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Archive for February, 2009

More Classical Literature

Written by Jevgeni Kabanov on February 21, 2009 – 1:50 pm

The Early History of Smalltalk: Philosophically, Smalltalk’s objects have much in common with the monads of Leibniz and the notions of 20th century physics and biology. — reading texts like that reminds me that at some point I need to delve into classic literature again.


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Is the Relational Database Not an Option in Cloud Computing?

Written by Jevgeni Kabanov on February 20, 2009 – 10:16 pm

InfoQ: Though scalability is a key factor, cloud computing has other advantages that makes it attractive for vendors that do not need to deliver highly scalable applications or services. — you know when something is way overhyped, when it starts to be presented as a solution to all of your problems, even the ones it was never intended for. Also the keywords like “doomed” give a strong hint…


Posted in creative | No Comments »

The Ultimate Java Puzzler

Written by Jevgeni Kabanov on February 16, 2009 – 6:01 pm

Why is this particular one the ultimate? Two reasons:

  • It’s at the very core of the Java language, not some obscure piece of API.
  • It melted my brain when I hit it.

UPDATE 2: If you want to test yourself before reading the post take this test. Results are not saved (it’s a paid feature apparently and I just don’t care enough), but you can post them in the comments.

Let’s start by setting up the puzzler environment. We’ll have three classes in two packages. Classes C1 and C2 will be in package p1:

PLAIN TEXT
JAVA:
  1. package p1;
  2. public class C1 {
  3.   public int m() {return 1;}
  4. }
  5. public class C2 extends C1 {
  6.   public int m() {return 2;}
  7. }

Class C3 will be in a separate package p2:

PLAIN TEXT
JAVA:
  1. package p2;
  2. public class C3 extends p1.C2 {
  3.   public int m() {return 3;}
  4. }

We will also have the test class p1.Main with the following main method:

PLAIN TEXT
JAVA:
  1. public static void main(String[] args) {
  2.   C1 c = new p2.C3();
  3.   System.out.println(c.m());
  4. }

Note that we’re calling the method of C1 on an instance of C3. The output for this example is “3″ as you’d expect. Now let’s change the m() visibility in all three classes to default:

PLAIN TEXT
JAVA:
  1. public class C1 {
  2.   /*default*/ int m() {return 1;}
  3. }
  4. public class C2 extends C1 {
  5.   /*default*/ int m() {return 2;}
  6. }
  7. public class C3 extends p1.C2 {
  8.   /*default*/ int m() {return 3;}
  9. }

The output will now be “2″!

Why is that? The Main class that invokes the method does not see the m() method in the C3 class, it being in a separate package. As far as it cares the chain ends with C2. But as C2 is in the same package it overrides the m() method in C1. This does not seem too intuitive, but that’s the way it is.

Now let’s try something different, let’s change the modifier of C3.m() back to public. What will that do?

PLAIN TEXT
JAVA:
  1. public class C1 {
  2.   /*default*/ int m() {return 1;}
  3. }
  4. public class C2 extends C1 {
  5.   /*default*/ int m() {return 2;}
  6. }
  7. public class C3 extends p1.C2 {
  8.   public int m() {return 3;}
  9. }

Now Main can clearly see the C3.m() method. But amazingly enough output is still “2″!

Apparently C3.m() is not considered to override C2.m() at all. One way to think about it is overriding methods should have access to the super methods (via super.m()). However in this case C3.m() wouldn’t have access to its super method, as it it not visible to it, being in another package. Therefore C3 is considered to be in a completely different invocation chain from C1 and C2. Were we to call C3.m() directly from Main the output would actually be “3″.

Now let’s look at one last example. Protected is an interesting visibility. It behaves like default for members in the same package and like public for subclasses. What will happen if we change all of the visibilities to protected?

PLAIN TEXT
JAVA:
  1. public class C1 {
  2.   protected int m() {return 1;}
  3. }
  4. public class C2 extends C1 {
  5.   protected int m() {return 2;}
  6. }
  7. public class C3 extends p1.C2 {
  8.   protected int m() {return 3;}
  9. }

My reasoning goes like this: as Main is not a subclass of any classes protected should behave as default in this case and output should be “2″. However that is not the case. The crucial thing is that C3.m() has access to super.m() and thus the actual output will be “3″.

Personally, when I first encountered this accessibility issue I got thoroughly confused and couldn’t get it until I did all of this examples through. The intuition I got from this is that if and only if you can access super.m() the subclass is a part of the invocation chain.

UPDATE: Apparently even though the whole thing is obvious to anyone, the intuition I came up with was wrong. A mysterious commenter know only as “C” has provided the following example:

PLAIN TEXT
JAVA:
  1. public class C1 {
  2.   /*default*/ int m() {return 1;}
  3. }
  4. public class C2 extends C1 {
  5.   /*default*/ int m() {return 2;}
  6. }
  7. public class C3 extends p1.C2 {
  8.   /*default*/ int m() {return 3;}
  9. }
  10. public class C4 extends p2.C3 {
  11.   /*default*/ int m() {return 4;}
  12. }

Note that C4 is in the package p1. If we now change the Main code as follows:

PLAIN TEXT
JAVA:
  1. public static void main(String[] args) {
  2.   C1 c = new C4();
  3.   System.out.println(c.m());
  4. }

Then it will output “4″. However super.m() is not accessible from C4 and putting @Override on the C4.m() method will stop the code from compiling. At the same time if we change the main method to:

PLAIN TEXT
JAVA:
  1. public static void main(String[] args) {
  2.   p2.C3 c = new C4();
  3.   System.out.println(c.m());
  4. }

The output will be “3″. This means that C4.m() overrides C2.m() and C1.m(), but not C3.m(). This also makes the issue even more confusing, and the amended intuition is that a method in a subclass overrides a method in a superclass if and only if the method in the superclass is accessible from the subclass. Here superclass can be any ancestor, not necessarily the direct parent and the relation has to be transitive.

For the kicker try reading all of this out from the JVM specification that selects the method to be invoked:


Let C be the class of objectref. The actual method to be invoked is selected by the following lookup procedure:

  • If C contains a declaration for an instance method with the same name and descriptor as the resolved method, and the resolved method is accessible from C, then this is the method to be invoked, and the lookup procedure terminates.
  • Otherwise, if C has a superclass, this same lookup procedure is performed recursively using the direct superclass of C; the method to be invoked is the result of the recursive invocation of this lookup procedure.
  • Otherwise, an AbstractMethodError is raised.


Posted in Featured, meme | 54 Comments »

The Java-Scala Interop

Written by Jevgeni Kabanov on February 10, 2009 – 2:43 am

The Java-Scala Interop — Daniel Spiewak sheds some light on the (quite shady) interoperability options between Java and Scala. A must-read for anyone planning to mix Scala and Java.


Posted in cool | No Comments »

Correcting the Billion Dollar Mistake

Written by Jevgeni Kabanov on February 1, 2009 – 11:04 pm

Last week I visited Stockholm to speak at the JFokus 2009. The event was quite spectacular, but for me the most interesting part occurred on the evening before the conference. I was sitting at the speaker’s dinner with Rickard Öberg, Kirk Pepperdine, Simon Ritter and a couple of others. For some reason or other I started talking to Simon about the problem that’s recently been on my mind. Perhaps it’s been eating me after legendary Tony Hoare said this:

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. [...] This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.

Now since I have a background in functional languages, I know that null references are not necessary. Null value is a member of all types, but some types don’t have or need a natural notion of absence. In fact in Haskell there is no such thing as an “absent” value generic to all types — types either have to declare it explicitly or wrap the underlying value into a Maybe type that has a dedicated Nothing value.

So e.g. the List type has a notion of an empty list encoded into the type:

// Roughly: a list is an empty list
// or a pair of a value and a list
data List a = Nil | Cons a (List a)

While the only way to encode an absent structure is to use Maybe:

// To build a car you need at most one engine
// and zero or more wheels :)
buildCar :: Maybe Engine -> [Wheel]  -> Car

So while I was discussing that with Simon, Kirk banged me on the head to get my attention (just joking, banging me on the head is nowhere near enough to get my attention). Apparently they were discussing more or less the same topic with Rickard, and he had a similar solution for Java. I assumed it was the widely discussed @NotNull annotation, but it was way cooler than that.

Basically in the beginning Rickard started by adding @NotNull support to Qi4J (BTW an amazing piece of engineering, definitely check it out), so that they could automatically insert runtime assertions for the parameters having the @NotNull annotations, something like that:

PLAIN TEXT
JAVA:
  1. class Test {
  2.   void transfer(
  3.     @NotNull Account from,
  4.     Account to,
  5.     double amount) {
  6.       //…
  7.   }
  8.  
  9.   public static void main(String[] args) {
  10.     // Succeeds
  11.     transfer(
  12.       new Account(“Bugs Bunny”), null, 1000000.0);
  13.     // Throws NullPointerException
  14.     transfer(
  15.       null, new Account(“Bugs Bunny”), 1000000.0);
  16.   }
  17. }

However soon after that they discovered that they were inserting @NotNull-s everywhere. So they reverted the notion and decided that they will generate the not null assertions for all parameters except the ones marked as @Optional. And this was the point when I thought to myself “Jevgeni, this is exactly what you were looking for!” So I turn to Rickard and I say “Rickard, this is exactly what I was looking for! This is amazing!” In fact this truly is amazing as it’s exactly captures the semantics of the Maybe type that I liked so much in the functional languages.

After that we go into a discussion whether or not it is possible to validate this assertion during compile-time. Opinions were mixed on this one, though I personally am convinced that it shouldn’t be too easy (your opinion on the topic is welcome, also it will make a great master’s thesis topic). Eventually I get an idea and I say “Rickard, I bet I could implement a JavaRebel plugin that would check this at runtime in about half an hour in 50 lines of code”. And of course Rickard goes “No way!”, so I’m challenged. Next day I sit down for half an hour, then for another half an hour (there was no internet!) and voila — I have a working JavaRebel plugin (in less that 50 lines of code) that will make your methods throw an exception if you try to pass a null reference to a parameter not marked as @Optional (the plugin code deserves another post altogether). Of course I have to show this to Rickard (and he goes “No way!”) and we agree to somehow join forces to promote this sane approach (starting with having a single namespace for the @Optional annotation).

So what do we get? The previous example now looks like this:

PLAIN TEXT
JAVA:
  1. @OptionalCheck
  2. class Test {
  3.   void transfer(
  4.     Account from,
  5.     @Optional Account to,
  6.     double amount) {
  7.       //…
  8.   }
  9.  
  10.   public static void main(String[] args) {
  11.     // Succeeds
  12.     transfer(
  13.       new Account(“Bugs Bunny”), null, 1000000.0);
  14.     // Throws NullPointerException
  15.     transfer(
  16.       null, new Account(“Bugs Bunny”), 1000000.0);
  17.   }
  18. }

As you can understand if we make all of the code without the @Optional annotation to throw a NullPointerException for nulls we’ll break a lot of existing code. Therefore at the moment you also have to annotate the class where you want to enable such semantics with @OptionalCheck.

That’s pretty much it — you can download the plugin right away and just drop it in the classpath when JavaRebel is enabled (you’ll need a 2.0 milestone as 1.x was missing the necessary APIs). At the moment both annotations are in the org.optionalalliance package, but when it changes all you have to do is Organize Imports, so I won’t sweat the naming too much. Please do let us know what do you think of the approach and feel free to advocate it further :)

Cheers,
Jevgeni Kabanov

P.S. The plugin along with the source is also avalable in our Maven repository:


Posted in Featured, creative | 31 Comments »