# Wednesday, January 3, 2007
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In the entry about object schizophrenia I used java.lang.Comparable as an example in J#, but for IKVM I did not mention it as a ghost interface. In the comments Stuart Ballard asked why Comparable isn't a ghost interface.

Unlike what Stuart claimed, this is definitely not a stupid question. He also figured out part of the answer: java.lang.Comparable maps to System.IComparable.

That's not the whole story though. The reason that I originally mapped Comparable to IComparable was that System.String conveniently implements IComparable and that IComparable.CompareTo is semantically (nearly) identical to Comparable.compareTo. Back then I hadn't yet "invented" the ghost interface idea and this seemed to solve most of the issues (the other two ghost interface candidates, Cloneable and Serializable are only used as marker interfaces, so it didn't seem like a big deal that System.String didn't implement these interfaces. java.lang.CharSequence only appeared in Java 1.4 and I vaguely knew about it, but at the time I chose not to worry about that yet.)

 A nice side effect of mapping Comparable to IComparable is that you get better interop (e.g. Java objects that implement Comparable will sort correctly when used in .NET applications and .NET objects that implement IComparable will sort correctly in Java), so that meant that when I developed ghost interfaces I didn't really want to go back and change Comparable into a ghost interface.

Finally, there is one more complication. Even though Comparable.compareTo and IComparable.CompareTo have identical semantics, there still is a problem with String. Java's String.compareTo is specified more strictly than Comparable.compareTo. The interface method is specified to simply return zero or a signed or unsigned integer, but the String version is actually specified to return the difference between the mismatching characters (or zero, if the strings match.) To handle this correctly, when you call Comparable.compareTo, you're actually calling a static method Comparable.__Helper.compareTo that first does a check to see if the object you're comparing is a String and, if so, it calls a static helper method that implements the specified Java comparision algorithm.

BTW, the reason that the static compareTo helper method lives inside a nested __Helper class is because Reflection.Emit (on .NET 1.x) incorrectly prohibits adding static methods to interfaces (which is entirely legal according to the CLI specification).

Performance Impact

The following table compares the direct methods calls with the interface method calls. Note that these basically represent a worst case estimate, because of limit amount of work the actual compareTo method does. Note also that for notational convenience I used the zero digit, but in the actual benchmark I used a local variable containing new Integer(0).

Method Call Time (ns)
"".compareTo((Object)"") 140
((Comparable)"").compareTo("") 220
0.compareTo(0) 6
((Comparable)0).compareTo(0) 35

These results are on .NET Framework 2.0 (x86). Averaged over 10,000,000 calls and includes the loop overhead.

For comparison, here are the results for JDK 1.5 HotSpot Client VM:

Method Call Time (ns)
"".compareTo((Object)"") 17
((Comparable)"").compareTo("") 21
0.compareTo(0) 7
((Comparable)0).compareTo(0) 15
Wednesday, January 3, 2007 12:12:02 PM (W. Europe Standard Time, UTC+01:00)  #    Comments [0]