Once Upon a Coding ...

Java and Beyond, a New Year Resolution ...

2011-12-27T07:59:00.001-08:00

Maybe this should be my NewYear resolution! I’m changing this blog from a just-java blog to my generalcoding experiment blog. First, I’m going to share a lot more especiallyregarding my Fall 2011 experience with C++. I’m also finding it more and moredifficult to tag every post with just one technology. When a simple chatapplication calls for PHP, JavaScript and Ajax, wouldn’t it be useless todivide a post on it in three different blogs? I guess I’m going to keep theaddress. After all, I’m still very Java-based and even a C++ programmer won’tmind a cup of Java once in a while.

Adding Java Data Types to Visio

2011-12-12T12:39:00.001-08:00

If you want to create UMLdiagrams using Microsoft Visio but you cannot find Java data types in it, there’sa very quick way of getting them. As you know, Java data types are very similarto the ones of C# - or may I say C# is very similar to Java – You can duplicateC# data types and make any changes required later, e.g. I would change string to String and bool to boolean. For this, assuming you’reusing Visio 2010, Go to UML menu, click on Packages, select C# Data Typesand push Duplicate. A new package will be added named Package1 orsomething like that. Change the name to something like Java Data Typesand click ok. You’re all set!

Using Processing for simpler Graphics

2011-11-27T15:07:00.001-08:00

Creating animated graphicsand artworks is not easy in Java – ok, it’s a lot easier than C++ but still notclean – fortunately you can use Processingand export your code as a java jar file, applet or application, and hence useit in your java application. Processing takes care of many dirty details youhave to do manually in a regular java program. I’ve written a code to run asimple key catching game, which you can download from here. The game keepsgenerating random character and gives you a score every time you hit the rightcharacter. Simply download processing, copy my code – it comes with detailedcomments - and run the game. I know it’s not beautiful but it’s really easy tomake it look good, in terms of fonts and colors. You might also want to look atexamples from this workshop, Cate Huston created and after helpingher a couple of times, I’ve continued teaching it. Once you save your projectand export it as an applet, go under the applet folder of your project and haveyour browser open the file index.html.

An Example of using Static Members

2011-10-13T15:15:00.000-07:00

Static members, we all know,are class members that are shared amongst all instances of the class and thatyou can access them without instantiating the class – well, the compiler does adefault instantiation but that’s hidden from our point of view. Here’s a goodexample I just encountered today while programming a web application:

I have a list of text labels– which I call them term buttons - that look like hyperlinks but they’re notreally. I wanted to change the color of every label when I click on it. Butwhen I click on another label, I want the previously clicked one to go back toits normal color (See the figure below). This way, User can always say whichlabel is being processed right now. So, I need to define a variable that alwaysholds the lastly clicked button but where to keep it? I have a controller forevery button and the GRASP patterns suggest that the controller is the bestplace to keep that because the controller is the one that needs it and will useit anyways.

But how can I make sure thistracking variable has a consistent value and all instances of controller haveaccess to it? The answer is simply this, define it as a static member! This isthe code fragment of where I defined this:

public classTermSummaryController implements ClickHandler {

private static TermButton selectedTermButton=null;

It is null at the beginning andhere’s how I control it:

@Override

public voidonClick(ClickEvent event) {

TermButtontermButton=(TermButton)event.getSource();

if (selectedTermButton!=null) {

selectedTermButton.setStyleName("termButton");

}

selectedTermButton=termButton;

termButton.setStyleName("termButtonClicked");

if selectedTermButtonis not null, which means a button has been clicked before, I simply set its styleback to normal. Then I assign the newly clicked button to selectedTermButtonand change its style to my clicked style.

* This code is written for Google WebToolkit and does not work with other Java-based frameworks but the idea remainsthe same.

Max Heap using PriorityQueue

2011-09-29T18:55:00.000-07:00

Java has a niceimplementation of Heaps named PriorityQueue. Briefly, a heap is a tree datastructure, where the root is always either the minimum or maximum. The defaultin Java is minimum. Hence, if you need a maximum heap, you should somehowimplement your own comparator. For example the following comparator returns anumber greater than zero if the second parameter is bigger – unlike the naturalordering:

importjava.util.Comparator;

public class MyComparator implements Comparator<Integer>

{

public int compare( Integer x, Integer y )

{

return y - x;

}

};

A min heap on integers issimply defined as:

PriorityQueueheap=new PriorityQueue();

Whereas a max heap onintegers, using MyComparator, is implemented as:

PriorityQueueheap=new PriorityQueue(n, newMyComparator());

n is the initial size of theheap.

Declaring variables using Unicode

2011-09-22T06:29:00.001-07:00

I just had this studentcoming to me this morning asking if identifier names have a lot of constraintsin Java? The answer is almost none – there are a few but compared to old daysis really nothing! The length is now unlimited – well virtually! – The name caninclude and start even with a dollar sign. And even more, you can have Unicode inyour variable names. In the lattercase, you will need to use the compiler directive –encoding though, that isif you use UTF-8 in your code then your compile line will look like

javac–encoding UTF-8 MyJavaProgram.java

In IDEs like Eclipse youmight face problems with just saving the file, so your main issue would just beto change the general encoding option. For example, in Eclipse, go to projectproperties and click on Resource. Then change the Text file encodingto whatever your desired encoding is.

A more effective type checking approach ...

2011-09-18T18:55:00.001-07:00

I said herethat you can use ClassCastException to identify the type of an object. Iwas correct but now I realized this isn’t very effective. It is much moreeffective to use a direct class comparison. So to find out if the type of anobject matches the desired class type simply use this check:

anObject.getClass().equals(TheType.class)

ClassNotFoundException while running from command prompt

2011-09-15T15:38:00.001-07:00

Many Java beginners are advisedor tempted to try running Java from command prompt. As a beginner you mightset-up everything correctly and yet get the exception ClassNotFoundExceptionwhile running a successfully complied program. In some Windows versions youmight have this problem even if you .class file exists within the samefolder. Simply try the following to remind JVM – and perhaps Windows! – that theclass file exists within the same directory – I’m assuming your java class isnamed MyJavaProgram:

java –classpath .MyJavaProgram

One more thing, if you haveto avoid IDEs for writing programs – Java or others – at least use Notepad++. It helps a lot, you’ll foryourself!

less function calls wouldn't kill ...

2011-09-12T19:28:00.000-07:00

I know this may sound verysimplistic but perhaps a common mistake everyone may make – at least I did. You’reto find out what is the maximum length between two strings. You might betempted to do the following:

intmaxLength=s.length()>t.length() ? s.length() : t.length();

Alright this looks nice,maybe professional and C-style for sure, but how about the following:

intmaxLength=s.length();

if(t.length()>maxLength)

maxLength=t.length();

Note that in the lattercase, your doing only two function calls.

Implementing a customized event listener

2011-07-22T15:02:00.001-07:00

You’ve been perhaps using listeners/handlers but it would be nice to implement your own. A good example is when you have a class for managing global settings in your application, which many other classes refer to retrieve different types of settings such as default language, color or size. This is one way to implement this pattern:

First, you need an interface:

public interface SettingsListener {

public void onChange();

}

Every Class implementing this interface will then use its own specific strategy to deal with new changes.

Second, in your settings class add a collection to hold all listeners:

private Set<SettingsListener> settingsListeners=new HashSet<SettingsListener>();

Using the interface Set, we ensure only one instance of each object is added here. Now, in every method of the settings class, where a change occurs, you need to call the onChange method of all listeners:

for (SettingsListener sl : settingsListeners) {

sl.onChange();

}

Oops! I almost forgot. The settings class needs a method to add change listeners:

public void addChangeListener(SettingsListener settingsListener) {

settingsListeners.add(settingsListener);

}

Finally, you’ll make all classes that refer to settings to read a change implement the SettingsListener interface. For example:

public class A implements SettingsListener {

public A() {

...

Settings.addChangeListener(this);

}

...

@Override

public void onChange() {

...

language=Settings.getLanguage();

...

}

Other classes may need taking color or size or anything else within their onChange method. Note that you need to add this object to settings’ collection of change listeners. Settings might be an object or class depending how you implement it. But perhaps the best way is to implement it as a singleton.

Java For-Each Loop

2011-07-14T13:22:00.000-07:00

The conventional loop has another face in Java. This is sometimes useful especially when you’re doing the same thing to all members of a collection or array. That is you don’t really need a loop counter or collection iterator. Instead you can just use the for loop as follows

Example 1 – An Array:

String[] languages={"Chinese", "Korean", "Japanese"} ;

for (String l : languages) {

System.out.println(l);

}

Example 2 – A Collection:

HashSet<string> languages=new HashSet<string>();

languages.add("Chinese");

languages.add("Korean");

languages.add("Japanese");

for (String l : languages) {

System.out.println(l);

}

This perhaps yields to better performance too as it imposes a data-level parallelism given that the application is running on multi-core – I’m not sure though ;)

The new Java enum type

2011-07-13T13:33:00.000-07:00

Traditionally, programmers use static fields within classes to represent enum types. For example, in order to show a list of languages, a programmer would define a class like the following:

public class Language {

public static int UNKNOWN=0;

public static int PERSIAN=1;

public static int ENGLISH=2;

public static int FRENCH=3;

public static int ARABIC=4;

public static String[] getAllLanguages() {

String[] languages={"Unknown", "Persian", "English", "French", "Arabic"};

return languages;

}

public static int getLanguageIndex(String string) {

// TODO Auto-generated method stub

if (string.compareToIgnoreCase("Persian")==0)

return 1;

if (string.compareToIgnoreCase("English")==0)

return 2;

if (string.compareToIgnoreCase("French")==0)

return 3;

if (string.compareToIgnoreCase("Arabic")==0)

return 4;

return 0;

}

Since Java 1.5, you can define enums using a more efficient, more flexible and more beautiful structure called enum. A simple enum for languages looks like this:

public enum Language {

UNKNOWN,

PERSIAN,

ENGLISH,

FRENCH,

ARABIC

}

In the above code, every constant is in fact an instance of the enum Language. Hence, they can all have their own constructor, fields and methods. For example, to add an string value representing every constant, you would do the following:

public enum Language {

UNKNOWN(“Unknown”),

PERSIAN(“Persian”),

ENGLISH(“English”),

FRENCH(“French”),

ARABIC(“Arabic”)

private final String languageName;

public Language(String name) {

lanuageName=name.toUpperCase();

}

@Override

public String toString() {

return languageName;

}

Also, to obtain the numeric value, you can add an abstract method and implement it for every constant to return the value you wish as follows:

public enum Language {

UNKNOWN(“Unknown”) {public int index() {return 0} },

PERSIAN(“Persian”) {public int index() {return 1} },

ENGLISH(“English”) {public int index() {return 2} },

FRENCH(“French”) {public int index() {return 3} },

ARABIC(“Arabic”) {public int index() {return 4} };

private final String languageName;

public Language(String name) {

lanuageName=name.toUpperCase();

}

@Override

public String toString() {

return languageName;

}

Public abstract in index();

}

The following code uses the Language enum to fill in a list of languages:

for (Language l : Language.values()) {

languageList.addItem(l.toString());

}

The method values() returns all members of the enum. Accessing a certain member is as easy as the following:

Language.FRENCH

And accessing the numberic value of it would be follows:

Language.FRENCH.index();

SAX Exception while deploying GWT Application on GAE

2011-06-29T14:18:00.000-07:00

I just faced this problem while trying to deploy my GWT application on Google Application Engine. This is the message

An internal error occurred during: "Deploying ... to Google".

Received SAXException parsing the input stream for .../war\WEB-INF/web.xml

This, often, doesn’t have anything to do with the XML file but just in case, check the validity of your xml first. If the file is ok then the problem is simply that one of the websites your XML is referring to is down! In my case, java.sun.com was done, so go check those out. With the growing number of XMLs referring to reference sites such as Java’s this is becoming a typical problem so though sharing it would help.

Sorting arrays using comparator

2010-12-14T14:26:00.001-08:00

Sorting arrays could be simply performed using the function sort of the class Arrays. This, however, needs implementing the interface Comparator for the specific type you’re storing in the array. Consider the following Class:

public class Car {

double speed;

int cylinders;

double gas_per_mile;

public Car(double s, int c, double g) {

speed=s;

cylinders=c;

gas_per_mile=g;

}

public String toString() {

return "speed="+speed+" sylinders="+cylinders+"gas="+gas_per_mile;

}

When sorting an arrays of cars we’d like to see cars sorted from minimum speed to maximum speed but from maximum gas_per_mile to minimum one. We, thus, implement two comparators. Notice that the usage of comparator enables the developer to sort the array in any custom order required:

import java.util.Comparator;

public class GasComparator implements Comparator<Car> {

@Override

public int compare(Car arg0, Car arg1) {

// TODO Auto-generated method stub

if (arg0.gas_per_mile>arg1.gas_per_mile)

return -1;

if (arg0.gas_per_mile<arg1.gas_per_mile)

return 1;

return 0;

}

import java.util.Comparator;

public class SpeedComparator implements Comparator<Car> {

@Override

public int compare(Car arg0, Car arg1) {

// TODO Auto-generated method stub

if (arg0.speed<arg1.speed)

return -1;

if (arg0.speed>arg1.speed)

return 1;

return 0;

}

The interface Comparator implements the function compare, which returns an integer value. The most common use is to return -1 if arg0 is smaller than arg1, return 1 if arg0 is bigger than arg1 and 0 elsewhere but as implemented in GasComparator one may use other ways of comparison as well. Try the following test program:

import java.util.*;

public class Test {

/**

* @param args

public static void main(String[] args) {

// TODO Auto-generated method stub

Car[] cars={new Car(225.0,6,10.5),new Car(215.5,4,12.5),new Car(320,6,15.5)};

Arrays.sort(cars, new SpeedComparator());

System.out.println("sorting based on maximum speed");

for (int i=0;i<cars.length;i++) {

System.out.println(cars[i].toString());

}

System.out.println("now sorting based on gas usage");

Arrays.sort(cars, new GasComparator());

for (int i=0;i<cars.length;i++) {

System.out.println(cars[i].toString());

}

The output is as follows:

sorting based on maximum speed

speed=215.5 sylinders=4 gas=12.5

speed=225.0 sylinders=6 gas=10.5

speed=320.0 sylinders=6 gas=15.5

now sorting based on gas usage

speed=320.0 sylinders=6 gas=15.5

speed=215.5 sylinders=4 gas=12.5

speed=225.0 sylinders=6 gas=10.5

note that the array is first sorted based on speed from min to max, then sorted base on gas usage from max to min.

Sub-packaging in Eclipse

2010-12-11T07:02:00.000-08:00

I created a UI widget for a GWT project in Eclipse. But I wanted to separate logic from UI, so I created a UI package and moved the widget class to this new package. This resulted in an error saying GWT compiler cannot access the code for the widget: "No source code is available for ...". I realized, the widget code should be in the same folder/package of the package client. But Eclipse doesn’t allow creating a sub-package. So the problem turned out to be this: In a project, e.g. proj1, how can I create a sub-package, e.g. ui under another package, e.g. client? The answer is as simple as this, rename the package ui to client.ui or if your package names are trailed by the project name, change its name from proj1.ui to proj1.client.ui. Having done this, your package still appears as an independent one in Eclipse but if you soar to your eclipse workspace, you’ll see that the folder ui appears under the folder client.

GWT and Database? Huh?

2010-06-23T21:57:00.000-07:00

I’m assuming you’re using Eclipse-based Google Web Toolkit (GWT). If you try to connect to database using JDBC in GWT you will face several problems. First thing, if you receive a ClassNotFoundException on your JDBC driver, make sure you have done the followings to get rid of it:

1- Add the driver JAR to your application classpath

2- Copy the driver JAR under you application’s war/web-inf/lib folder

But that’s not even the half of the problem. The real issue is with getting connected to the database, which is not possible – at least as far as I’ve tried – under GWT’s default server simply because it does not permit socket connections to proceed and there’s no way to add such permissions as a java policy file or something like that.

I suggest using Tomcat instead. Here’s what you do (In Eclipse – I’m using Europa):

1- Create an XML file under your project folder with the following contents:

</webinf>

</war>

</target>

</target>

</project>

2- 2- Make sure your code is good to the best of your knowledge

3- Clean your project: Project Menu – Clean

4- Right Click on your project. Select Google -> GWT Compile. This might take a few moments to complete.

5- Right Click on you project. Select Export – Jar File – Make sure only the src folder of your project is selected. The destination should be under your project folder in …/war/web-inf/lib.

6- Right click on the xml file created in the first step and select Ant Build.

If everything goes well, you are going to see a war file under your project folder. Copy this under the webapps folder of your tomcat; I suggest downloading a recent version of Tomcat. If this is not your first time, stop Tomcat, delete the project under webapps devoted to your project if any, start it again. Check if a folder for you project is created under tomcat and if it’s not empty. Open you browser and go to http://localhost:8080/yourprojectname

p.s. GWT has its own internal datastore mechanism, which is less know to most developers. This post addresses those who wish to use GWT with their own preference of database.

Reading an XML File in Java

2009-02-22T19:50:00.000-08:00

The following code is all you need to open an XML file using Java:

public static void openXML(String xmlFile) {
try {
DocumentBuilderFactory docBuilderFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder docBuilder = docBuilderFactory.newDocumentBuilder();
Document xmlProfile = docBuilder.parse (new File(xmlFile));

...

} catch (ParserConfigurationException pce) {
System.out.println("ParserConfigurationException: " + pce.toString());
} catch (IOException ioe) {
System.out.println("IOException: " + ioe.toString());
} catch (SAXException sxe) {
System.out.println("SAXException: " + sxe.toString());
}
}

You would need the following inclusions:

import java.io.IOException;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.w3c.dom.Document;
import org.xml.sax.SAXException;

xmlProfile provides you the facilities you require to read XML elements. Some useful methods are:
getChildNodes(), returns a list of all children.
getElementsByTagName(String aName), finds all elements tagged as aName
getAttributes(), returns a list of attributes.

Methods usually return a list such as NodeList. Items can be accessed using item method. For example the following code prints all the attributes of a selected node:

for (int i=0;i
String attrName=node.getAttributes().item(i).getNodeName();
String attrVal=node.getAttributes().item(i).getNodeValue();
System.out.println(attrName+ “ “+ attrVal);
}

Input from Console

2009-01-14T17:51:00.000-08:00

It's amazingly a very common question. The truth is it was an annoying job to read things from command line in Java before Java 5.x. Now the Scanner class has made things easy. This is how you do it for a simple integer input:

Scanner scanner=new Scanner(System.in);

System.out.println("Eneter a number between 1 to 100:");
int num=scanner.nextInt();

One may even use the following loop to read more than one input at once:

while (scanner.hasNext()) {
num=scanner.nextInt();

...
}

the loop reads from input while there is input to read. Other methods such as nextString, nextLong, nextDouble, ... read other types of data. It is also possible to provide a pattern to read according to: scanner.read(“a*b”); hasNext method used in the above loop could also be applied to specific types or patterns.

Java Dialog Box

2009-01-11T09:19:00.000-08:00

One may always extend a complex window class such as Jframe to create a dialogh box in Java. However, there is a quick way for building simple dialog boexs such as message box or input box. This is provided using JoptionPane, a class accessible through javax.swing. JoptionPane has four useful methods:

showConfirmDialog: Asks a confirming question, like yes/no/cancel.
ShowInputDialog: Prompt for some input.
ShowMessageDialog: Tell the user about something that has happened.
ShowOptionDialog: a mix of all three above.

Exampes of ShowInputDialog, which I use a lot:

String name=JOptionPane.showInputDialog("What's your name?");
Inputs a name

String[] selectionValues={"ONE", “TWO”, “THREE”};
String defaultSelection = "ONE";
Object selection = JOptionPane.showInputDialog(null, "Select a number",
"NUMBER SELECTION", JOptionPane.QUESTION_MESSAGE, null, selectionValues, defaultSelection);

shows a dropdown selection box. There are 7 parameters:

1- Just leave it null. Indicates the parent component though.
2- Message
3- Title
4- Type of ICON. Could be any/mix of ERROR/INFORMATION/WARNING/QUESTION/PLAIN_Message
5- The buttons you'd like in your dialog. Leave it null for a simple OK button. Other options are: DEFAULT_OPTION/YES_NO_OPTION/YES_NO_CANCEL_OPTION/OK_CANCEL_OPTION
6- The array of selection values
7- default item

Confirm Dialog is also interesting. This is how I use a simple instance:

int login=JOptionPane.showConfirmDialog(null, "Is login required to browse this page?", "Login Check", JoptionPane.YES_NO_OPTION);
if (login==JOptionPane.YES_OPTION)
...
else
...

Open File Dialog in Java

2008-12-20T21:51:00.000-08:00

The easiest way to create an Open File Dialog - that I'm aware of - in java is to use JFileChooser. This is relatively easy. Just look at the following exerpt of code:

import javax.swing.*;
import javax.swing.filechooser.*;
...
1: JFileChooser fd = new JFileChooser(".");
2: ExampleFileFilter filter = new ExampleFileFilter();
3: filter.addExtension("tst");
4: fd.setFileFilter(filter);
5: int returnVal = fd.showOpenDialog(null);
6: if(returnVal == JFileChooser.APPROVE_OPTION) {
7: String fileName=fd.getSelectedFile().getName();
8: }

Line 1 creates a simple file chooser that opens a dialog in the current path/directory. “.” stands for the current path; that is the path in which your program runs. One may change this according to his/her program requirements.
Line 2 creates a file filter. Line 3 adds an extension to this file filter and Line 4 adds the filter to the dialog.
Line 5 shows the the dialog. Return value is stored within an integer variable that shows the status of the dialog. If the a file was successfully chosen, selected file name may be obtained as shown in Line 7.
But wait a minute ... what is the example file filter? frankly speaking, creating a file filter could be a headache. Fortunately, an ExampleFileFilter is available from Sun's website. My Suggestion, just download it and use it as it is.

File Buffered Read/Write

2008-12-05T16:33:00.000-08:00

Reading/writing files in Java may be done in a variety of ways. The most efficient way is probably to use serializable objects and IO streams. But for this post I’ll review an even simpler approach using buffered writer/readers. The following example contains the methods required for writing and reading a file. One may add as much as information required. The problem with this approach is that it’s not efficient and that both the reader and writer must be aware of each other, which is a bit against object-oriented ideas behind Java but it’s ok for a small experience:

import java.io.BufferedReader;
import java.io.File;
import java.io.BufferedWriter;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;

public class Test {

private static void save() {
File helloFile=new File("HelloWorld.txt");

BufferedWriter output = null;
try {
output = new BufferedWriter( new FileWriter(helloFile) );
output.write("Helo World!");

} catch(FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
finally {
try {
if (output!= null) {
output.close();
}
}
catch (IOException ex) {
ex.printStackTrace();
}
}
}

private static String load() {
File helloFile=new File("HelloWorld.txt");

BufferedReader input = null;
try {
input = new BufferedReader( new FileReader(helloFile) );
String helloWorld=input.readLine();

return helloWorld;
} catch(FileNotFoundException e) {
e.printStackTrace();
return "FileNotFound";
} catch (IOException e) {
e.printStackTrace();
return "IO Error";
}
finally {
try {
if (input!= null) {
input.close();
}
}
catch (IOException ex) {
ex.printStackTrace();
}
}
}

public static void main(String[] args) {
save();
String hello=load();
System.out.println(hello);
}

}

Singletons in Java

2008-11-29T15:45:00.000-08:00

Singleton is a design pattern (ok, design patterns are to be discussed in another note!) that guarantees a certain object will only have one single instance at runtime. This is sometimes required if different versions/threads/extensions of the program might try to re-initialize a specific object without knowledge of each other. There are serious discussions if Singleton is required in Java at all and I will discuss this later in a separate post but for now just consider it as yet another way of doing things. The basic way of creating a singleton is as follows:

public class Singleton {
private static Singleton instance=null;

public static Singleton getInstance() {
if (instance==null)
instance=new Singleton();
return instance;
}
}

Some good questions:
- What if someone tries to make a new instance of the singleton? Nothing because method and attribute are both static.
- What if two applications call getInstance() simultaneously? Good question, you may make it a synchronized entity. Just change the declaration to

public static synchronized Singleton getInstance()

- What if a naughty girl tricks the program by cloning the singleton object? Just override the clone method of your Singleton class as follows:

public Object clone() throws CloneNotSupportedException {
throw new CloneNotSupportedException();
}

Flexible Type Checking with Java Exceptions

2008-11-21T11:30:00.000-08:00

If you have always thought that exceptions are only good to catch errors, this post may change your understanding of Java exceptions. Today, I’m going to mention a different application of exceptions that is to use them for identifying the type of an object. Consider a situation, where you have different objects inheriting from an upper-hand object. For example, a profile creation system that registers different types of members may have the following hierarchy:

class Member {
}

class Student extends Member {
}

class Teacher extends Member {
}

The main registration facility is the same for both types so the developer decides to create one register method. However, in each case the system should take a different strategy for setting up members’ profile. Amongst other ways to doing so I describe the technique of using Java exceptions:

public Profile createProfile(Member newMember) {
try {
Student newStudent=(Student)newMember;
…
} catch (ClassCastException cce) {
try {
Teacher newTeacher=(Teacher)newMember;
…
} catch (ClassCastException cce) {
…
}
}}

The first class casting tries to convert the newMember object to a Student object; if not successful the flow is trapped in the catch block, where there is another class cast operation towards a Teacher object. The cascade may go on for as many objects as extend the main Member object.

Iterators

2008-10-27T03:55:00.000-07:00

You might have heard of Iterators; if not you should know they’re used to iterate over a collection. Iterators can replace the traditional way of defining a counter for looping. This is a more object-oriented way of doing things. Collections usually have an operation, iterator(), which provides the iterator for further use. The following example shows how to use an iterator over an array:

Import java.util.ArrayList;
Import java.util.Iterator;

…

ArrayList students;

…

Iterator studentIterator=students.iterator();
While (studentIterator.hasNext()) {
Student currentStudent=(Student)studentIterator.next(); // The next() method returns an object. You should typecast it before using it.
// do something with currentStudent
}

Once Upon a Java

2008-09-24T06:40:00.000-07:00

Once upon a time there was ME, MY PHD THESIS and A LOT OF JAVA WORK! ME was really in love with open source, open mind, open doors, open .... So ME decided to open A LOT OF JAVA WORK that I had to do in MY PHD THESIS to others. Hopefully, this way Liberals will win all the upcoming elections!!!

Ok! Java has been programmers' favorite taste for a while now. I'm only here to expose simple small tips in Java as occurs in my own expereince. The first tip is obviosuly to not drink your coffee when it's so hot nor to drink it all at once. A cup of coffee must be drunk gently so that it cools down and coooools you down! Java is no exception. Stay tuned ... be back soooooooon!