other.isInstanceOf[Inner]
    
    def compareSharpWithTypeMatch(other: Any) = {
      other match {
        case _: Outer#Inner => true
        case _              => false
      }
    }
    def compareSharpWithInstanceOf(other: Any) = other.isInstanceOf[Outer#Inner]
    
    def comparePathWithTypeMatch(other: Any) = other match {
      case _: Outer.this.Inner  => true
      case _                    => false
    }
    def comparePathWithInstanceOf(other: Any) = other.isInstanceOf[Outer.this.Inner]    
  }
  
  def f() = {
    class MethodInner { 
      def passOuter(other: Outer) = ()                  // pass any Outer
      def passThisType(other: Outer.this.type) = ()     // pass only this Outer instance
      def passInner(other: Inner) = ()                  // pass only Inners from this Outer instance
      def passInner2(other: Outer.this.Inner) = ()      // same as above
      def passInnerSharp(other: Outer#Inner) = ()       // pass any Inner
      def passMethodInner(other: MethodInner) = ()      // pass only MethodInners from this Outer instance
      // is there any way to refer to Outer#MethodInner? Not that there should be.
      
      def compareWithInstanceOf(other: Any) = other.isInstanceOf[MethodInner]
      def compareWithTypeMatch(other: Any) = other match {
        case _: MethodInner => true
        case _              => false
      }
    }
    
    new MethodInner
  }
}

object Test {
  val outer1 = new Outer
  val outer2 = new Outer
  val inner1 = new outer1.Inner
  val inner2 = new outer2.Inner
  val method1 = outer1.f()
  val method2 = outer2.f()
  
  def testInnerStatic = {
    // these should all work
    inner1.passOuter(outer1)
    inner1.passOuter(outer2)
    inner1.passThisType(outer1)
    inner1.passInner(inner1)
    inner1.passInner2(inner1)
    inner1.passInnerSharp(inner1)
    inner1.passInnerSharp(inner2)
    
    // these should all fail to compile, and do
    //
    // inner1.passThisType(outer2)
    // inner1.passInner(inner2)
    // inner1.passInner2(inner2)
  }
  def testInnerRuntime = {
    println("testInnerRuntime\n")
    
    List("These should be true under any scenario: ",
      inner1.isInstanceOf[outer1.Inner] , 
      inner1.isInstanceOf[Outer#Inner] ,
      (inner1: Any) match { case _: Outer#Inner => true ; case _ => false } ,
      (inner1: Any) match { case _: outer1.Inner => true ; case _ => false } ,
      inner1.compareSharpWithTypeMatch(inner2) ,
      inner1.compareSharpWithInstanceOf(inner2)
    ) foreach println
    
    List("These should be true under current proposal: ",
      inner1.compareSimpleWithInstanceOf(inner2) 
    ) foreach println
    
    List("These should be false under current proposal: ",
      inner1.compareSimpleWithTypeMatch(inner2) ,
      inner1.comparePathWithTypeMatch(inner2) 
    ) foreach println
    
    List("These return true but I think should return false: ", 
      inner1.isInstanceOf[outer2.Inner] ,               // true
      inner1.comparePathWithInstanceOf(inner2)          // true
    ) foreach println
    
    List("These are doing the wrong thing under current proposal",
      (inner1: Any) match { case _: outer2.Inner => true ; case _ => false }    // should be false
    ) foreach println
  }

  def testMethodInnerStatic = {
    // these should all work
    method1.passOuter(outer1)
    method1.passOuter(outer2)
    method1.passThisType(outer1)
    method1.passInner(inner1)
    method1.passInner2(inner1)
    method1.passInnerSharp(inner1)
    method1.passInnerSharp(inner2)
    // This fails with:
    //
    // a.scala:114: error: type mismatch;
    //  found   : Test.method1.type (with underlying type MethodInner forSome { type MethodInner <: java.lang.Object with ScalaObject{def passOuter(other: Outer): Unit; def passThisType(other: Test.outer1.type): Unit; def passInner(other: Test.outer1.Inner): Unit; def passInner2(other: Test.outer1.Inner): Unit; def passInnerSharp(other: Outer#Inner): Unit; def passMethodInner(other: MethodInner): Unit} })
    //  required: MethodInner where type MethodInner <: java.lang.Object with ScalaObject{def passOuter(other: Outer): Unit; def passThisType(other: Test.outer1.type): Unit; def passInner(other: Test.outer1.Inner): Unit; def passInner2(other: Test.outer1.Inner): Unit; def passInnerSharp(other: Outer#Inner): Unit; def passMethodInner(other: MethodInner): Unit}
    //     method1.passMethodInner(method1)
    //                             ^
    method1.passMethodInner(method1)
    
    // these should all fail to compile, and do
    //
    // method1.passThisType(outer2)
    // method1.passInner(inner2)
    // method1.passInner2(inner2)
    // method1.passMethodInner(method2)
  }
  
  def testMethodInnerRuntime = {
    println("\ntestMethodInnerRuntime\n")
    
    List("These should be true under any scenario: ",
      method1.compareWithInstanceOf(method1) ,
      method1.compareWithTypeMatch(method1) 
    ) foreach println
    
    List("These should be true under current proposal: ",
      method1.compareWithInstanceOf(method2)
    ) foreach println
    
    List("These are doing the wrong thing under current proposal",
      method1.compareWithTypeMatch(method2)    // should be false
    ) foreach println
  }
  
  def main(args: Array[String]): Unit = {
    testInnerRuntime
    testMethodInnerRuntime
  }
}

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