Is the CLR incompatible with Java (as a language)?

Question

I have wondered for a while about the feasibility of having Java run on the CLR.

After seeing a previous question here, I realize that there are quite a few differences between the Sun Java platform and the .NET runtime that would make cross-compiling impossible in all but the most trivial cases.

That being said, isn't IL a Turing-complete language? Couldn't you write a JVM in IL? Of course, the answer is yes, but why even go that far?

My question is:

Is the CLR (as a platform) incompatible with Java as a language (not a platform)?

How much of Java would have to be contorted or broken in order to make it fit?

---

Surely, this could be compiled for the CLR + .NET:

import System.*;

public class HelloWorldExample
{
    public static void main(String args[])
    {
        Console.WriteLine("Hello World !");
    }
}

---

Clarification:

What I'm getting at, is that I want to know what language features of java are incompatible with their CLR counterparts.

For example, I think that Generics are somehow in-congruent. I believe that it is as similar story with exceptions.

Answer 1

A quick search reveals IKVM and the in-progress project Ja.NET.

That said, I could see value in going the other way.

Answer 2

Yes, but given that Java is much more widely used and more standard... wouldn't you want it the other way around, i.e. compile C# to Java bytecode? The answer of course is that it is possible to write a Java compiler that will emit bytecode for CLR or a C# compiler that will emit Java bytecode. The trickiness, though, is that you would need to provide a custom implementation of the standard library / libraries that would defer to the library or libraries for that platform. And given the size of both APIs that would be a fairly significant undertaking. This would probably require a fairly large team to get it done in a reasonable amount of time. I would suggest starting an opensource project if you intend to do that, although given that C#/CLR is a Microsoft-ism, I'm not sure how much enthusiam can be generated.

Answer 3

The CLR was designed with the assumption that people would have to be able to transition to .Net from Java by being able to compile their Java source code largely unmodified. Thus, things in Java you might consider to be problematic in a runtime (like covariant arrays) are included as features in the CLR.

The J# project implements enough of the Java standard library that most Java apps will compile with minimal changes. Note that J# can load JVM bytecodes for compatibility, but J# source always compiles to CLR bytecodes.