Skip to content

Tag: programming

Example of Unix commands implemented in Java

I created some illustrative and simple implementations of common Unix commands. For those who are familiar with Unix-like systems them make easier to understand Java. For those who are familiar with Java them make easier to understand Unix-like systems. :-)

1. PWD

The first one is pwd that show the current working directory.

public class Jpwd {
    public static void main(String[] args) {
        String pwd = System.getProperty("user.dir");
        System.out.println(pwd);
    }
}

Running this at /home/silveira directory gives us as output:

$ java Jpwd

/home/silveira

1. CAT

The command cat is usually utilized for displaying files.

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
 
public class Jcat {
    public static void main(String[] args) {
        if(args.length==1){
            try {
                FileReader fileReader = new FileReader(args[0]);
                BufferedReader in = new BufferedReader(fileReader);
                String line;
                while((line = in.readLine())!= null){
                    System.out.println(line);
                }
            } catch (FileNotFoundException ex) {
                System.out.println(args[0]+", file not found.");
            }
            catch (IOException ex) {
                System.out.println(args[0]+", input/output error.");
            }
        }
    }
}

$ java Jcat /etc/timezone

America/Fortaleza

3. LS

The command ls is to list files. The File API (java.io.File) is very flexible and portable, but in this example I want just list files and directories of the current directory.

import java.io.File;
 
public class Jls {
    public static void main(String[] args) {
        File dir = new File(System.getProperty("user.dir"));
        String childs[] = dir.list();
        for(String child: childs){
            System.out.println(child);
        }
    }
}

Usage:

$ java Jpwd

/home/silveira/example

$ java Jls

directoryA

fileA

.somefile

4. CD

The cd command changes the current working directory.

import java.io.File;
 
public class Jcd {
    public static void main(String[] args) {
        if(args.length==1){
            File dir = new File(args[0]);
            if(dir.isDirectory()==true) {
                System.setProperty("user.dir", dir.getAbsolutePath());
            } else {
                System.out.println(args[0] + "is not a directory.");
            }
        }
    }
}

Usage:

$ java Jpwd
/home/silveira
$ java Jcd /tmp
$ java Jpwd
/tmp

JavaFX, Draggable Nodes

One thing that I like a lot to do with JavaFX is draggable objects. Due to some recent changes in the JavaFX syntax my old codes for that are no longer working. Joshua Marinacci from Sun’s JavaFX engineering team and other guys from the JavaFX community gave me some tips. Here some strategies I’m using for making draggable nodes in JavaFX.

In this first example, a simple draggable ellipse.


video url: http://www.youtube.com/watch?v=pAJHH-mPLaQ

import javafx.application.*;
import javafx.scene.paint.*;
import javafx.scene.geometry.*;
import javafx.input.*;
 
Frame {
    width: 300, height: 300, visible: true
    stage: Stage {
        content: [
            Ellipse {
                var endX = 0.0; var endY = 0.0
                var startX = 0.0; var startY = 0.0
                centerX: 150, centerY: 150
                radiusX: 80, radiusY: 40
                fill: Color.ORANGE
                translateX: bind endX
                translateY: bind endY
                onMousePressed: function(e:MouseEvent):Void {
                    startX = e.getDragX()-endX;
                    startY = e.getDragY()-endY;
                }
                onMouseDragged: function(e:MouseEvent):Void {
                    endX = e.getDragX()-startX;
                    endY = e.getDragY()-startY;
                }
            }
        ]
 
    }
}

When you need to create a group of draggable objects, you can try thie approach of a draggable group like this. Inside on it, you can put whatever you want.


Video url: http://www.youtube.com/watch?v=mHOcPRrgQCQ

import javafx.application.*;
import javafx.scene.paint.*;
import javafx.scene.geometry.*;
import javafx.input.*;
import javafx.scene.*;
import javafx.scene.effect.*;
import javafx.scene.image.*;
import javafx.animation.*;
 
// a graggable group
public class DragGroup extends CustomNode{
    public attribute content: Node[];
 
    private attribute endX = 0.0;
    private attribute endY = 0.0;
 
    private attribute startX = 0.0;
    private attribute startY = 0.0;
 
    public function create(): Node {
        return Group{
            translateX: bind endX
            translateY: bind endY
            content: bind content
        }
    }
 
    override attribute onMousePressed = function(e:MouseEvent):Void {
        startX = e.getDragX()-endX;
        startY = e.getDragY()-endY;
    }
 
    override attribute onMouseDragged = function(e:MouseEvent):Void {
        endX = e.getDragX()-startX;
        endY = e.getDragY()-startY;
    }
}
 
// angle animation, cycles between 0 to 360 in 36 seconds
var angle = 0.0;
var angleAnimation = Timeline {
    repeatCount: Timeline.INDEFINITE
    keyFrames : [
        KeyFrame {
            time: 0s
            values: 
                    angle => 0.0
        },
        KeyFrame{
            time: 36s
            values :  
                    angle => 360.0 tween Interpolator.LINEAR
        }
    ]
}
 
// some pictures from my Flickr albums 
var me    = "http://farm4.static.flickr.com/3042/2746737338_aa3041f283_m.jpg";
 
 
 
var dog   = "http://farm4.static.flickr.com/3184/2717290793_ec14c26a85_m.jpg";
var plant = "http://farm4.static.flickr.com/3014/2731177705_bed6d6b8fa_m.jpg";
var bird  = "http://farm4.static.flickr.com/3190/2734919599_a0110e7ce0_m.jpg";
var me_89  = "http://farm3.static.flickr.com/2138/2308085138_7b296cc5d0_m.jpg";
 
 
Frame {    
    width: 640, height: 480, visible: true
    stage: Stage {
        fill: Color.BLACK
        content: [
            DragGroup{
                content: ImageView {
                    anchorX: 120, anchorY: 90
                    rotate: bind 30 + angle
                    image: Image { backgroundLoading: true, url: me }
                }
            },
            DragGroup {
                translateX: 300, translateY: 50
                content: ImageView {
                    anchorX: 120, anchorY: 90
                    rotate: bind -30 + angle
                    image: Image { backgroundLoading: true, url: dog }
                }
            },
            DragGroup {
                translateX: 300, translateY: 300
                content: ImageView {
                    anchorX: 120, anchorY: 90
                    rotate: bind 90 + angle
                    image: Image { backgroundLoading: true, url: plant }
                }                
            },
            DragGroup {
                translateX: 200
                translateY: 200
                content: ImageView {
                    anchorX: 120, anchorY: 90
                    rotate: bind 90 + angle
                    image: Image { backgroundLoading: true, url: bird }
                }                
            },
            DragGroup {
                translateX: 30
                translateY: 200
                content: ImageView {
                    anchorX: 85, anchorY: 120
                    rotate: bind angle + 180
                    image: Image { backgroundLoading: true, url: me_89 }
                }                
            },
        ]
 
    }
 
    closeAction: function() { 
        java.lang.System.exit( 0 ); 
    }
}
 
angleAnimation.start();

One more example, using the same class DragGroup, we can put multiple nodes using lists.


Video url: http://www.youtube.com/watch?v=gJqy7EdtEqs

import javafx.application.*;
import javafx.scene.*;
import javafx.scene.geometry.*;
import javafx.scene.paint.*;
import javafx.input.*;
import javafx.animation.*;
import java.lang.Math;
 
// Class to create a draggable group of objects
public class DragGroup extends CustomNode{
    public attribute content: Node[];
 
    private attribute endX = 0.0;
    private attribute endY = 0.0;
 
    private attribute startX = 0.0;
    private attribute startY = 0.0;
 
    override attribute onMousePressed = function(e:MouseEvent):Void {
        startX = e.getDragX()-endX;
        startY = e.getDragY()-endY;
    }
 
    override attribute onMouseDragged = function(e:MouseEvent):Void {
        endX = e.getDragX()-startX;
        endY = e.getDragY()-startY;
    }
 
    public function create(): Node {
        return Group{
            translateX: bind endX
            translateY: bind endY
            content: bind content
        }
    }
}
 
// angle animation, cycles between 0 to 360 in 10 seconds
var angle = 0.0;
var angleAnimation = Timeline {
    repeatCount: Timeline.INDEFINITE
    keyFrames : [
        KeyFrame {
            time: 0s
            values: angle => 0.0
        },
        KeyFrame{
            time: 10s
            values :  angle => 360.0 tween Interpolator.LINEAR
        }
    ]
}
 
// breath animation, go and back from 0.0 to 10.0 in 2 seconds
var breath = 0.0;
var breathAnimation = Timeline {
    repeatCount: Timeline.INDEFINITE
    autoReverse: true
    keyFrames : [
        KeyFrame {
            time: 0s
            values: breath => 0.0
        },
        KeyFrame{
            time: 1s
            values :  breath => 10.0 tween Interpolator.LINEAR
        }
    ]
}
 
// Creates n multi colored floating circles around a bigger circle
var n = 12;
var colors = [
    Color.BLUE, Color.AQUA, Color.MAGENTA, Color.RED,
    Color.YELLOW, Color.ORANGE, Color.HOTPINK, Color.LIME
];
var chosen = Color.YELLOW;
var floatingCircles = Group{
    rotate: bind angle
    content: for (i in [1..n]) 
    Circle {
        fill: colors[i mod sizeof colors]
        radius: 10
        centerX: Math.cos(i * 2 * Math.PI/n) * 70
        centerY: Math.sin(i * 2 * Math.PI/n) * 70
        onMouseClicked: function( e: MouseEvent ):Void {
            chosen = colors[i mod sizeof colors];
        }
    }
}
var circle = Circle{
    radius: bind 50 + breath
    fill: bind chosen
}
 
 
Frame {
    width: 400, height: 400, visible: true
    stage: Stage {
        fill: Color.BLACK
        content: [
            DragGroup{
                translateX: 200, translateY: 200
                content: [circle, floatingCircles]
            }
        ]
    }
 
    closeAction: function() { 
        java.lang.System.exit( 0 ); 
    }
}
 
// starts all animations
angleAnimation.start();
breathAnimation.start();

Compiling Inkscape

Inkscape running

Inkscape is a Open Source vector graphics editor that works with SVG (Scalable Vector Graphics) format, Inkscape works with transparency, gradients, node editing, pattern fills, PNG export, and more. It also runs on Linux, Windows and OSX, those three are officially supported, but also runs in a broad list of Operational Systems. Is a software that I work daily and frequently is featured here in my blog.

You can download Inkscape or directly install it via some package system like Apt:

sudo apt-get install inskcape

But sometimes we need some special feature that is not available yet in the repositories or we want gain speed by having special binaries for our platforms or we want to help developing a new feature. In those cases we need to compile the software by ourself.

Those tips are valid for Ubuntu 8.04 but some part of them can be applied in others distributions. The Inkscape compiled here is the version 0.46+devel so newest versions can have compiling procedures slightly different.

Getting sources via APT.The easiest way to compile Inkscape on Ubuntu is

sudo su
apt-get build-dep inkscape
apt-get source inkscape
cd inkscape
./autogen.sh
./configure
make
make install

This will get a version of inkscape, compile it and install. If the first step doesn’t work well, you can try install all necessary packages by yourself using:

sudo apt-get install autotools-dev fakeroot dh-make build-essential autoconf automake intltool libglib2.0-dev libpng12-dev libgc-dev libfreetype6-dev liblcms1-dev libgtkmm-2.4-dev libxslt1-dev libboost-dev libpopt-dev libgsl0ldbl libgsl0-dev libgsl0-dbg libgnome-vfsmm-2.6-dev libssl-dev libmagick++9-dev libwpg-dev

Getting sources via SVN. The recipe I showed above will compile a stable version of Inkscape but not the last version of Inkscape. For that we need to grab the source directly from the Subversion repositories and so compile it.

At your home folder:

sudo apt-get install subversion
svn checkout https://inkscape.svn.sourceforge.net/svnroot/inkscape/inkscape/trunk inkscape

A alternative way to subversion is getting sources from here. Those are tarballs built every hour after someone change something in the development repositories. Download a tarball, and decompress it on your home folder.

Install all tools we need to compile Inkscape, this should fits:

sudo apt-get install autotools-dev fakeroot dh-make build-essential autoconf automake intltool libglib2.0-dev libpng12-dev libgc-dev libfreetype6-dev liblcms1-dev libgtkmm-2.4-dev libxslt1-dev libboost-dev libpopt-dev libgsl0ldbl libgsl0-dev libgsl0-dbg libgnome-vfsmm-2.6-dev libssl-dev libmagick++9-dev libwpg-dev

Enter in the directory with the Inkscape source and do:

./autogen.sh
mkdir build
cd build
../configure
make
sudo make install

In both cases, grabbing sources via svn or via apt, or can set the place where the software will be installed so it not cause conflicts with you already installed version of Inkscape. You can do that replacing the ./configure step with something like:

./configure –prefix=/home/yourname/inkscape

If you had some trouble in one of those steps, consider reading some of those other tutorials:

ps: thanks guys from the inkscape-devel@lists.sourceforge.net specially heathenx.

Ruby: A Simple Example of Meta Programming

A really cool piece of code I read today in the book Ruby on Rails: Up and Running.

class Talker
  def method_missing(method)
     if method.to_s =~ /say_/
       puts $'
     end
  end
end
 
t = Talker.new
 
t.say_hello
t.say_something
t.say_goodbye

And running

$ ruby Talker.rb
hello
something
goodbye

But why do that when we have parameters? Because you can do some methods really dynamic with a clear semantic, just looking for them you know are they are doing.

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