Can a local variable be static..? uhh..why not?

No! - that's a very straight forward answer to that question and most simple one. In java the local variables can not be static - burn this in your mind. But Why??. The local variables are those which are declared inside a method body, so they have scope only for the method execution period. Any static thing in java belongs to the class and not to the instance. Whenever the method is called the method variables are in the memory area of the thread executing that method, so they are on stack. On the other hand the objects and static members of a class are on heap. Static things are to live throughout the method calls and when they are inside the method body (on stack) they can't live outside the scope of method, hence its ridiculous and illogical to have static variables inside a method body.

For the exam, you need not know why, just remember that a local variable can never be static - compiler will complain.

File name and class name should be same?

There is a relation between the name of java source file and the public classes you can declare in that file - only the public classes..okay..not classes with any other access level. Hey by the way do you know which access modifiers can be applied to the outer classes/non-inner classes?...that will be a fantastic post in itself...coming soon.. :) So, coming back to the point, yes there is a constraint on the file name when you want to declare a class in the file as public. The constraint is that the file name should be same as that of your public class name.

But it's not that simple to understand by this single sentence, when I heard first time about this thing I had following doubts:

1. Can I have more than one public classes in my java file?

2. Should the class be public to be called by the JVM ?

3.If a class is not public but has a main method can it be called by the JVM? If yes then how the hell is it possible??? And if not ...why?? what's the problem in that??

There is no simple solution to these questions than coding for yourself and thats what I did!

Take a look at the code carefully, it's in the java source file named as example6.java

Let me tell you some things in this example. The source file name is example6.java and there is no class called as example6 in the source file. There are two classes class1 and class2 which are not public!

Doubts Answered:

1. NO! I tried to give public access to class1 and class2 and compiler scolded me for two things - one he said that class1 if public should be declared in file named class1.java, other is - there can't be any two public classes in a single java file.

2. Not al all! You can see that class1 and class2 are not public, still I am calling both of them!

3. Yes it can be called if it has a main method! You can see that class1 is not public but has a main method, so I can call it by giving parameter class1 to the java command.

I hope you have throughly understood the constraints. Just to revise all, here is the conclusion...

Conclusion: The above program proves that...

1.There is no need that the classes whose main method you are going to call should be public.

2.File name can be anything as long as you dont have any public classes in your file.

3.Which main method will execute only depends upon which classname you pass to the JVM that is as argument to java command.

4. If you want global access of a class and hence declare that class as public then it should be in the same file as the class name, and as you cant have two different names for a single file you can't have two public classes in a single .java file!

Difference: positive zero 0 and negative zero -0 ?

Recently I have come across some strange things. I know that java language takes positive and negative zero differently than our logical mathematical brain will take. Well the two are certainly different but how does java identifies them? Or more basic question... does java even differenciate between positive and negative zero? The truth is that IEEE floating point specification differentiates between positive and negative zero, but the java specification says that positive zero is equal to negative zero. Please go through the following code and see if you can satisfy your questions...

I am still not able to understand why I am not getting -0 at 2nd and 4th place. I have posted this in javaranch forums, lets see if I can get some clarification. If you have the answer please comment.

Java is pass by value.....Always! - Part2

In the last post we saw what exactly pass by value and pass by reference means and strictly speaking its a fact that there is nothing like pass by reference in java. So we can say that java is pass by value...always! This post is a continuation so if you havn't please go through the first part - Java is pass by Value.....Always! - Part1

However note that, for the clarity of understanding the pass by value is when we pass a value and pass by reference is when we pass a reference to something.
Now lets look at the code which demonstrates pass by value and pass by reference.

Suppose there is a class TV, which contains a string which shows its state and its setters and getters.(Note:For the time please forget about the access modifiers..etc because this is just to make you understand one concept..I havnt run this program so it may have some errors/exceptions. But this code is perfect to make you understand what you need to at this point of time.)


class TV

{
string state; // this stores whether TV is currently ON or OFF

TV(string h)
{
this.state=h; //constructor assign a state for new TV object
}

setState(string s)
{
this.state=s;
}

string getState()
{
return(state);
}
}

The following code will demonstrate the pass by value with variable i and pass by reference with references remote1 and remote2.

class example1
{
int i=2;
public static void main()
{
TV remote1= new TV("OFF");
func2(i);
func3(remote1);
}

func2(int j)
{
System.out.println(j); // this will print 2
j=3;
System.out.println(j); // this will print 3
System.out.println(i); // this will print 2
}


func3(TV remote2)
{
System.out.println(remote2.getState()); //this will print OFF
remote2.setState("ON");
System.out.println(remote1.getState()); //this will print ON
System.out.println(remote2.getState()); //this will print ON
}

}


Here i and j are variables and they are independant of eachother. remote1 and remote2are references and they point to the same object, so both can turn on and off the same TV!

Have some doubts still? Go..on.. post a comment.. I will try my best to answer.

Rules for legal identifiers...

The SCJP exam wants you to know perfectly the rules which define legal and illegal identifiers names. Make sure you know this perfectly as there is no excuse for this. Read the following carefully and you will understand it's not that difficult:

Rules for Legal Identifier Names:

1. Identifiers must start with a letter, a currency character ($), or a connectingcharacter such as the underscore ( _ ). Identifiers cannot start with a number!

2. After the first character, identifiers can contain any combination of letters, currency characters, connecting characters, or numbers.

3. In practice, there is no limit to the number of characters an identifier cancontain.

4. You can't use a Java keyword as an identifier. Following figure lists all of the Java keywords including one new one for 5.0, enum.

5. Identifiers in Java are case-sensitive; so pen and PEN are two different identifiers.

The list of Java 1.5 Keywords follow -

Some of the legal identifiers are:

$aturn, _$23, _23, $_aturn, there_is_no_limit_to_the_number_of_characters

Some illegal identifiers are:

23$, -saturn, :saturn, saturn#, .saturn

If you have any questions...let them come through the comments, I will gladly answer each one of them.

Java is pass by value.....Always! - Part1

Surprised? Well, lets look at what is pass by value and pass by reference. Let us understand first what is a value and what is a reference. A value of any variable/or java object is the value assigned to it....huh...now what does this means? More technically, a variable is nothing but a name given to a memory location and the value of x is what is stored at the memory location of x. A reference is a pointer to an object. Say its like a remote control through which you can handle the target object. Take help of following figures...

Variable X storing value 7

Reference X storing a remote control

Clearly understand the difference between a value and reference and then you can understand what is pass by value and pass by reference. Well basically as far as java is concerned there is no difference!!Thats why I said java is pass by value always. Let's see why there is no difference. In a pass by value you always pass the underlying value of that variable...that means what ever is stored at the memory place. So if X is a variable and it has value 7 then the memory location contains the number 7. And when you pass by value you actually pass what is stored in the memory..and so 7 is passed! When X is a reference then also same thing happens...whatever is stored at the memory location is passed. But hey wait..in earlier case memory contained 7, but what is here? Here X is a reference, hence it contains a remote control to someone whom it is pointing to..so the memory contains the remote control! So here the remote control is passed! In both cases the value is passed, in first case the value is the actual value and in next case the value is actually the remote control. When you pass the remote control the entity (to which you are passing the remote control) can manipulate the object. In the next post I will demonstrate code for both the cases and then you will be able to point out the exact difference between pass by value and pass by reference. Hey, by pass by reference we mean passing the value..which is infact the remote control(the reference!) :)

Other than SCJP...what lies ahead?

This post will tell you about other Sun Java Certifications. Although many know only SCJP there are other plethora of Sun Java Certifications. Lets look at them, hey this knowledge isn't necessary for an SCJP taker to know but we should know where we are heading, right?

Sun Java Certification is itself a ladder and as you climb up the technology becomes more advanced and powerful. Many consider that SCJP is the 1st and most basic exam in the ladder but there is one more, its called SCJA - Sun Certified Java Associate Exam. These two are entry level exams in the ladder and you need to pass SCJP before you can give any of further examinations. There are core differences between the syllabus of SCJA and SCJP, like the "swing" is not included in SCJP but it is in SCJA. There is a difference between a java programmer and a java developer. You can call yourself a java developer after you give the special java developer exam called SCJD - the Sun Certified Java Developer exam. You are supposed to have the basic knowledge of java as a language after you complete SCJP and SCJD but you still dont know how to use the java language to the fullest i.e: developing web/standalone commercial products/applications from it - thats the path ahead. There are 4 certifications up the ladder each gives you a special knowledge about the corresponding technology.
They are...

1. SCWCD - the Sun Certified Web Component Developer exam.

2. SCBCD - the Sun Certified Business Component Developer exam.

3. SCDJWS - the Sun Certified Developer for Java Web Services exam.

4. SCMAD - the Sun Certified Mobile Application Developer exam.

The SCWCD portion is about the web components you need to build a web application. You should know how to develope a web application from scratch, before giving SCWCD. The SCBCD concentrates upon the business components required in an application, it includes in-depth study of the EJB (Enterprise Java Beans) technology. I donn't know about the SCDJWS. The next one SCMAD is concerned about knowledge about developing the java mobile applications. Hey the mobile in your pocket if it's java enabled (if it's cost is above 8k, mostly it is.) can run java applications developed by you! You may be thinking these certifications aren't that famous (so may be unimportant) but let me tell you that in the software industry, these special level certifications are valued the most. As SCJP is a pre-requisite for all these you need to pass SCJP before giving any of these. The ladder doesn't end here, there is one more certification at the top, and this is the highest one, it called SCEA - the Sun Certified Enterprise Architect exam. I really don't know what the portion is at this level and what knowledge we need to have... :)

I hope this post has helped you to know where we are and how far the journey is!

My level of java knowledge right now...i.e.: before starting preparation...

Now after knowing Java from past 3 years my teachers will be ashamed if I say "I am pretty new to java." So I am not new to java, I know java from past 3 years and yes have done pretty good amount of coding in it. But knowing java and passing the Sun certification examinations are two different things. You don't need to have working knowledge of java to do programs and even projects in it, but the Sun certification examinations demand that you should not only know good ways of programming in java but also know bad ways of programming in java. A normal java programmer will write a code which runs fine, but a SCJP programmer is supposed to write the most efficient code - and that's where you should know the intracacies and details of the programming language. So, here I am, I am able to write java programs..sometimes very long ones, but nah nah.. I don't even know were they efficient or not.

Talking to the point, I have gone through most basic Java programming books like the Complete Reference by Herbert Schildt, some chapters of Core Java and even some chapters of SCJP preparation books by Kathy Seirra & Bert Bates. I havn't given even a single Mock Exam and havn't started preparation for the examinations as such. So, this is where I am beginning. Let's start now.... join me.

What this blog is about?

Hello friends,

Today I have decided to give the SCJP exam. You will find anything and everything related to SCJP over here. You will find the SCJP information, posts stating my journey of study..yeh you got it right...technical posts (and no philosophy..eek..!:) ) and in the process indirectly I will be sharing the knowledge with you all. This is a perfect place for people who are planning to give SCJP in near future or are preparing for it. The most interesting thing about this blog is that it will contain "my journey of facing SCJP" and that means I havn't yet given the exam and I will start preparing for SCJP. It's named "facing SCJP" because who knows I will pass or fail! :)

Warm Regards,
Saurabh