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Month: March 2008

Java assertion to prevent time travelers

cc broken pocket watches
Creative Commons photo by jekemp

public class PreventTimeTravelers {
    public static void main(String args[]){
        long startTime = System.currentTimeMillis();
        long endTime = System.currentTimeMillis();
        assert endTime >= startTime: "We came back in time!";
    }
}

Please do not execute this code in a time machine.

NumberFormatException Example

NumberFormatException: Thrown to indicate that the application has attempted to convert a string to one of the numeric types, but that the string does not have the appropriate format.

An simple example:

public class SummationExample {
    public static void main(String args[]){
        int sum = 0;
        for(String arg: args){
            System.out.println("+" + arg);
            sum += Integer.parseInt(arg);
        }
        System.out.println("= " + sum);
    }
}

$ javac SummationExample.java
$ java SummationExample 1 2 3 4 5
+1
+2
+3
+4
+5
= 15

but

$ java SummationExample 1 2 3 4 five
+1
+2
+3
+4
+five
Exception in thread “main” java.lang.NumberFormatException: For input string: “five”
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:48)
at java.lang.Integer.parseInt(Integer.java:447)
at java.lang.Integer.parseInt(Integer.java:497)
at SummationExample.main(SummationExample.java:6)

So

public class SummationExample {
    public static void main(String args[]){
        int sum = 0;
        for(String arg: args){
            try {
                sum += Integer.parseInt(arg);
                System.out.println("+" + arg);
            } catch (NumberFormatException e) {
                // nothing
            }
        }
        System.out.println("= " + sum);
    }
}

now

$ javac BetterSummationExample.java
$ java BetterSummationExample 1 2 3 4 5
+1
+2
+3
+4
+5
= 15

and also

$ java BetterSummationExample 1 2 3 4 five
+1
+2
+3
+4
= 10

Java Duke animated gif waving

JRuby simple example

JRuby is a Java implementation of the Ruby interpreter, with an tightly integrated with Java. In this examples you will call swing components.

On Ubuntu you can get it installing the package jruby (a meta package for jruby1.0 or jruby0.9 packages).

To test it save this code as test.rb and invocate it with the jruby interpreter:

require 'java'

frame = javax.swing.JFrame.new()
frame.getContentPane().add(javax.swing.JLabel.new('Hello, JRuby World!'))
frame.setDefaultCloseOperation(javax.swing.JFrame::EXIT_ON_CLOSE)
frame.pack()
frame.set_visible(true)

$ jruby1.0 test.rb

Hello JRuby World

ps: We did not define a close method, so to close this windows you’ll need to kill its process or press Ctrl+C in your terminal.

List of popular programming book acronyms

I found cool this list on reddit.

  • AIMA Artificial Intelligence a Modern Approach by Stuart Russell and Peter Norvig
  • AMOP The Art of the Meta Object Protocol by Gregor Kiczales
  • ATTAPL Advanced Topics in Types and Programming Languages by Benjamin C. Pierce
  • AWDWR Agile Web Development with Rails by by Dave Thomas and David Heinemeier Hansson
  • EOPL Essentials of Programming Languages by Daniel P. Friedman, Mitchell Wand, and Christopher T. Haynes
  • CLR Introduction to Algorithms by Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest
  • CLRS Introduction to Algorithms, 2nd Edition by Thomas H. Cormen, Charles E. Leiserson, Ronald L. Rivest, and Cliff Stein
  • CLtL Common Lisp the Language by Guy L. Steele Jr.
  • CTM Concepts, Techniques, and Models of Computer Programming by Peter Van Roy and Seif Haridi
  • GOF Design Patterns by Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides (the Gang of Four)
  • HOP Higher-Order Perl by Mark Jason Dominus
  • HTDP How To Design Programs by Matthias Felleisen, Robert Bruce Findler, Matthew Flatt and Shriram Krishnamurthi
  • K&R The C Programming Language by Brian Kernighan and Dennis Ritchie
  • LiSP Lisp In Small Pieces by Christian Queinnec
  • LL Lessons Learned in Software Testing by Cem Kaner, James Bach and Bret Pettichord
  • PAIP Paradigms of Artificial Intelligence Programming by Peter Norvig
  • PCL Practical Common Lisp by Peter Seibel
  • PLP Programming Language Pragmatics by Michael L. Scott
  • SICP Structure and Interpretation of Computer Programs by Hal Abelson and Gerald J. Sussman, with Julie Sussman
  • TAOCP The Art Of Computer Programming by Donald E. Knuth
  • TAPL Types And Programming Languages by Benjamin C. Pierce
  • TCPL The C++ Programming Language by Bjarne Stroustrup
  • TCS Testing Computer Software by Cem Kaner, Jack Falk, and Hung Quoc Nguyen
  • TLS The Little Schemer by Daniel P. Friedman and Matthias Felleisen
  • TSPL The Scheme Programming Language by R. Kent Dybvig
  • TSS The Seasoned Schemer by Daniel P. Friedman and Matthias Felleisen
  • TRS The Reasoned Schemer by Daniel P. Friedman, William E. Byrd and Oleg Kiselyov
  • WELC Working Effectively with Legacy Code by Michael Feathers

Introdução ao Solaris e opensolaris.org

Você lembra que eu falei sobre Cursos de Java de Graça para Estudantes?

Opensolaris Logo

O Sun Student Courses abriu mais um curso (Introduction to Solaris and opensolaris.org), agora sobre Solaris e opensolaris.org no mesmo estilo do curso anterior (Real World Technologies: NetBeans GUI Builder, JRuby, JavaFX, and JavaME).

Sun Students Courses

São 5 tópicos:

  1. Introduction to the course and opensolaris
  2. Java Desktop System
  3. SMF – Service Management Framework

  4. Solaris Containers, OS level virtualization for Solaris

  5. Introduction to ZFS
  6. DTrace – Dynamic instrumentation of system and applications

E só para lembrar:

  • É de graça!
  • No momento os cursos só estão disponíveis em inglês.
  • Cada tópico tem uma parte em texto, uma em slides e um questionário no final. Quando você responder corretamente todos os questionários você pode pegar um certificado de conclusão de curso.
  • Não perca essa ótima chance de estar em contato com tecnologias de ponta como ZFS e Dtrace. ;)