Programming with Passsion

Begin to Read The Art of Computer Programming

2009-05-04T07:52:00.000-07:00

I baugh the book more than one year ago. Due to busy development work and other stuff to learn, I have not read it. Now it is time for me to read this great book. I will write notes when reading it.

Reading of Programming Language Pragmatics

2009-05-04T03:01:00.000-07:00

After more than a year, I have finally finished reading PLP 2e. It is a wonderful book. Every programmer who want a complete and deep of programming language theory should read it. It teaches you compiler techniques. But unlike the dragon book which is for writing compilers, it teaches you the compiler concepts.

The book is written by Miachael Scott who is academic. I like his writing style, succint and precise. For one sentence of his writing, I often need a page to describe it. After his writing has a academic flavor, it is easy for non-academics to understand.

But there are two problems with this book. Only an instructor using this book can obtain the solutions to the exercises in the book. As a software professional outside of schools, I am not entitled to get these solutions. It is really frustrated if I can't solve one exercise and can't get the solution provided by the author. As a result, I did not do the exercises in the book.

The other problem is that the author often mention some technique in a short sentence. I often need some considerable time to understand the sentence because I am teaching myself.

During the reading of this book, I find some bugs of the book. I have emailed to the author. The author has listed these bugs in the errdata page.

Seeking in PrintStream

2009-05-03T05:18:00.001-07:00

I played with maven today. I found the following output message when maven is downloading something. The xxx changes to indicate the download progress.


xxx/yyyK

I am curious about how this is done. As far as I know, System.out is PrintStream. In PrintStream, there is no method to seek. I searched for the answer on the web. Unfortunately, I can't find the answer. I downloaded the maven source code. After some browsing, I found that printing \r will position the following printings at the beginning of a line. The following Java code shows how it is done.


import java.io.PrintStream;

public class Print {

    public static void main(String[] args) {
        PrintStream ps = System.out;        
        ps.print("100");
        sleep2s();
        ps.print("\r200");
        sleep2s();
        ps.print("\r300");
    }

    private static void sleep2s() {
        try {
            Thread.sleep(2 * 2000);
        } catch (InterruptedException ie) {
            throw new RuntimeException(ie);
        }
    }
}

I think that Javadoc should have documented this kind of usage, which can save developers a lot of time to scratch their heads.

Install PHP 5.2.9-2 (Windows)

2009-04-17T22:32:00.000-07:00

I have already had Apache 2.2 installed.

Download php-5.2.9-2-Win32.zip. Extract it to C:/free/php

Make a copy of php.ini-dist and rename it to php.ini

Add the following text to C:/free/Apache2.2/conf/httpd.conf

Opening PDF files within Firefox

2009-04-16T00:42:00.000-07:00

I can't open pdf files side Firefox recently. I am using Firefox 3 on Windows XP.

After done some googling, I found
Opening PDF files within Firefox which provides a complete solution of this problem. The following method works for me.


   1. Close Firefox.
   2. Navigate to my Firefox profile folder.
   3. Delete the mimetypes.rdf file.
   4. Restart Firefox.

For how to find profile folder, refer to to How to find your profile.

Handle Multi-key to One-value Property File

2009-04-13T08:35:00.000-07:00

One of my friend asked how to implement a configuration file which contains key to value mappings. One special thing is that multiple keys are mapped to one value. If Java property is used, the following text shows the property file.


# [v1]
K11=v1
K12=v1
K13=v1

# [v2]
K21=v2
K22=v2
K23=v2

# [v2]
K31=v3
K32=v3
K33=v3

It contains a lot of duplications. v1, v2 and v3 appears many times in the property file. DIY (Don't Repeat Yourself) principle is violated. One improvement is to write the following property file:


# [v1]
K11,K12,K13 = v1

# [v2]
K21, K22, K23 = v2

# [v2]
K31, K32, K33 = v3

For this property file, I need to write additional code. The following steps are one option:

get the entry set from the property file

create a hash map

iterate over the entry set. k is the map entry key, v is the map entry value

split the iterated k with , into key[]

add (key[0], v), (key[1], v), ... to the hash map

After the above steps done, the hash map can be used to get a value for a given key. There still one problem. The property file and the Java code are separated. Then I thought about Scheme. Here is the solution in Scheme. It is tested on DrScheme, version 4.1.4.


#lang scheme
(define (find-value k)
   (let* ([ks2v '(

; multi-key to one-value mappings
((K11 K12 K13) V1)
((K21 K22 K23 K24) V2)
((K31 K32 K33 K34) V3)

                 )]
         [contain? (lambda (k keys)
                (let ([equal2k? (lambda (a) (equal? k a))])
                  (ormap equal2k? keys)))]
         [k2v (lambda (pair)
                 (if (contain? k (car pair))
                     (cadr pair)
                     #f))])
     (ormap k2v ks2v)))

This solution is elegant. Code as data characteristic of Scheme makes such kind of solutions possible. And when I was coding this solution, I had the same feeling as the quote SQL, Lisp, and Haskell are the only programming languages that I've seen where one spends more time thinking than typing.

Disable Translating Chinese Character into Character Entities in Tidy and Xmllint

2009-04-08T07:46:00.000-07:00

For Tidy, set char-encoding to raw. The following text is my setting.


tidy -q --char-encoding raw -indent --show-warnings n --tidy-mark n -w 80

For xmllint, set encode to UTF-8 (The encoding of my XML files is UTF-8). Here is my xmllint setting.

xmllint --encode UTF-8 --format

Building DSL with Scheme

2009-04-06T05:38:00.000-07:00

I have listened a talk made by Shriram Krishnamurthi. It is called The Swine Before Perl . The talk is about using scheme to do heavy lifting which other languages can't handle. It is a great talk. It shows me the real amazing strength of Scheme programming language. Shriram centers his talk around an example of automaton. The Scheme source code is based on the old version of PLT Scheme. As a result, the source code can't run against the current implementation. I have made a little change to the source code. I post it here for reference.

The following source code is tested in DrScheme, version 4.1.4.
The first version of automaton.


#lang slideshow
(define b-machine-states
  '((init (c more))
    (more (a more)
          (d more)
          (r end))
    (end (r end))))

(define (b-machine stream)
  (letrec ([walker (lambda (state stream)
                     (or (empty? stream)
                         (let ([transitions
                                 (cdr (assv state b-machine-states))])
                           (let ([1st (first stream)])
                             (let ([new-state (assv 1st transitions)])
                               (if new-state
                                 (walker (cadr new-state) (rest stream))
                                 false))))))])
    (walker 'init stream)))

(b-machine '(c a r))
(b-machine '(c a d r))
(b-machine '(c a x))

The goto version of automaton.


#lang scheme
(define b-machine
  (letrec ([init
             (lambda (stream)
               (or (empty? stream)
                   (case (first stream)
                     [(c) (more (rest stream))]
                     [else false])))]
           [more
             (lambda (stream)
               (or (empty? stream)
                   (case (first stream)
                     [(a) (more (rest stream))]
                     [(d) (more (rest stream))]
                     [(r) (end (rest stream))]
                     [else false])))]
           [end
             (lambda (stream)
               (or (empty? stream)
                   (case (first stream)
                     [(r) (end (rest stream))]
                     [else false])))])
    init))

(b-machine '(c a r))
(b-machine '(c a d r))
(b-machine '(c a x))

The second version of automaton.


#lang scheme
(define-syntax automaton
  (syntax-rules (-> :)
                [(_ init-state
                    (state : (cndn -> new-state) ...)
                    ...)
                 (letrec ([state
                            (lambda (stream)
                              (or (empty? stream)
                                  (case (first stream)
                                    [cndn (new-state (rest stream))]
                                    ...
                                    [else false])))]
                          ...)
                   init-state)]))

(define b-machine 
  (automaton init
             (init : ((c) -> more))
             (more : ((a) -> more)
                     ((d) -> more)
                     ((r) -> end))
             (end  : ((r) -> end))))



(b-machine '(c a r))
(b-machine '(c a d r))
(b-machine '(c a x))

The following code is really an elegant DSL for automaon.


(automaton init
     (init : ((c) -> more))
     (more : ((a) -> more)
             ((d) -> more)
             ((r) -> end))
     (end  : ((r) -> end)))

I have also implemented the first and goto version of automaton in Ruby.
The first version of automaton in Ruby


$b_machine_states = {
  "init" => {"c" => "more"},
  "more" => {"a" => "more",
             "d" => "more",
             "r" => "end"},
  "end" => {"r" => "end"}}

def walker(state, stream)
  if stream.empty?
    return true
  end
  first = stream[0]
  transitions = $b_machine_states[state]
  new_state = transitions[first]
  if new_state == nil
    return false
  else
    rest = stream[1, stream.length()]
    return walker(new_state, rest)
  end
end

def b_machine(stream)
  walker("init", stream)
end

print b_machine("car") , "\n"
print b_machine("cadr") , "\n"
print b_machine("cax") , "\n"

The goto version of automaton in Ruby


class Automaton
  
  def accept(stream)
    return init(stream)
  end

  private
  def init(stream)
    if stream.empty?
      return true
    end
    first = stream[0]
    case first
    when "c"
      return more(rest(stream))
    else
      false
    end
  end

  def more(stream)
    if stream.empty?
      return true
    end
    first = stream[0]
    case first
    when "a"
      return more(rest(stream))
    when "d"
      return more(rest(stream))
    when "r"
      return terminate(rest(stream))
    else
      return false
    end
  end

  def terminate(stream)
    if stream.empty?
      return true
    end
    first = stream[0]
    case first
    when "r"
      return terminate(rest(stream))
    else
      return false
    end
  end

  def rest(stream)
    return stream[1, stream.length()]
  end
end

auto = Automaton.new()
print auto.accept("car") , "\n"
print auto.accept("cadr") , "\n"
print auto.accept("cax"), "\n"

For the second version of automaton, I can't find a way to implement it in Ruby because Ruby does not have a macro system similar to Scheme. I am happy to know if anyone has a solution.

Add Fonts to Windows Command Line Console

2009-03-16T06:34:00.000-07:00

By default, the command line console only has two fonts for Windows XP Simple Chinese version. They are Roster Font and Lucida Console. I am unhappy with them. I like Consolas and Bitstream Vera Sans Mono for command line. So I decided to add these fonts to the command line console. I have done it following Necessary criteria for fonts to be available in a command window. I have added two registry values as shown in the following registry file.


Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Console\TrueTypeFont]
"00"="Consolas"
"000"="Bitstream Vera Sans Mono"

The machine needs to be restarted after adding these stuff to registry. And I need to set the current code page to 437 in Option tab of the command line console shortcut if I want to select Consolas and Bitstream Vesa Sans Mono. The code page 936 does not work for these 2 fonts.

Subroutine Pointer in C

2009-02-27T05:11:00.000-08:00

C has subroutine pointers. It does not require closures because it does not have nested scopes and it is static scoped. Every subroutine is at global scope. As a result, the referencing environment for a subroutine is the same whether it is created when the subroutine is first passed a parameter or when the subroutine is finally called. In the following code, plus_one has the same referencing environment whether the referencing environment is created when it is passed as a parameter to caller or when it is called in caller.


#include 
int plus_one(int n) {
    return n + 1;
}
void caller(int (*f) (int)) {
    int result = f(3);
    printf("3 plus 1 is %d\n", result);
}
int main() {
    caller(plus_one);
    return 0;
}

For a language which supports nested scope, the situation is different. In the following Common Lisp code, a closure must be created for plus_2 to make it a function which addes 2 to its sole parameter.


(let ((one 1))
  (defun plus_1 (x)
     (+ x one)))

(let ((one 5))
  (plus_1 3))

Dynamic Scope in LISP

2009-02-26T06:50:00.000-08:00

The following program will print (3 5) in newlisp which is dynamic scoped. But it will print (3 7) in CLISP which is static scoped.


(let ((y 7)) (define (scope-test x) (list x y)))
(let ((y 5)) (scope-test 3))

Java's file.encoding property on Windows platfor

2009-02-22T05:17:00.000-08:00

This property is used for the default encoding in Java, all readers and writers would default to using this property. file.encoding is set to the default locale of Windows operationg system since Java 1.4.2. System.getProperty("file.encoding") can be used to access this property. Code such as System.setProperty("file.encoding", "UTF-8") can be used to change this property. However, the default encoding can be not changed dynamically even this property can be changed. So the conclusion is that the default encoding can't change after JVM starts. java -dfile.encoding=UTF-8 can be used to set the default encoding when starting a JVM. I have searched for this option Java official documentation. But I can't find it.

Produce the "TeX: The Program" from tex.web

2009-02-17T06:11:00.000-08:00

This is how I have produced the TeX: The Program from tex.web

Install the full package of MikTex 2.7

Get tex.web from http://www.tug.org/svn/texlive/trunk/Build/source/texk/web2c/tex.web

Place tex.web in D:/book.

Open a command console. Change directory to D:/book.

Make a empty file change-file in the current directory.

Run "weave tex.web change-file program.tex".

Run "pdfTex program"

The result is a PDF file program.pdf and CONTENTS.tex. These files are the book.

A Quick Start of SWI-Prolog for Windows

2009-02-09T02:00:00.000-08:00

Recently, I have been reading chapter 12 of Programming Language Pragmatics. This chapter is about logic programming. So I need a prolog implementation to try the code in the book. I choose SWI-Prolog. SWI-Prolog has a thorough reference manual. But what I want is only some quick start. After searching on Google, I don't find a decent SWI-Prolog quick start. So I have to do my work by scanning the reference book. It is kind of painful. Here, I write down what I think it is enough to do some simple things with SWI-Prolog, hoping that it will help you if you only do some simple things with SWI-Prolog.

My platform is Windows. After the installation of SWI-Prolog, add the SWI-Prolog bin directory to PATH environment varialbe. In my case, the bin directory is C:\Program Files\pl\bin.

SWI-Prolog has a plwin.exe which is GUI environment for SWI-Prolog. I prefer to use the SWI-Prolog console environment which is plcon.

Open a command line

change to directory containing my prolog source code. In my case, it is D:\documents\Prolog.

type plcon to launch prolog

Now the command console looks as follows.


D:\Documents\Prolog>plcon
Welcome to SWI-Prolog (Multi-threaded, 32 bits, Version 5.6.64)
Copyright (c) 1990-2008 University of Amsterdam.
SWI-Prolog comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to redistribute it under certain conditions.
Please visit http://www.swi-prolog.org for details.

For help, use ?- help(Topic). or ?- apropos(Word).

1 ?- consult(woman).
% woman compiled 0.00 sec, 1,060 bytes
true.

2 ?-

Type consult(women) to load D:\documents\Prolog\women.pro. It contains the following clauses.


:-dynamic pregnant/.
human(susan).
human(jane).
pregnant(susan).
woman(X) :- pregnant(X), human(X).

Assert can only be used to add facts to dynamic predicates in the prolog interpreter. I want to add facts for predicate pregnant. So I add the dynamic directive in the prolog source code. pregnant/1 is a predicator indicator. A predicate indicator is used to denote predicates. It's form is Name/Arity where Name is an atom denoting the name of a predicate and Arity is a integer denoting the number of its arguments. Add one fact to the prolog clause databse.


2 ?- assert(pregnant(jane)).
true.

Issue a query. Type ; to ask for more answers.


3 ?- woman(X).
X = susan ;
X = jane.

Type halt. to exit SWI-Prolog


4 ?- halt.

Iterator in Icon

2009-02-08T06:36:00.000-08:00

In Icon programming language, iterator is very powerful. As a result, Programming Language Pragmatics dedicates a sub section to explain it. The following shows the complete running code corresponding to the code snippets in the book.


procedure main()
 write("=================================================")
 every i := 1 to 10 by 2 do {
  write(i)
 }
 write("=================================================")
 every i := 1 + upto(' ', "a ss ss") do {
  write(i)
 }
 write("=================================================")
 i := 1
 every write(i + upto(' ', "a ss ss"))
 write("=================================================")
 if 2 < 3 then {
  write("2 < 3")
 }
 write("=================================================")
 if (i := find("abc", "abcabcabc")) > 6 then {
  write(i)
 }
 write("=================================================")
 if (i := find("x", "xaax")) = find("x", "bxxx") then {
  write(i)
 } 
end

The following code explains the description at the end of the sub section. It is If the expression following suspend contains an invocation of a generator, then the subroutine will suspend repeatedly, once for each generated value.


# suspend is followed by a value
procedure FollowedByValue()
 i := 1
 while i <= 3 do {
  suspend i
        i +:= 1
 }
end

# suspend is followed by a generator expression
procedure FollowedByGenExpr()
 suspend(1|2|3)
end

# suspend is followed by a generator function invocation
procedure FollowedByGenFuncInvocation()
 suspend FollowedByValue() 
end

procedure main()
 write("=================================================")
 every write(FollowedByValue())
 write("=================================================")
 every write(FollowedByGenExpr())
 write("=================================================")
 every write(FollowedByGenFuncInvocation())
end

Iterator in Ruby and Python

2009-02-08T02:41:00.000-08:00

Unlike Java, both languages support real iterator not just iterator object. The following code shows how to create and use iterator in Python.


def from_1_to_5():
  for i in [1, 2, 3, 4, 5]:
    yield i

for num in from_1_to_5():
  print num

The following code shows how to make it in Ruby. Desides a style similar to Python, Ruby can use iterator with block.


def iterator
  for i in [1, 2, 3, 4, 5]
    yield(i)
  end 
end

# invoke iterator in a way similar to Python
call_block do |num|
  puts "#{num}"
end
  
# invoke iterator with the use of block 
call_block { |num| puts "#{num}" }

Accumulator Generator in Common Lisp

2009-01-22T18:42:00.000-08:00

The following code helps me understand Accumulator Generator in Common Lisp. I am still not used to lisp's true meaning. I often need to struggle to understand a little piece of lisp code. So I write down the code.


(defun foo (n) (lambda (i) (incf n i)))
(setq starting 10)
(setq acc (foo starting))
(funcall acc 1)

For a complete description of Accumulator Generator, refer to Accumulator Generator

"define-syntax" and "syntax-rules" in Scheme

2009-01-21T01:09:00.000-08:00

Scheme has a macro mechanism which allow us to extend the Scheme language syntax. The description in Revised5 Report on the Algorithmic Language Scheme is precise. But the description has a mathematical flavor.

I refer to the book The Scheme Programming Language. It uses the following example to explain "define-syntax" and "syntax-rules". The following code is to define let. let is defined as (let <bindings> <body>) in Revised5 Report on the Algorithmic Language Scheme.


(define-syntax let
  (syntax-rules ()
    ((_ ((x v) ...) e1 e2 ...)
     ((lambda (x ...) e1 e2 ...) v ...))))

I still find the description hard for me to understand because I am very new to Scheme. After some time of struggling, I find the following way to understand it. (let <bindings> <body>) is just like a function invocation. bindings create a local binding scope. In some sense, it binds formal parameters to actual parameters. Take (let ((x 1) (y 2)) (+ x y)) as an example, x is bound to 1 and is bound to 2. body is just like the body of an function. (+ x y) will produce 3.

Back to the "define-syntax" example, we see that ((lambda (x ...) e1 e2 ...) v ...) define what we have described as function invocation. For (_ ((x v) ...) e1 e2 ...), we can see that (x v) indicate a binding. ((x v) ...) indicates any number of bindings. e1 indicates a expression in the body. e1 e2 ... indicates all the expressions in the body.

Great Programming Book

2009-01-18T23:12:00.000-08:00

This book (Programming Language Pragmatics) by Scott is really great. Everyone who want to have a complete and deep understanding of programming languages should read this book. I bought this book from US. It costs me more than $60 which is a big amount for a Chinese citizen. Although I have not finished it yet, this book has taught me much.

Use GREP or ACK to Search for Chinese Characters

2008-11-22T17:11:00.000-08:00

I have been developing a solution for a Chinese customer. As a result, I need to search for Chinese characters in text files. I use GREP and ACK. There is some subtlety when searching for Chinese characters.

In order to input Chinese characters, the windows command line's code page needs to be set to 936. Issuing chcp 936 can do this. 936 is for GBK encoding.

There are two files a.txt and b.txt in d:/text. Both of these 2 files contain the following text:


中国
中国abc

The encoding for a.txt is GBK. The encoding for b.txt is UTF-8. The grep 中国 d:/test/*.txt only finds 中国 in a.txt.

The conclusion is that GREP can only find Double Byte characters such as Chinese only if the command line console and the file to be searched has the same encoding. This conclusion also applies to ACK.

Use PAR to format XML comment

2008-11-22T16:50:00.000-08:00

PAR is fantastic for formatting text files. It can be used to give XML comment a pretty layout. For the following XML comment:


<!-- You can recognize truth by its beauty and -->
<!-- simplicity. When you get it right, it is obvious that it is right. -->

par 50 produces


<!-- You can recognize truth by its beauty    -->
<!-- and simplicity. When you get it right,   -->
<!-- it is obvious that it is right.          -->

But for the following text


<!-- You can recognize truth by its beauty and simplicity. When you get it right, it is obvious that it is right. -->

par 50 produces


<!-- You can recognize truth by its beauty and
simplicity. When you get it right, it is obvious
that it is right. -->

It is not what we want. Instead, par 50 -p5 -s5 can be used to produce


<!-- You can recognize truth by its beauty    --> 
<!-- and simplicity. When you get it right,   --> 
<!-- it is obvious that it is right.          -->

For the details of using PAR, you can refer to Par.

Paste Multiple Lines of Code into Clisp Console

2008-11-17T22:07:00.000-08:00

There is a little problem when pasting multiple lines of code into clisp console. If TAB is used for code indentation, there will be the following error when pasting the code into CLISP console.


You are in the top-level Read-Eval-Print loop.

Using spaces for code indentation will fix this problem.

Display Chinese characters in Eclipse console

2008-10-15T00:36:00.000-07:00

Recently, I have been working on a project for a Chinese company. As a result, I want to display Chinese characters in Eclipse console. I am using Eclipse 3.3. I don't want to change Regional and language settings for Windows XP since it will influence other programs. After searching with google, I find the following solution work for me. First, we need to configure the JRE we are using in Eclipse.

In Preferences->Java->Installed JREs, choose the JRE you are using.

Click Edit to show Edit JRE dialog

Input -Dfile.encoding=UTF-8 in Default VM Arguments

Second, we need to configure all the Run (Debug) configurations.

In Common tab, select UTF-8 as Console Encoding

Eclipse 3.4 does not have this prolem. It can display Chinese correctly by default.

Notebook and Ballpoint Pen

2008-09-22T07:52:00.000-07:00

My favorite note book and ballpoint pen

Swap CAPSLOCK and ESC key in Windows

2008-08-27T07:10:00.000-07:00

I decide to do the swap because I use VIM a lot. Save the following text into a file such as swap.reg. Double click the file import the registry values.
REGEDIT4 [HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout] "Scancode Map"=hex:00,00,00,00,00,00,00,00,03,00,00,00,3a,00,01,00,01,00,3a,00,00,00,00,00

Build Ruby on Windows

2008-08-16T02:27:00.000-07:00

Get the Ruby 1.8.7 source code from ftp://ftp.ruby-lang.org/pub/ruby/1.8/ruby-1.8.7-p71.tar.gz. Extract the source code to a directory such as D:/Ruby 1.8.7-p71 . Open a Visual Studio .NET 2003 Command Prompt. Run the following commands to compile ruby.

cd /d D:/Ruby 1.8.7-p71
win32\configure.bat
nmake

If the following error occurs, its means that Cygwin find is invoked from the make file. Make sure that windows find precedes Cygwin find in PATH environment variable.

Creating Makefile find: `=': No such file or directory NMAKE : fatal error U1077: 'cl' : return code '0x1' Stop.

HTTP Status 404 when running tomcat inside Eclipse

2008-08-02T06:32:00.000-07:00

HTTP Status 404 will show up when I running tomcat inside Eclipse. And my resource exists. The error will disappear when I restart tomcat for some times. How many times I need to restart tomcat to remove the error is random. And the dialogs of Run on server wizard is random.

Sometimes, I get this error when using welcome-file-list. For some reasons, the welcome-file can be found.

I have used tomcat in Eclipse for some. My overall experience is bad. Maybe some improvements should be made on WTP. For now, I use ANT to deal with Tomcat. And it works well.

Classpath subtlety when using tomcat inside Eclipse

2008-08-02T06:17:00.000-07:00

First, an eclipse dynamic web project only recognized classes produced by source folders configured in Java Build Path->Source and classes in the libraries under WebContent/WEB-INF/lib folder. The locations of source folders and class output folders do not matter if they are inside this project. The count of source folders and output folders also does not matter. By default, the output class folder is build/classes. It does not recognize the jar libraries which are not under WebContent/WEB-INF/lib and class folder configured in Java Build Path->Libraries. For class compilation, it is Ok. ClassNotFound exception will be thrown if you begin to use Run on server.

Running the Eclipse 3.4 formatter application

2008-07-02T06:41:00.000-07:00

D:\gnu\eclipse\eclipse.exe -application org.eclipse.jdt.core.JavaCodeFormatter -config D:\gnu\jee\workspace\Concurrency\.settings\org.elipse.jdt.core.prefs d:\Foo.java

variable type checking in C

2008-06-12T07:28:00.001-07:00

Put all the files in a directory. Run gcc *.c -o main.exe. The compilation will succeed. The reason is that C does not check type consistency of external variable declaration and reference.
1 one.c


int abc = 1;

2 main.c


#include <stdio.h>
extern float abc;
int main(void) {
    printf( "%f", abc );
}

Function type checking in C

2008-06-12T07:01:00.000-07:00

Put all the following 3 files in a directory. Run gcc *.c. The compilation will succeed. And running the resulted excecutable file will print I am here, man!. Run g++ *.c The compilation will fail. The reason is that during linking, C only check function names. But C++ check function type.
1. caller.c


#include "callee.h"
int main(void) {
    foo();
}

2. callee.h


int foo(void);

3. callee.c


#include 
void foo(int v) {
    printf( "I am here, man!" );
}

Index overflow in Java for loop

2008-05-14T06:36:00.000-07:00

The following little code snippet shows the overflow of for loop index in Java.


public class Out {
    public static void main(String[] ars) {
        for(int i = 0; i <= 2147483647 ; i+=100000000 )
        {
            System.out.println( i + "\n" );
        }        
    }
}

You may expect that the iteration count is 22. But it is not true. i is 2100000000 after the 22nd iteration. Then 100000000 is added to i(2100000000) again. 2200000000 is bigger than 2147483647. So int overflow will happen. i will become a negative integer. So the loop will continue.

For details of loop index overflow, you can refer to Programming Language Pragmatics.

Genenared methods values and valueOf for Enum

2008-05-02T08:06:00.000-07:00

I have recently tried Java Enum when doing software development. The following code is an example:


public Enum FooType {
    ABC, DEF;
}

I found two methods valueOf and values in the Javadoc for my enum code. I did not write these 2 methods. And it's superclass java.lang.Enum<FooType> also does not have these 2 methods. Then I checked JLS 3.0. 8.9 Enums says these 2 methods are automatically generated. It is a little magic. One Java class can has methods which do not exist as source code. Too many magic which do not have a consistent rationale can make a language hard to learn and use.

Modula-2

2008-04-26T23:51:00.000-07:00

I tried a little Modula-2 programming today. I used XDS 2.5. I built the following little program successfully in XDS IDE.


MODULE hello;
FROM InOut IMPORT WriteString, WriteLn;
BEGIN
    WriteString("Hello, world!");
    WriteLn;
END hello.

And I could use xc to compile it. The following error showed up when I used xlink to link it.


XDS Link  Version 2.6 Copyright (c) 1995-2001 Excelsior
Fatal error (13): No program entry point

I have googled answers for it and read XDS documentation. Unfornately, I can't find an answer. Again, it shows a quick start guide with some simple examples is extremely important for a beginner to use some new tools.

After wresting with xlink for some time , I worked out the following solution.


xlink hello.obj C:\free\XDS\LIB\x86\libxds.lib C:\free\XDS\LIB\x86\import32.lib C:\free\XDS\LIB\x86\xstart.lib

Reading of Programming Language Pragmatics

2008-04-15T01:12:00.000-07:00

Programming Language Syntax

The following syntax shows dangling else problem of Pascal.

stmt -> if condition then_clause else_clause | other_stmt

then_clause -> then stmt

else_clause -> else stmt | ε

Sometimes, we have a hard time to use some language. Sometimes it is not because we are not smart enough. It is because of the flaws of the programming language.

Hiding and overriding in Java

2008-04-12T06:34:00.000-07:00

For some technical terms in this post, please refer to JLS 3.0. And the discussion is not very precise. I have looked for some explicit rules governing this topic in JLS. But I failed. So I am just trying to summarize my personal understanding. Any feedbacks are welcomed.

In the following discussion, A and B are used. Type A extends type B (type means class or interface).

Field

Only hiding applies tojava fields. Hiding happens in the following situation.Both A and B have a variable named as var. And var is of the same kind of variables ( static fields or instance fields).
Java allows a variable belonging to different variable kinds appears in both A and B. But hiding does not happen in such situations.

Method

Method m1 in A . Method m2 in B.

m1 is subsignature of m2

m1's visibility is no less than m2

Both m1 and m2 are instance methods. Or both m1 and m2 are static methods. Otherwise, there will some compile errors.

Overriding

Overriding applies to instance methods.

Hiding

Overriding applies to static methods.

blank in shell script

2008-04-09T06:17:00.000-07:00

Try to avoid the use of blank in pathnames. It will drive you crazy under some circumstances. Take the following script as an example.


#!/bin/sh
opt="-path \"*/a bc\" -prune"
find $opt -o -print0
bash -c "find $opt -o -print0"

find $opt -o -print0 does not work. But bash -c "find $opt -o -print0" works. I have tried very hard to find a way to make the former work. But I failed. I will be happy to hear if anybody has a solution.

Little hacking with bash shell

2008-04-09T05:20:00.000-07:00

I have recently tried locate from findutils in cygwin. I wanted to use updatedb to create a file name database. And I wanted to exclude all the files and directories under .svn directories . So I tried updatedb --localpaths='D:/gnu/ws/ctre/space' --findoptions='-path "*/.svn" -prune -o'. But I could't exclude the files in .svn directories with this command. I am not very experienced with bash. But I wanted to solve this problem. So I checked update script and did some debugging to it. The problem is that bash shell will do pathname expansion to "*/.svn". My first fix is to add set -f in updatedb script. It works.

Then I wanted to open a bug for this script. For opening a bug, I need to do further investigation. So I learned more about bash and try more scripts. Finally, I found that I can use updatedb --localpaths='D:/gnu/ws/ctre/space' --findoptions='-path */.svn -prune -o' or updatedb --localpaths='D:/gnu/ws/ctre/space' --findoptions='-path '*/.svn' -prune -o'. Bash shell will preserve the literal value of every chacter within the qoutes. But bash shell will do expansion to characters enclosed by double quotes.

As a result, I did not open an invalid bug. I have learned more. It tells me that contributing to community will let me learn more.

Sometimes I was frustrated during this hacking. But the overall experience is good.

Solution to one Design Puzzle in Head First Design Pattern

2008-04-05T08:08:00.000-07:00

I am reading Head First Design Pattern. It is a great book.

Generally, I don't like exercises in a book which don't have answers provided. I have run into the design puzzle on page 468. At first sight, I knew that I should use state pattern. I have never applied state pattern in real software work before. So I took this exercise as an opportunity to practice state pattern. Here is my solution.

State.java


package headfirst.proxy.virtualproxy;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public interface State {
 public int getIconWidth();
 public int getIconHeight();
 public void paintIcon( Component c, Graphics g, int x, int y );
}

NullState.java


package headfirst.proxy.virtualproxy;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class NullState implements State {

    private ImageProxy proxy;
    private Thread retrievalThread;
    private URL imageURL;
 boolean retrieving = false;

    public NullState( ImageProxy p, URL url) {
        proxy = p;
        imageURL = url;
    }

    public int getIconWidth() {
        return 800;
    }

    public int getIconHeight() {
        return 600;
    }

    public void paintIcon( final Component c, Graphics g, int x, int y ) {
        g.drawString("Loading CD cover, please wait...", x+300, y+190);
        if (!retrieving) {
            retrieving = true;
            retrievalThread = new Thread(new Runnable() {
                public void run() {
                    try {
                        ImageIcon imageIcon = new ImageIcon(imageURL, "CD Cover");
                        proxy.setState( new ImageState( imageIcon )  );
                        c.repaint();
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            });
            retrievalThread.start();
        }
    }
}

ImageState.java


package headfirst.proxy.virtualproxy;
import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ImageState implements State {
    
    private ImageIcon imageIcon;
    
    public ImageState( ImageIcon i ) {
        imageIcon = i;
    }

    public int getIconWidth() {
        return imageIcon.getIconWidth();
    }

    public int getIconHeight() {
        return imageIcon.getIconHeight();
    }

    public void paintIcon( final Component c, Graphics g, int x, int y ) {
        imageIcon.paintIcon(c, g, x, y);
    }
}

ImageProxy.java


package headfirst.proxy.virtualproxy;

import java.net.*;
import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class ImageProxy implements Icon {

    private NullState ns;
    private State state;
     
 public ImageProxy(URL url) {
        ns = new NullState( this, url );
        state = ns;
    }
     
 public int getIconWidth() {
        return state.getIconWidth();
 }
 
 public int getIconHeight() {
        return state.getIconWidth();
 }
     
 public void paintIcon(final Component c, Graphics  g, int x,  int y) {
        state.paintIcon( c, g, x, y);
 }

    public void setState( State s ) 
    {
        state = s;
    }
}

ClassNotFoundException problem with rmiregistry

2008-04-05T02:34:00.000-07:00

It is very possible that you will get ClassNotFoundException when you try Getting Started tutorial for RMI in JDK documentation. There is a easy way to fix it. Start rmiregistry and the server implementation in the same command line console on Windows (assuming that the current directory is the root directory of the class file tree for the example).

start rmiregistry

java example.hello.Server

The CLASSPATHs for both rmiregistry and java example.hello.Server must contains the path for the class file tree for the example if you want to run them from different command line console.

I have run into this problem twice. And JDK documentation does not explicitly specified. I searched the web. Remote Method Invocation (RMI) - RMI server ClassNotFoundExceptionsolves my problem.

So documentation is vital to software quality. Good documentation can save developers a lot of precious time.

Working Directory

2008-03-24T02:23:00.000-07:00

To decide the working directory for a process is difficult. I have found a entry in wikipedia for working directory. And I have added some rules for deciding working directory in the wikipedia entry Working directory. You can refer to it for details.

For this reason, we should always try to use pathname relative to classpath instead of working directory in Java.

Unicode & Java

2008-03-19T09:22:00.000-07:00

Endianess

In computing, endianness is the byte (and sometimes bit) ordering used to represent
some kind of data.
Most modern computer processors agree on bit ordering "inside" individual bytes (this was not always the case). This means that any single-byte value will be read the same on almost any computer one may send it to.
Integers are usually stored as sequences of bytes, so that the encoded value can be obtained by simple concatenation. The two most common of them are:

increasing numeric significance with increasing memory addresses, known as little-endian

its opposite, most-significant byte first, called big-endian.

Inter x86 use little-endian. JVM use big-endian.(The above content is from Wikipedia Endianess Entry)

Unicode

Code Point. Any value in the Unicode codespace; that is, the range of integers from 0 to 10FFFF16.
Mapping of Unicode character planes is a good explanation of Unicode planes and code points.

UTF-16

To represent the complete range of characters using only 16-bit units, the Unicode standard defines an encoding called UTF-16. In this encoding, supplementary characters are represented as pairs of 16-bit code units, the first from the high-surrogates range, (U+D800 to U+DBFF), the second from the low-surrogates range (U+DC00 to U+DFFF). For characters in the range U+0000 to U+FFFF, the values of code points and UTF-16 code units are the same.

Java

In the Java SE API documentation, Unicode code point is used for character values in the range between U+0000 and U+10FFFF, and Unicode code unit is used for 16-bit char values that are code units of the UTF-16 encoding.

It is not true that there is simply no way to extend an instantiable class and add an aspect while preserving the equals contract.

2008-02-24T06:54:00.000-08:00

In Effective Java, Item 7 is "Obey the general contract when overriding equals". There is one sentence "There is simply no way to extend an instantiable class and add an aspect while preserving the equals contract." Josh uses the following classes to illustrate Transitivity. The following text is excerpted from the book.
The Point class.


public class Point {
private final int x;
private final int y;
public Point(int x, int y) {
    this.x = x;
    this.y = y;
}
public boolean equals(Object o) {
    if (!(o instanceof Point))
        return false;
    Point p = (Point)o;
    return p.x == x && p.y == y;
}
... // Remainder omitted
}

The ColorPoint class.


public class ColorPoint extends Point {
private Color color;
public ColorPoint(int x, int y, Color color) {
    super(x, y);
    this.color = color;
}

//Broken - violates transitivity.
public boolean equals(Object o) {
    if (!(o instanceof Point))
        return false;
    // If o is a normal Point, do a color-blind
    // comparison
    if (!(o instanceof ColorPoint))
        return o.equals(this);
    // o is a ColorPoint; do a full comparison
    ColorPoint cp = (ColorPoint)o;
    return super.equals(o) && cp.color == color;
}

... // Remainder omitted
}

This approach does provide symmetry, but at the expense of transitivity:


ColorPoint p1 = new ColorPoint(1, 2, Color.RED);
Point p2 = new Point(1, 2);
ColorPoint p3 = new ColorPoint(1, 2, Color.BLUE);

At this point, p1.equals(p2) and p2.equals(p3) return true, while p1.equals(p3) returns false, a clear violation of transitivity. The first two comparisons are “color-blind,” while the third takes color into account.

So what's the solution? It turns out that this is a fundamental problem of equivalence relations in object-oriented languages. There is simply no way to extend an instantiable class and add an aspect while preserving the equals contract.

Then Josh gives a a workaround where ColorPoint does not extend Point. I thinks that we can allow ColorPoint to extend Point and preserve the equals contract. Here is my solution.
The Point class.


public class Point {
    private final int x;
    private final int y;

    public Point( int x, int y ) {
        this.x = x;
        this.y = y;
    }

    public boolean equals( Object o ) {
        if( this == o )
            return true;
        if( !(this.getClass() == o.getClass()) )
            return false;
        Point p = (Point) o;
        return p.x == x && p.y == y;
    }

... // Remainder omitted

}

The ColorPont class.


public class ColorPoint extends Point {
private Color color;
public ColorPoint(int x, int y, Color color) {
    super(x, y);
    this.color = color;
}

public boolean equals( Object o ) {
    if( this == o )
        return true;
    if( o == null )
        return false;
    if( !(this.getClass() == o.getClass()) )
        return false;
    ColorPoint cp = (ColorPoint) o;
    return super.equals( o ) && cp.color == color;
}
... // Remainder omitted
}

First, equals method use the == operator to check if the argument is a reference to this object. Secondly, it check whether the argument and this object are of the same type. I have tested this piece of code. It works. But I think that there may be some situations I have not taken into account.

Any comments are welcome.