Java is the future !: 2011

Tuesday, August 16, 2011

Quiz of the day | OCPJP - Aug 16, 2011

What will be the result of compiling and running the following code:

Options :
a) true
b) false
c) false
d) None of the above

Solution: Correct choice is option B.
Creating a new instance of the class File, you're not yet making an actual file, you're just creating a filename. So file.exists() return false.
createNewFile() method created an actual file. So file.exists() return true.

How to become a Java developer?

"I want to be a java developer, What are the all skills that I need to have. I have learned the core java in college but what now?"
Does this sound a familiar question! You too may have a similar question in your mind. Don't worry, This article explains what are all the skills that you will need to be a Java web developer, and what are the skills that you are expected to have when you appear for an interview for a Java/J2EE web developer.

Java developer! But what kind of?

Let me clarify, this article is about the Java web development. As most of the time, when beginner say, 'I want to be a java developer', he actually means he wants to be a java web developer and wants to get involved in developing web applications. Otherwise, Java development in general is a vast field, today Java is used in many types of platforms, that includes Personal computers, Main frames, Hand held portable devices, Mobiles, Smart cards, Games and many more thing. But the biggest stream of java development that creates more jobs than any other stream is the JavaEE platform. The JavaEE platform is used to create the enterprise applications (It includes web applications as well).
So be clear in your mind, This article is about java web development or enterprise application development. Most of the time when you read a job classified for skills, “Spring, Spring MVC, Struts, Hibernate, EJB” they are for java web development.
So, now it’s clear that you want to be a Java web developer and looking for a jump start. So here are the skills that will get you going.

Step 1 : First thing first, know the basics, learn core Java

The first thing you must learn is Core Java. Having a solid foundation in core java will help you when you jump into the advanced java. No matter what Java technologies you plan to work on, you will need to use the core java every day. So it is absolutely must to have a good understanding of the core concepts. Learn the basic programming concepts like variables, branching and looping, logical conditions, data types etc.

Good understanding of core java library is a must

Learn core java library, especially collection framework, Threads, java.lang, packages. If you are targeting the web development, you may not want to waste time on the swing.
Get yourself a good core java book that covers the latest java edition JDK 1.6. (When I was a beginner, I read “The complete reference java”)

Step 2 : Learn to use an IDE (Integrated Development Environment)

While working as a professional, you won’t use the plain text editors to write the code, you will have to know how to use IDE. IDE A.K.A code editor will greatly improve your productivity, and make your life easier. Code editors are used to write, compile, run and debug code. They provide features like syntax highlighting, auto completion, can identify the problems in code even before compiling. These are some of the most common features, but there are many more than just these. I will recommend you to learn eclipse which is one of the most popular IDE amongst java developer, and its free.

Step 3 : Your first step towards web development, HTML, CSS and JavaScript

I will not explain what HTML or CSS is, there are plenty of resources available on the internet. But understand, this is the most important parts of any web application. Second most important thing to learn is JavaScript, let me clarify, the JavaScript is not java, the similarity in name is for historical reasons. Get yourself into the world of web applications with this three skills, learn some tutorials on the internet or get some good book.

Step 4 : The java web components, Servlet API and JSP

Almost every web application running on internet or intranet which is developed in Java has their roots in Servlet API. So next thing to learn is servlets. Servlet API is the only standard API for java web application development. JSP is also based on the servlet API. Almost all the web frameworks, like Spring MVC, Struts, Wicket uses the servlet API under the hood. Get yourself a good book like “head first servlet”. Having the solid understanding of the Servlet API is quite important. Understand the core concepts like, life cycle, request, response, filters, session, config, context etc.

Step 5 : Learn a web container

Web container is a run time environment or server that is used to deploy and run the web applications or more specifically Web components (Eg servlets or JSP)
Web containers does not know how to run struts or Spring MVC, they just understand the servlet API, but still you can run them on a web container, because ultimately they are based on the Servlet API. You can learn the tomcat, that would be sufficient initially.

Step 6 : Learn JDBC

Almost every web application will need to connect to a database for storing and retrieving persistent data. JDBC is the standard API for database connectivity. So it is quite important that you understand the JDBC well.

Step 7 : Learn a web application framework and an ORM framework

A web application framework is actually a set of libraries that makes developing web applications easier. Struts and Spring MVC are good choices. If you are just starting, I would recommend going with Spring MVC.
ORM stands for object to relational mapping, they are based on JDBC and used for storing and querying the database. But unlike JDBC, ORM frameworks operates on objects and provides ways for storing and retrieving objects. (But be clear, they are ultimately using JDBC under the hood). ORM frameworks reduces lots of boilerplate code, improves productivity and makes developers life easier. Hibernate is the best choice. If you are just starting, read and understand JPA (Java persistence API) which is the standard java ORM API. You can use hibernate for running your JPA code.
That's it. If you know everything I mentioned above well, you are all set to get a job as java web developer.

Step 8 : What next!

Once you are familiar with the things mentioned above, The next step would be to learn the JavaEE (Java enterprise edition). Learn EJB (Enterprise java beans), JSF (Java server faces) and other APIs that are part of the JavaEE. Get a good book that covers the latest JavaEE edition 1.6


Quiz of the day | OCPJP - 16 Aug 2011

What is the output for the above code ?

Options :
a) Compile with error - Not allowed to override the return type of a method with a subtype of the original type.
b) class A - return C
c) class B - return D
d) Runtime Exception

Saturday, May 14, 2011

char a = '\u000A'. Why is this invalid?

Unicode escape characters of the form '\Uxxxx', where xxxx is a hexadecimal value, are processed very early in the translation process (see JLS 3.10.4 ). As a result, the special characters '0A' (line feed) and '0D' (carriage return) are interpreted literally as "end of line."
For example, the expression...

char A = '\u000A';
...therefore becomes...

char A =;
...which results in a compile-time error.
To avoid this error, always use the special escape characters '\n' (line feed) and '\r' (carriage return).

Is it allowed to declare the main method private?

Former JVM versions (pre-1.4) allowed the main method to have any accessibility (private, etc). This incompatibility with Section 12.1.4 of the Java Language Specification has been fixed as of version 1.4. In order to invoke a main method from the command-line, it is now mandatory to declare the main method as follows:

public static void main (String[] args)
If the main method has any access level other than public, it will no longer run from the command-line.

Sunday, April 24, 2011

What are some potential trips/traps in the SCJP exam?

  • Two top-level public classes cannot be in the same source file.
  • main() cannot call an instance (non-static) method.
  • Methods can have the same name as the constructor(s).
  • Watch for thread initiation with classes that don't have a run() method.
  • Local classes cannot access non-final variables.
  • Case statements must have values within permissible range.
  • Watch for Math class being an option for immutable classes.
  • instanceOf is not the same as instanceof.
  • Constructors can be private.
  • Assignment statements can be mistaken for a comparison; e.g., if(a=true)...
  • Watch for System.exit() in try-catch-finally blocks.
  • Watch for uninitialized variable references with no path of proper initialization.
  • Order of try-catch-finally blocks matters.
  • main() can be declared final.
  • -0.0 == 0.0 is true.
  • A class without abstract methods can still be declared abstract.
  • Map does not implement Collection.
  • Dictionary is a class, not an interface.
  • Collection (singular) is an Interface, but Collections (plural) is a helper class.
  • Class declarations can come in any order (e.g., derived first, base next, etc.).
  • Forward references to variables gives a compiler error.
  • Multi-dimensional arrays can be "sparse" -- i.e., if you imagine the array as a matrix, every row need not have the same number of columns.
  • Arrays, whether local or class-level, are always initialized
  • Strings are initialized to null, not empty string.
  • An empty string is not the same as a null reference.
  • A declaration cannot be labelled.
  • continue must be in a loop (e.g., for, do, while). It cannot appear in case constructs.
  • Primitive array types can never be assigned to each other, even though the primitives themselves can be assigned. For example, ArrayofLongPrimitives = ArrayofIntegerPrimitives gives compiler error even though longvar = intvar is perfectly valid.
  • A constructor can throw any exception.
  • Initializer blocks are executed in the order of declaration.
  • Instance initializers are executed only if an object is constructed.
  • All comparisons involving NaN and a non-NaN always result in false.
  • Default type of a numeric literal with a decimal point is double.
  • int and long operations / and % can throw an ArithmeticException, while float and double / and % never will (even in case of division by zero).
  • == gives compiler error if the operands are cast-incompatible.
  • You can never cast objects of sibling classes (sharing the same parent).
  • equals() returns false if the object types are different. It does not raise a compiler error.
  • No inner class (non-static inner class) can have a static member.
  • File class has no methods to deal with the contents of the file.
  • InputStream and OutputStream are abstract classes.

What's up with integers not being equal to one another? The following code prints "true, true, false, true". Shouldn't it be "true, true, true, true"?

The key to understanding this is that the JVM uses a process called "boxing" (or "auto-boxing") when converting an int (like 127) to an Integer object. This involves calling the Integer.valueOf(127) method. The JavaDoc:java.lang.Integer#valueOf(int) says: "Returns a Integer instance representing the specified int value. If a new Integer instance is not required, this method should generally be used in preference to the constructor Integer(int), as this method is likely to yield significantly better space and time performance by caching frequently requested values." What that means is that the valueOf() method has a cache of Integer objects, and if the primitive being boxed is in that range, the cached object is returned. It just so happens that 127 is in that range, but 128 is not. So i and j are the same object, while i1 and j1 are not.

Source: CodeRanch

How to convert a String array to ArrayList?




Note: In the solution no 1 Arrays.asList() is efficient because it doesn't need to copy the content of the array. This method returns a List that is a "view" onto the array - a wrapper that makes the array look like a list. When you change an element in the list, the element in the original array is also changed. Note that the list is fixed size - if you try to add elements to the list, you'll get an exception.

If you only need read access to the array as if it is a List and you don't want to add or remove elements from the list, then only use solution no 1.

Source: JavaRanch

Difference between Class reference and Interface reference

Interface provides the base for the concrete classes. It defines the contract which must be full filled by the implementing classes. So it can be use full in case where you need polymorphism. for example if you have some classes and interface like:

Here if you want to do some generic work which is common for both the doctors (doCheckUp) you should use the interface like

In this case no matter which object you are passing it will call the doCheckUp method of corresponding object. But your concrete class can have some extra methods and if you want to use those methods then you have to call the methods using corresponding concrete class objects.

So If you want to use the generic common methods, you should use the interface instance.

Sunday, January 30, 2011

Top 10 errors java programmers make

Whether you program regularly in Java, and know it like the back of your hand, or whether you're new to the language or a casual programmer, you'll make mistakes. It's natural, it's human, and guess what? You'll more than likely make the same mistakes that others do, over and over again. Here's my top ten list of errors that we all seem to make at one time or another,  how to spot them, and how to fix them.

10. Accessing non-static member variables from static methods (such as main)

Many programmers, particularly when first introduced to Java, have problems with accessing member variables from their main method. The method signature for main is marked static - meaning that we don't need to create an instance of the class to invoke the main method. For example, a Java Virtual Machine (JVM) could call the class MyApplication like this :-
MyApplication.main ( command_line_args );
This means, however, that there isn't an instance of MyApplication - it doesn't have any member variables to access! Take for example the following application, which will generate a compiler error message.
public class StaticDemo
        public String my_member_variable = "somedata";
public static void main (String args[])
  // Access a non-static member from static method
                System.out.println ("This generates a compiler error" +
   my_member_variable );
If you want to access its member variables from a non-static method (like main), you must create an instance of the object. Here's a simple example of how to correctly write code to access non-static member variables, by first creating an instance of the object.
public class NonStaticDemo
        public String my_member_variable = "somedata";

        public static void main (String args[])
                NonStaticDemo demo = new NonStaticDemo();

  // Access member variable of demo
                System.out.println ("This WON'T generate an error" +
                        demo.my_member_variable );

9. Mistyping the name of a method when overriding

Overriding allows programmers to replace a method's implementation with new code. Overriding is a handy feature, and most OO programmers make heavy use of it. If you use the AWT 1.1 event handling model, you'll often override listener implementations to provide custom functionality. One easy trap to fall into with overriding, is to mistype the method name. If you mistype the name, you're no longer overriding a method - you're creating an entirely new method, but with the same parameter and return type.
public class MyWindowListener extends WindowAdapter {
 // This should be WindowClosed
 public void WindowClose(WindowEvent e) {
  // Exit when user closes window
Compilers won't pick up on this one, and the problem can be quite frustrating to detect. In the past, I've looked at a method, believed that it was being called, and taken ages to spot the problem. The symptom of this error will be that your code isn't being called, or you think the method has skipped over its code. The only way to ever be certain is to add a println statement, to record a message in a log file, or to use good trace debugger (like Visual J++ or Borland JBuilder) and step through line by line. If your method still isn't being called, then it's likely you've mistyped the name.

8. Comparison assignment (  = rather than == )

This is an easy error to make. If you're used other languages before, such as Pascal, you'll realize just how poor a choice this was by the language's designers. In Pascal, for example, we use the := operator for assignment, and leave = for comparison. This looks like a throwback to C/C++, from which Java draws its roots.
Fortunately, even if you don't spot this one by looking at code on the screen, your compiler will. Most commonly, it will report an error message like this : "Can't convert xxx to boolean", where xxx is a Java type that you're assigning instead of comparing.

7. Comparing two objects ( == instead of .equals)

When we use the == operator, we are actually comparing two object references, to see if they point to the same object. We cannot compare, for example, two strings for equality, using the == operator. We must instead use the .equals method, which is a method inherited by all classes from java.lang.Object.
Here's the correct way to compare two strings.
String abc = "abc"; String def = "def";

// Bad way
if ( (abc + def) == "abcdef" )
// Good way
if ( (abc + def).equals("abcdef") )

6. Confusion over passing by value, and passing by reference

This can be a frustrating problem to diagnose, because when you look at the code, you might be sure that its passing by reference, but find that its actually being passed by value. Java usesboth, so you need to understand when you're passing by value, and when you're passing by reference.
When you pass a primitive data type, such as a char, int, float, or double, to a function then you are passing by value. That means that a copy of the data type is duplicated, and passed to the function. If the function chooses to modify that value, it will be modifying the copy only. Once the function finishes, and control is returned to the returning function, the "real" variable will be untouched, and no changes will have been saved. If you need to modify a primitive data type, make it a return value for a function, or wrap it inside an object.
When you pass a Java object, such as an array, a vector, or a string, to a function then you arepassing by reference. Yes - a String is actually an object, not a primitive data type.  So that means that if you pass an object to a function, you are passing a reference to it, not a duplicate. Any changes you make to the object's member variables will be permanent - which can be either good or bad, depending on whether this was what you intended.
On a side note, since String contains no methods to modify its contents, you might as well be passing by value.

5. Writing blank exception handlers

I know it's very tempting to write blank exception handlers, and to just ignore errors. But if you run into problems, and haven't written any error messages, it becomes almost impossible to find out the cause of the error. Even the simplest exception handler can be of benefit. For example, put a try { .. } catch Exception around your code, to catch ANY type of exception, and print out the message. You don't need to write a custom handler for every exception (though this is still good programming practice). Don't ever leave it blank, or you won't know what's happening.
For example
public static void main(String args[])
    try {
 // Your code goes here..
    catch (Exception e)
 System.out.println ("Err - " + e );

4. Forgetting that Java is zero-indexed

If you've come from a C/C++ background, you may not find this quite as much a problem as those who have used other languages. In Java, arrays are zero-indexed, meaning that the first element's index is actually 0. Confused? Let's look at a quick example.
// Create an array of three strings
String[] strArray = new String[3];

// First element's index is actually 0
strArray[0] = "First string";

// Second element's index is actually 1
strArray[1] = "Second string";

// Final element's index is actually 2
strArray[2] = "Third and final string";
In this example, we have an array of three strings, but to access elements of the array we actually subtract one. Now, if we were to try and access strArray[3], we'd be accessing the fourth element. This will case an ArrayOutOfBoundsException to be thrown - the most obvious sign of forgetting the zero-indexing rule.
Other areas where zero-indexing can get you into trouble is with strings. Suppose you wanted to get a character at a particular offset within a string. Using the String.charAt(int) function you can look this information up - but under Java, the String class is also zero-indexed. That means than the first character is at offset 0, and second at offset 1. You can run into some very frustrating problems unless you are aware of this - particularly if you write applications with heavy string processing. You can be working on the wrong character, and also throw exceptions at run-time. Just like the ArrayOutOfBoundsException, there is a string equivalent. Accessing beyond the bounds of a String will cause a StringIndexOutOfBoundsException to be thrown, as demonstrated by this example.
public class StrDemo
 public static void main (String args[])
        String abc = "abc";

        System.out.println ("Char at offset 0 : " + abc.charAt(0) );
        System.out.println ("Char at offset 1 : " + abc.charAt(1) );
        System.out.println ("Char at offset 2 : " + abc.charAt(2) );

 // This line should throw a StringIndexOutOfBoundsException
        System.out.println ("Char at offset 3 : " + abc.charAt(3) );
Note too, that zero-indexing doesn't just apply to arrays, or to Strings. Other parts of Java are also indexed, but not always consistently. The java.util.Date, and java.util.Calendar classes start their months with 0, but days start normally with 1. This problem is demonstrated by the following application.
import java.util.Date;
import java.util.Calendar;

public class ZeroIndexedDate
        public static void main (String args[])
                // Get today's date
                Date today = new Date();
  // Print return value of getMonth
  System.out.println ("Date.getMonth() returns : " +

  // Get today's date using a Calendar
  Calendar rightNow = Calendar.getInstance();

  // Print return value of get ( Calendar.MONTH )
  System.out.println ("Calendar.get (month) returns : " +
   rightNow.get ( Calendar.MONTH ));
Zero-indexing is only a problem if you don't realize that its occurring. If you think you're running into a problem, always consult your API documentation.

3. Preventing concurrent access to shared variables by threads

When writing multi-threaded applications, many programmers (myself included) often cut corners, and leave their applications and applets vulnerable to thread conflicts. When two or more threads access the same data concurrently, there exists the possibility (and Murphy's law holding, the probability) that two threads will access or modify the same data at the same time. Don't be fooled into thinking that such problems won't occur on single-threaded processors. While accessing some data (performing a read), your thread may be suspended, and another thread scheduled. It writes its data, which is then overwritten when the first thread makes its changes.
Such problems are not just limited to multi-threaded applications or applets. If you write Java APIs, or JavaBeans, then your code may not be thread-safe. Even if you never write a single application that uses threads, people that use your code WILL. For the sanity of others, if not yourself, you should always take precautions to prevent concurrent access to shared data.
How can this problem be solved? The simplest method is to make your variables private (but you do that already,  right?) and to use synchronized accessor methods. Accessor methods allow access to private member variables, but in a controlled manner. Take the following accessor methods, which provide a safe way to change the value of a counter.
public class MyCounter
 private int count = 0; // count starts at zero

 public synchronized void setCount(int amount)
  count = amount;
 public synchronized int getCount()
  return count;

2. Capitalization errors

This is one of the most frequent errors that we all make. It's so simple to do, and sometimes one can look at an uncapitalized variable or method and still not spot the problem. I myself have often been puzzled by these errors, because I recognize that the method or variable does exist, but don't spot the lack of capitalization.
While there's no silver bullet for detecting this error, you can easily train yourself to make less of them. There's a very simple trick you can learn :-
  • all methods and member variables in the Java API begin with lowercase letters
  • all methods and member variables use capitalization where a new word begins e.g - getDoubleValue()
If you use this pattern for all of your member variables and classes, and then make a conscious effort to get it right, you can gradually reduce the number of mistakes you'll make. It may take a while, but it can save some serious head scratching in the future.

(drum roll)

And the number one error that Java programmers make !!!!!

1. Null pointers!

Null pointers are one of the most common errors that Java programmers make. Compilers can't check this one for you - it will only surface at runtime, and if you don't discover it, your users certainly will.
When an attempt to access an object is made, and the reference to that object is null, a NullPointerException will be thrown. The cause of null pointers can be varied, but generally it means that either you haven't initialized an object, or you haven't checked the return value of a function.
Many functions return null to indicate an error condition - but unless you check your return values, you'll never know what's happening. Since the cause is an error condition, normal testing may not pick it up - which means that your users will end up discovering the problem for you. If the API function indicates that null may be returned, be sure to check this before using the object reference!
Another cause is where your initialization has been sloppy, or where it is conditional. For example, examine the following code, and see if you can spot the problem.
public static void main(String args[])
 // Accept up to 3 parameters
 String[] list = new String[3];

 int index = 0;

 while ( (index < args.length) && ( index < 3 ) )
  list[index++] = args[index];

 // Check all the parameters 
 for (int i = 0; i < list.length; i++)
  if (list[i].equals "-help")
   // .........
  if (list[i].equals "-cp")
   // .........
  // else .....
This code (while a contrived example), shows a common mistake. Under some circumstances, where the user enters three or more parameters, the code will run fine. If no parameters are entered, you'll get a NullPointerException at runtime. Sometimes your variables (the array of strings) will be initialized, and other times they won't. One easy solution is to check BEFORE you attempt to access a variable in an array that it is not equal to null.


These errors represent but some of the many that we all make. Though it is impossible to completely eliminate errors from the coding process, with care and practice you can avoid repeating the same ones. Rest assured, however, that all Java programmers encounter the same sorts of problems. It's comforting to know, that while you work late into the night tracking down an error, someone, somewhere, sometime, will make the same mistake!

Source : javacoffeebreak