Archive for: January 2012

Java Blocks–Advanced–Part 1

Following a previous post I will deep into the discussion about anonymous blocks mixed with static variables.

Lets see a simple example, and then we will complicate a little bit.

public class TestStaticInitializer {

{
System.out.println(“block 1”);
}

public TestStaticInitializer() {
System.out.println(“static x = ” + x);
}

private static int staticInitialization() {

System.out.println(“static initialization of x”);

return 5;
}

{
System.out.println(“block 2”);
}

static int x = staticInitialization();

public static void main(String[] args) {

System.out.println(“static main block before calling the constructor”);

TestStaticInitializer test = new TestStaticInitializer();

System.out.println(“static main block after constructor”);
}

{
System.out.println(“block 3”);
}

}

You can observe that:
– There is a static variable ‘x’ that its initialized calling a static method.
– There are 3 anonymous blocks.
– The constructor calls the static variable.

The main method references the class.

At first sight we can think the anonymous blocks are called in sequence, which it’s true. Although if there is a static variable initialized in anyway, this initialization runs before any anonymous block.(Why? Because a class is loaded in 2 phases: load class, run class. At load the JVM after some verification, initialized every static variable and runs any static anonymous initializer, and only after runs the class, or our instance.)

You can observe this by looking to the execution output:

static initialization of x
static main block before calling the constructor
block 1
block 2
block 3
static x = 5
static main block after constructor

The anonynous blocks are executed by sequence, but the initialization variable “static initialization of x” runs first, although its declaration is after “block 2”.

The sequence is:

1 – Initialize the static variable x

2 – run main method

3 – initialize the class (because I’m calling the constructor)

4 – run anonymous blocks

5 – runs the constructor (only after the static blocks)

6 – runs the other “main” statements.

You must be wondering why I underlined “The main method references the class” above.

Lets see if I remove (I will just comment it) the call to the class constructor.

public class TestStaticInitializer {

{
System.out.println(“static block 1”);
}

public TestStaticInitializer() {
System.out.println(“static x = ” + x);
}

private static int staticInitialization() {

System.out.println(“static initialization of x”);

return 5;
}

{
System.out.println(“static block 2”);
}

static int x = staticInitialization();

public static void main(String[] args) {

System.out.println(“static main block before calling the constructor”);

//TestStaticInitializer test = new TestStaticInitializer();

System.out.println(“static main block after constructor”);
}

{
System.out.println(“static block 3”);
}

}

and the output is:

static initialization of x
static main block before calling the constructor
static main block after constructor

Well, where are the anonymous blocks initializers? Thet are not needed! We haven’t crated any instance of the class, so the JVM just considered the static stuff!

And if a static block references the static variable x?

public class TestStaticInitializer {

{
System.out.println(“block 1”);
x = 7;
System.out.println(“block 1 after x = 7”);
    }

public TestStaticInitializer() {
System.out.println(“static x = ” + x);
}

private static int staticInitialization() {

System.out.println(“static initialization of x”);

return 5;
}

{
System.out.println(“block 2”);
}

static int x = staticInitialization();

public static void main(String[] args) {

System.out.println(“static main block before calling the constructor”);

//TestStaticInitializer test = new TestStaticInitializer();

System.out.println(“static main block after constructor”);
System.out.println(“x = ” + x);
    }

{
System.out.println(“block 3”);
}

}

The output:

static initialization of x
static main block before calling the constructor
static main block after constructor
x = 5

The initialization of “x = 7” in the first anonymous block is simple ignored, because the code was never ran!

static int x;// = staticInitialization();

The output will show you “x = 0”.

I hope this post will help you understand the mix between anonymous and static initialization blocks. In a future post I will bring some class hierarchy to this discussion.

Anonymous Java Classes and Interfaces

In java you can use Anonymous Classes and Anonymous Interfaces.

Anonymous classes

class MyMessage {
public void printMessage() {
System.out.println("Original message");
}
}

public class TestAnonymousClass {

public static void main(String[] args) {
TestAnonymousClass tac = new TestAnonymousClass();

MyMessage m = new MyMessage();
m.printMessage();

tac.showGeneralMessage(new MyMessage() {
public void printMessage() {
System.out.println("Override message");
}
}
);
}

void showGeneralMessage(MyMessage msg) {
msg.printMessage();
}
}

The Bold source shows how to create the anonymous class. As you can see it’s not so anonymous as we expected, it’s just the name that is anonymous.

In anonymous classes the only think we can do it’s to instantiate an existing class, and override or add some methods. What are we really doing is extending another class(in our example we are extending “MyMessage”) with the possibility to override or add new methods.

If you run the code above the output will be:

Original message

Override message

Anonymous Classes with interfaces

interface MyMessage {
void printMessage();
}

public class TestAnonymousClass {

public static void main(String[] args) {
TestAnonymousClass tac = new TestAnonymousClass();

tac.showGeneralMessage(new MyMessage() {
public void printMessage() {
System.out.println("Override message");
}
}
);
}

void showGeneralMessage(MyMessage msg) {
msg.printMessage();
}
}

Now with interfaces, you can observe that the syntax it’s the same, but we are creating a class by “implementing” the interface MyMessage.

Look that I only changed the MyMessage class to be and interface, but the remaining code is still the same and works with the same behavior (ok, now the output doesn’t have the “Original Message”, because we don’t have one! We lost it when we changed the class to be an interface.)

Hope it helps you in your projects, to implement callbacks, using swing components to register listeners, etc..

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Java Initialization Blocks

Java has some features that are not well know from a lot of Java Developers, even developers with 5 and 10 years of Java Experience.As a Oracle/Java trainer I must study all of them to teach my students and prepare them well for the Java Certification.

One of these bot well know features are java Initialization blocks. One of these are blocks runs before the JVM runs any code (at class loader level) and the other one runs at constructor time.

Lets see an example and an explanation:

Instance Block
class MyClass {
private int x;
{
x = 7;
System.out.println(“This is called inside constructor, just after super(…)”);
}

public MyClass() {
System.out.println(“the constructor”);
}
}
Static Block
class MyClass {
private int x;
public static int y;
static {
y = 7; // y must be static!
System.out.println(“This is called by class loader before your program runs!”);
// x = 5; it’s illegal because this is class context and not instance context
}

public MyClass() {
System.out.println(“the constructor”);
}
}

So, there are 2 types of code initialization blocks:

Instance

When we talk about instances we talk about objects, something that was already created and has memory allocated.

The instance code block is executed right after the call to super in any constructor called.

It’s a way to initialize variables in all constructors without write code in any of them, which it’s a great way to organize initialization code, too.

Static

Static context it’s called class context too. Everything that is static it’s only visible when we use de class name before the field, method or inner class. (ok, unless we use import static…)

When we talk about classes we are talking about the blueprint or definition of the object. This definition “runs” before the instance is created, so the static initialization block runs before the java interpreter and before any constructor called. So, who runs the static initialization blocks? The class loader of the Java Virtual Machine.

This is the place to initialize ONLY static variables, because they are the only ones that are visible at this time and in this context.

In the future I will post more about not so know java features.

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Error starting jBoss inside eclipse

You install jBoss 7. Unzip it to a folder like d:\jboss.

Now install jboss tools 3.3 for eclipse indigo.

When you start the jboss from eclipse (servers tab) you get an error. The console in eclipse doesn’t show any log activity and jboss doesn’t start.

After some research you discover that eclipse cant “talk” with jboss. You can confirm this by reading jboss logs.

You start jboss with standalone.bat and it works.

I got this problem in our dev team, and after some research around I found this solution. It was so simple and at the same time one of that problems that make us thinking about changing to other version or product that I thought it was a good idea to share it.

Possible solution:

1) Go to runtime environment in eclipse options and add jdk runtime environment. Not jre.

2) Make sure that you remove the JRE probably already installed and configured from eclipse. (really! Remove it from eclipse and just let the jdk as an environment).

3) Now, start jboss from the tab servers. It must work by now.

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Learning PHP – Step 4 – Statements

In my last post I’ve talked about variables and operators. Now, lets see what can we do with them.

Statement: if

The “if” statement has 3 main forms:





Learning PHP – Step 2


$name = "José Cruz";
$even = 10 % 2;
$odd = 10 % 3;
$someValue = $even * 4;
$ifvar = 1;

// it’s even – simple "if"
if($even == 0) {
echo "10 its even!";
}

// it’s even – simple "if" with an else
if($even == 0) {
echo "10 its even!";
} else {
echo "10 its odd! Really?";
}

// it’s even – complex "if"
if($even == 0) {
echo "10 its even!";
} elseif($ifvar == 1) {
echo "Althougth true, the first branch has already taken the flow! :(";
} elseif($ifvar == 2) {
echo "Just to ensure you understand this construction!";
} else {
echo "10 its odd! Really?";
}
?>

Statement: switch

“switch” it’s a variation of “if”, but for cases where we have some kind of a list of values belonging to a common domain, like: weekdays, months, (1,3,4,5), etc.

The expression inside “switch()” its evaluated and the result will be compared with each of the “case ”. Each “case” can have the “break” statement, which terminates the statement. If its missing, the execution flow goes to the very next “case” (if its valid) until a “break” it’s found.

If a “case ” was not found, then the “default” its used (it it exists).





Learning PHP – Step 2


$weekday = 1; // assuming a week starts on sunday with value 1

switch($weekday) {
case 1:
echo "Sunday";
break;
case 2:
echo "Monday";
break;
case 3:
echo "Tuesday";
break;
case 4:
echo "Wednesday";
break;
case 5:
echo "Thursday";
break;
case 6:
echo "Friday";
break;
case 7:
echo "Saturday";
break;
default:
echo "big mistake1";
}


?>

And that’s it!

Next post about loops!

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Learning PHP – Step 3 Basics

Following previous posts, lets start with the basics.

Variables:

– Always start with $ and can have the following chars _ [a-z] [A-Z] and [0-9]. the first symbol must be an _ ou a letter.

$count = 0;
$name = “José Cruz”

What about types? If you know python, ruby or the “var x = 0;” construction from C# you know how PHP works. It converts the variable for the type defined by the value initialized. For instance: $count = 0; will be an “integer”.

Lets use some variables in our script:





Learning PHP – Step 2


$name = "José Cruz";
echo $name . " is starting to understand PHP!";
?>

If you run the script above it will display “José Cruz is starting to understand PHP!”

PS: The dot between $name and “ is starting… ” is the concatenation in strings. This is new. Other languages usually use the plus sign or some function to do it.

The operators are the same as C or mainstream languages:

Arithmetic: +, ++, –, –, *, /, %

Assignment: =, +=, –=, *=, /=, %=, .=

Comparasion: ==, <=, <, >= >, <>, !=

Logic: &&, ||, !

PS: You can use “(“ and “)”

Some examples:

$a = 5 + 2 * (7 + 5);
$a++;
$b = $a –4;
$c = $a / $b;
$even = 10 % 2;
$a+=7; // same as $a = $a + 7;
$even = 10;
$even %= 2;
$verytrue = 2 < 7;
$veryfalse = 6 >= 3;
if( $a > 3 ) …. // future post we talk about if and other statements.
if( $a > 3 && $b < 10 || $even == 0 ) …

As with other languages:

– Unary operators precede all other ones ( “!” )

– then: *, / and &&

– Last: +, –, ||

Lets try this in our php website:





Learning PHP – Step 2


$name = "José Cruz";
$even = 10 % 2;
$odd = 10 % 3;
$someValue = $even * 4;

echo $name . " is starting to understand PHP with some operators" . "
";
echo "even values: ". $even . "
";
echo "odd values: ". $odd . "
";
echo "some value = " + $someValue . "
";
?>

Run it!

Next post: statements.

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Learning PHP – Step 2

Following my previous post I’m now preparing an environment to develop in PHP.

1) Install XAMPP – There are other alternatives, but this one, after some research was my choice. Why? Because in the future I’m thinking developing some skills in Perl and with XAMPP it’s a 2 in 1! (Besides mysql included!)

2) IDE – I’m trying Netbeans 7.1. There are a lot others IDEs out there like: komodo, phpstorm, phpide, aptana, eclipse, phpdesigner. My choice probably it’s not the best one, but I develop in Visual Studio and Eclipse and as a Oracle trainer, sometimes I have to use Netbeans. I will try it, if the experience gets troubled I will change to another and keep this learning. I think, anyway, the experience will be always positive.

I’m now following the steps to create a project in Netbeans…

  • New Project PHP
  • Advantage – Netbeans detected my xampp instalation and discovered where to put the php files.
  • Project name: LearningPHP (in my drive sits at: D:\xampp\htdocs\LearningPHP)
  • Local Web Site
  • Project URL: http://localhost:8001/LearningPHP/
  • I will not use any PHP framework… maybe in a future post I’ll try one besides WordPress.

After the wizard I have the file below already created:

index.php (the first file in any web tech is always index.








// put your code here
?>

Now I will change the Title and put some basic code inside the





Learning PHP – Step 2


echo "My second step to learn php!";
?>

Let’s run it (In Netbeans just hit the “play”!

PS: Make sure xampp is running and the port it’s the correct one. In my example the url is: http://localhost:8001/LearningPHP/index.php

and the result, as expected, it’s the browser output:

My second step to learn php!

Now, I have an environment to run php and a first “hello word” example.

Next post: The usual language basics of php!

Learning PHP – Step 1

I develop in several programming languages like C#, VB.NET, VB, Java, JavaScript, Python, Groovy, Powershell, PL-SQL, T-SQL, a little of Ruby, C, C++, Assembly (in the last years not so often), Prolog, List/Scheme, F#, Perl and and few others less used. I’ve read and I can understand PHP, from the knowledge I have from “C” Languages, but I never developed a complete project in PHP, and I don’t know if in the future I will need to do it, but from what I’ve been reading PHP is used in the TOP CMS Frameworks/Applications as WordPress, Drupal, Joomla, etc.

I’m thinking to create a synergy here, I will deep my understanding and knowledge about PHP and WordPress, tweaking my blog, this blog! Which I think will enrich my CV and, in the future, my career, making me a better team leader and software architect.

Starting PHP.

1) It’s a language to use in web sites.

2) You mix HTML with PHP, which is different from ASP.NET and JSP for instance. To differentiate the HTML from PHP you use in the middle of the HTML. The server will know how to process the PHP and merge it with HTML.



My first example in PHP


This code will output “My first PHP!” in the browser.

Great! It’s the first step!

Next Post let’s install the software we need to run this example. Then, the sky it’s the limit!

The twin paradox

One of the most fascinating things to me are the paradoxes. Its something that’s true but its false, its something that’s right but it’s wrong, it’s something that it is not!

There are a lot all around us, and I will try to show some of them in future posts.

Lets see one very simple:

How many of you have already seen a page in a book with the sentence:

“This page was intentionally left blank”

This is a paradox! If the page was really blank there were no words in it. But the words are in it to say it, but they say the opposite. We understand it, but the meaning it is true and it is at the same time false.

The video below show the twin paradox. See if you understand it. (Clue: They are twins but…)

Physics in a Minute: The twin paradox

This video was taken from New Scientist

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