# Wednesday, 10 March 2004
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To Invert Or Not To Invert

Stuart commented:
I'm not convinced that cli.System.Object should be visible to Java at all. AIUI, Java code will never see instances of cli.System.Object, because all such objects appear to inherit from java.lang.Object instead.

If cli.System.Object *is* visible to Java code, it introduces a paradox: java.lang.Object inherits from cli.System.Object (per the way it's actually implemented) but cli.System.Object should appear to inherit from java.lang.Object (per Java's rule that *everything* inherits from java.lang.Object). Now, it may be possible to create magic glue code that inverts the apparent inheritance relationship like that, but do you really want to go there? :)

The inversion is exactly what I was thinking about. Stuart's analysis above contains a crucial mistake, java.lang.Object does not inherit from cli.System.Object. However, it is virtually impossible not to get confused about this stuff, so let's try to make the discussion a little easier by defining a naming convention:

  • java.lang.Object
    This is the base class of all Java classes (as seen from the Java side of the world).
  • [classpath]java.lang.Object
    This is an implementation artifact of IKVM, it is a .NET type that is used as the base type for all non-remapped Java classes.
  • System.Object
    This is the base class of all .NET classes (as seen from the .NET side of the world).
  • cli.System.Object
    This is the IKVM manifestation of the System.Object type on the Java side of the world.

The paradox is that [classpath]java.lang.Object inherits from System.Object and cli.System.Object inherits from java.lang.Object, but hopefully it is now clear that this isn't a problem. (BTW, one of the definitions of a paradox is "A seemingly contradictory statement that may nonetheless be true").

There are actually two reasons why I would want to do this:

  1. If a Java class extends a .NET type (that was exported using netexp) you see both the virtual methods in java.lang.Object as well as the ones in System.Object that the class in question happens to override (a fairly arbitrary set). By introducing cli.System.Object as the penultimate base class for all .NET types, this can be made much more consistent. cli.System.Object would have final implementations for all the virtual methods in java.lang.Object (to make sure that the essentially non-existing methods don't get overridden) and it would introduce the real virtual methods of System.Object.

  2. If you want to define your own "first class" .NET exception class in Java, you need to extend cli.System.Exception. In other words, it makes for a more powerful programming model to expose the remapped types in this way.

Wednesday, 10 March 2004 10:15:16 (W. Europe Standard Time, UTC+01:00)  #    Comments [6]