Q: What are the different kinds of enterprise beans?
A: Different kind of enterrise beans are Stateless session bean, Stateful session bean, Entity bean, Message-driven bean.
Q: What is Session Bean?
A: A session bean is a non-persistent object that implements some business logic running on the server. One way to think of a session object.
Q: What is Entity Bean?
A: The entity bean is used to represent data in the database. It provides an object-oriented interface to .
Q: What are the methods of Entity Bean?
A: An entity bean consists of 4 groups of methods, create methods.
Q: What is the difference between Container-Managed Persistent (CMP) bean and Bean-Managed Persistent(BMP) ?
A: Container-managed persistence (CMP) and bean-managed persistence (BMP). With CMP, the container manages the persistence of the entity bean.
Q: What are the callback methods in Entity beans?
A: Callback methods allows the container to notify the bean of events in
its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface.
Q: What is software architecture of EJB?
A: Session and Entity EJBs consist of 4 and 5 parts respectively, a remote interface.
Q: Can Entity Beans have no create() methods?
A: Yes. In some cases the data is inserted NOT using Java application
Q: What is bean managed transaction?
A: If a developer doesn't want a Container to manage transactions, it's possible to implement all database operations manually.
Q: What are transaction attributes?
A: The transaction attribute specifies how the Container must manage transactions for a method when a client invokes the method via the enterprise bean’s home.
Q: What are transaction isolation levels in EJB?
A: Transaction_read_uncommitted , Transaction_read_committed , Transaction_repeatable_read.
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Monday, July 27, 2009
JAVA QUESTION SET 2
Q: Does Java provide any construct to find out the size of an object?
A: No. There is not has sizeof() operator in Java. So there is not direct way to determine the size of an object directly in Java.
Q: What are wrapper classes?
A: Java provides specialized classes corresponding to each of the primitive data types. The classes also has methods. These are called wrapper classes. They are e.g. Integer, Character, Double, Float.
Note: String is not a wrapper class. It is a class.
Q: Why do we need wrapper classes?
A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types.
Q: What are checked exceptions?
A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.
Q: What are runtime exceptions?
A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.
Q: What is the difference between error and an exception?
A: An error is an irrecoverable condition occurring at runtime. Such as Syntax error. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception.
Q: How to create custom exceptions?
A: Your class should extend class Exception.
Q: If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.
Q: What are the different ways to handle exceptions?
A: There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. 2. List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.
Q: Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
Q: If I write return at the end of the try block, will the finally block still execute?
A: Yes. The finally block will execute and then the control return.
Q: If I write System.exit (0); at the end of the try block, will the finally block still execute?
A: No in this case the finally block will not execute because the control immediately goes out of the program.
Q: How are Observer and Observable used?
A: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
Q: What is synchronization and why is it important?
A:With respect to multithreading, synchronization is the capability to controlthe access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
Q: Does garbage collection guarantee that a program will not run out of memory?
A: Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected.
Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
Q:When a thread is created and started, what is its initial state?
A: A thread is in the ready state after it has been created and started.
Q: What is the purpose of finalization?
A:The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
Q: What is the Locale class?
A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
Q: What is the difference between a while statement and a do statement?
A: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
Q: What is the difference between static and non-static variables?
A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
Q: How are this() and super() used with constructors?
A: this() is used to invoke a constructor of the same class. super() is used to invoke a super class constructor.
Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Q: What is daemon thread and which method is used to create the daemon thread?
A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. SetDaemon() method is used to create a daemon thread.
Q: What is applets?
A: Applet is a java program that run on the web browser.
Q: What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following steps : Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
Q: How does a try statement determine which catch clause should be used to handle an exception?
A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception executed. The remaining catch clauses are ignored.
Q: What method must be implemented by all threads?
A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
Q: What is Externalizable?
A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
Q: What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in interfaces.
Q: What are some alternatives to inheritance?
A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
Q: What does it mean that a method or field is "static"?
A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
Q: What is the catch or declare rule for method declarations?
A: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
Q: Can a .java file contain more than one java classes?
A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.
Q: Is main a keyword in Java?
A: No, main is not a keyword in Java.
Q: Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System class object.
Q: What happens if you don’t initialize an instance variable of any of the primitive types in Java?
A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.
Q: What will be the initial value of an object reference which is defined as an instance variable?
A: The object references are all initialized to null in Java.
Q: What are the different scopes for Java variables?
A: Three scopes 1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.
Q: What is the default value of the local variables?
A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..
Q: How many objects are created in the following piece of code?MyClass c1, c2, c3;c1 = new MyClass ();c3 = new MyClass ();
A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.
Q: Can a public class MyClass be defined in a source file named YourClass.java?
A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.
Q: Can main method be declared final?
A: Yes, the main method can be declared final, in addition to being public static.
Q: What will be the default values of all the elements of an array defined as an instance variable?
A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.
A: No. There is not has sizeof() operator in Java. So there is not direct way to determine the size of an object directly in Java.
Q: What are wrapper classes?
A: Java provides specialized classes corresponding to each of the primitive data types. The classes also has methods. These are called wrapper classes. They are e.g. Integer, Character, Double, Float.
Note: String is not a wrapper class. It is a class.
Q: Why do we need wrapper classes?
A: It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types.
Q: What are checked exceptions?
A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.
Q: What are runtime exceptions?
A: Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.
Q: What is the difference between error and an exception?
A: An error is an irrecoverable condition occurring at runtime. Such as Syntax error. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception.
Q: How to create custom exceptions?
A: Your class should extend class Exception.
Q: If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.
Q: What are the different ways to handle exceptions?
A: There are two ways to handle exceptions, 1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions. 2. List the desired exceptions in the throws clause of the method and let the caller of the method handle those exceptions.
Q: Is it necessary that each try block must be followed by a catch block?
A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.
Q: If I write return at the end of the try block, will the finally block still execute?
A: Yes. The finally block will execute and then the control return.
Q: If I write System.exit (0); at the end of the try block, will the finally block still execute?
A: No in this case the finally block will not execute because the control immediately goes out of the program.
Q: How are Observer and Observable used?
A: Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by objects that observe Observable objects.
Q: What is synchronization and why is it important?
A:With respect to multithreading, synchronization is the capability to controlthe access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of using or updating that object's value. This often leads to significant errors.
Q: Does garbage collection guarantee that a program will not run out of memory?
A: Garbage collection does not guarantee that a program will not run out of memory. It is possible for programs to use up memory resources faster than they are garbage collected.
Q: What is the difference between preemptive scheduling and time slicing?
A: Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks. The scheduler then determines which task should execute next, based on priority and other factors.
Q:When a thread is created and started, what is its initial state?
A: A thread is in the ready state after it has been created and started.
Q: What is the purpose of finalization?
A:The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.
Q: What is the Locale class?
A: The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region.
Q: What is the difference between a while statement and a do statement?
A: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.
Q: What is the difference between static and non-static variables?
A: A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance.
Q: How are this() and super() used with constructors?
A: this() is used to invoke a constructor of the same class. super() is used to invoke a super class constructor.
Q: What are synchronized methods and synchronized statements?
A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method's object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.
Q: What is daemon thread and which method is used to create the daemon thread?
A: Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system. SetDaemon() method is used to create a daemon thread.
Q: What is applets?
A: Applet is a java program that run on the web browser.
Q: What are the steps in the JDBC connection?
A: While making a JDBC connection we go through the following steps : Step 1 : Register the database driver by using :
Class.forName(\" driver classs for that specific database\" );
Step 2 : Now create a database connection using :
Connection con = DriverManager.getConnection(url,username,password);
Step 3: Now Create a query using :
Statement stmt = Connection.Statement(\"select * from TABLE NAME\");
Step 4 : Exceute the query :
stmt.exceuteUpdate();
Q: How does a try statement determine which catch clause should be used to handle an exception?
A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exception executed. The remaining catch clauses are ignored.
Q: What method must be implemented by all threads?
A: All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.
Q: What is Externalizable?
A: Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in)
Q: What modifiers are allowed for methods in an Interface?
A: Only public and abstract modifiers are allowed for methods in interfaces.
Q: What are some alternatives to inheritance?
A: Delegation is an alternative to inheritance. Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).
Q: What does it mean that a method or field is "static"?
A: Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.
Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.
Q: What is the catch or declare rule for method declarations?
A: If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.
Q: Can a .java file contain more than one java classes?
A: Yes, a .java file contain more than one java classes, provided at the most one of them is a public class.
Q: Is main a keyword in Java?
A: No, main is not a keyword in Java.
Q: Is exit a keyword in Java?
A: No. To exit a program explicitly you use exit method in System class object.
Q: What happens if you don’t initialize an instance variable of any of the primitive types in Java?
A: Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.
Q: What will be the initial value of an object reference which is defined as an instance variable?
A: The object references are all initialized to null in Java.
Q: What are the different scopes for Java variables?
A: Three scopes 1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.
Q: What is the default value of the local variables?
A: The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized..
Q: How many objects are created in the following piece of code?MyClass c1, c2, c3;c1 = new MyClass ();c3 = new MyClass ();
A: Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.
Q: Can a public class MyClass be defined in a source file named YourClass.java?
A: No the source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.
Q: Can main method be declared final?
A: Yes, the main method can be declared final, in addition to being public static.
Q: What will be the default values of all the elements of an array defined as an instance variable?
A: If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type. e.g. All the elements of an array of int will be initialized to 0, while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.
Wednesday, January 7, 2009
Core java Interview question and answer
Q: What is the difference between an Interface and an Abstract class?
An abstract class can have instance methods that implement a default behavior.An Interface can only declare constants and instance methods, but cannot implement default behaviorand all methods are implicitly abstract. An interface has all public members and no implementation.An abstract class is a class which may have the usual flavors of class members (private, protected, etc.),but has some abstract methods.
Q: What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
Q: Describe synchronization in respect to multithreading.
Q: What is the purpose of garbage collection in Java, and when is it used?
The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused. A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.
Q: Describe synchronization in respect to multithreading.
With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.
Q: Explain different way of using thread? The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.
Q: What are pass by reference and passby value?
Q: Explain different way of using thread? The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, 'cause when you are going for multiple inheritance..the only interface can help.
Q: What are pass by reference and passby value?
Pass By Reference means the passing the address itself rather than passing the value. Passby Value means passing a copy of the value to be passed.
Q: What is HashMap and Map?
Q: What is HashMap and Map?
Map is Interface and Hashmap is class that implements that.
Q: Difference between HashMap and HashTable?
Q: Difference between HashMap and HashTable?
The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow). HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.
Q: Difference between Vector and ArrayList?
Q: Difference between Vector and ArrayList?
Vector is synchronized whereas arraylist is not.
Q: Difference between Swing and Awt?
Q: Difference between Swing and Awt?
AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.
Q: What is the difference between a constructor and a method?
Q: What is the difference between a constructor and a method?
A constructor is a member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.
Q: What is an Iterator?
Q: What is an Iterator?
Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn. Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.
Q: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
Q: State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.
public : Public class is visible in other packages, field is visible everywhere (class must be public too)
private : Private variables or methods may be used only by an instance of the same class that declares the variableor method, A private feature may only be accessed by the class that owns the feature.
protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature.This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.
default :What you get by default ie, without any access modifier (ie, public private or protected).It means that it is visible to all within a particular package.
Q: What is an abstract class?
Q: What is an abstract class?
Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract may not be instantiated (ie, you may not call its constructor), abstract class may contain static data. Any class with an abstract method is automatically abstract itself, and must be declared as such.A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.
Q: Wht is static in java?
Q: Wht is static in java?
Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can't override a static method with a nonstatic method. In other words, you can't change a static method into an instance method in a subclass.
Q: What is final?
Q: What is final?
A final class can't be extended ie., final class may not be subclassed. A final method can't be overridden when its class
Q: What if the main method is declared as private?
Q: What if the main method is declared as private?
The program compiles properly but at runtime it will give "Main method not public." message.
Q: What if the static modifier is removed from the signature of the main method?
Q: What if the static modifier is removed from the signature of the main method?
Program compiles. But at runtime throws an error "NoSuchMethodError".
Q: What if I write static public void instead of public static void?
Q: What if I write static public void instead of public static void?
Program compiles and runs properly.
Q: What if I do not provide the String array as the argument to the method?
Q: What if I do not provide the String array as the argument to the method?
Program compiles but throws a runtime error "NoSuchMethodError".
Q: What is the first argument of the String array in main method?
Q: What is the first argument of the String array in main method?
The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.
Q: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
Q: If I do not provide any arguments on the command line, then the String array of Main method will be empty or null?
It is empty. But not null.
Q: How can one prove that the array is not null but empty using one line of code?
Q: How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.
Q: Can an application have multiple classes having main method?
Q: Can an application have multiple classes having main method?
Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.
Q: Can I have multiple main methods in the same class?
Q: Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.
Q: What are Checked and UnChecked Exception?
Q: What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses.Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method·Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch theexception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.
Q: What is Overriding?
Q: What is Overriding?
When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.
Q: What are different types of inner classes?
Q: What are different types of inner classes?
Nested top-level classes, Member classes, Local classes, Anonymous classes
Nested top-level classes- If you declare a class within a class and specify the static modifier,the compiler treats the class just like any other top-level class.Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.
Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.
Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement amore publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.
Nested top-level classes- If you declare a class within a class and specify the static modifier,the compiler treats the class just like any other top-level class.Any class outside the declaring class accesses the nested class with the declaring class name acting similarly to a package. eg, outer.inner. Top-level inner classes implicitly have access only to static variables.There can also be inner interfaces. All of these are of the nested top-level variety.
Member classes - Member inner classes are just like other member methods and member variables and access to the member class is restricted, just like methods and variables. This means a public member class acts similarly to a nested top-level class. The primary difference between member classes and nested top-level classes is that member classes have access to the specific instance of the enclosing class.
Local classes - Local classes are like local variables, specific to a block of code. Their visibility is only within the block of their declaration. In order for the class to be useful beyond the declaration block, it would need to implement amore publicly available interface.Because local classes are not members, the modifiers public, protected, private, and static are not usable.
Anonymous classes - Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.
Q: What is the difference between declaring a variable and defining a variable?
In declaration we just mention the type of the variable and it's name. We do not initialize it. But defining means declaration + initialization.e.g String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.
Q: What is the default value of an object reference declared as an instance variable?
Q: What is the default value of an object reference declared as an instance variable?
null unless we define it explicitly.
Q: Can a top level class be private or protected?
Q: Can a top level class be private or protected?
No. A top level class can not be public or default
Q: What type of parameter passing does Java support?
In Java the arguments are always passed by value .
Q: Primitive data types are passed by reference or pass by value?
Primitive data types are passed by value.
Q: Objects are passed by value or by reference?
Q: Objects are passed by value or by reference?
Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .
Q: What is serialization?
Q: What is serialization?
Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.
Q: How do I serialize an object to a file? The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
Q: How do I serialize an object to a file? The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.
Friday, November 14, 2008
Ejb session bean develpment guide
EJB Development Guide in J2EE Server
First you install j2ee server.
download from sun.com
Coding the Enterprise Bean
Every enterprise bean requires the following code:
Remote interface
Home interface
Enterprise bean class
Coding the Remote Interface
A remote interface defines the business methods that a client may call. The business methods are implemented in the enterprise bean code. The source code for the Converter remote interface follows.
import javax.ejb.EJBObject;
import java.rmi.RemoteException;
public interface Converter extends EJBObject {
public double dollarToYen(double dollars) throws RemoteException;
public double yenToEuro(double yen) throws RemoteException;
}
Coding the Home Interface
A home interface defines the methods that allow a client to create, find, or remove an enterprise bean. The ConverterHome interface contains a single create method, which returns an object of the remote interface type. Here is the source code for the ConverterHome interface:
import java.io.Serializable;
import java.rmi.RemoteException;
import javax.ejb.CreateException;
import javax.ejb.EJBHome;
public interface ConverterHome extends EJBHome {
Converter create() throws RemoteException, CreateException;
}
Coding the Enterprise Bean Class
The enterprise bean in our example is a stateless session bean called ConverterEJB. This bean implements the two business methods, dollarToYen and yenToEuro, that the Converter remote interface defines. The source code for the ConverterEJB class follows.
import java.rmi.RemoteException;
import javax.ejb.SessionBean;
import javax.ejb.SessionContext;
public class ConverterEJB implements SessionBean {
public double dollarToYen(double dollars) {
return dollars * 121.6000;
}
public double yenToEuro(double yen) {
return yen * 0.0077;
}
public ConverterEJB() {}
public void ejbCreate() {}
public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext(SessionContext sc) {}
}
Compiling the Enterprise Bean's Source Code
Now you are ready to compile the remote interface (Converter.java), home interface (ConverterHome.java) , and the enterprise bean class (ConverterEJB.java).
Creating the J2EE Application
You cannot deploy an enterprise bean directly into an J2EE server. Instead, you add the bean to a J2EE application, which you then deploy. In this section, you will create a new J2EE application called ConverterApp and store it in the file named ConverterApp.ear.
1. At the command line prompt, start the J2EE server:
j2ee -verbose
(To stop the server, type j2ee -stop.)
2. In another terminal window, run the Application Deployment Tool:
deploytool
(To access the tool's context-sensitive help, press f1.)
3. Create a new J2EE application.
a. In the Application Deployment Tool, select the File menu.
b. From the File menu choose New Application.
c. Click Browse.
d. In the file chooser, locate the directory where you want to place the .ear file that contains the J2EE application.
e. In the File name field enter ConverterApp.ear.
f. Click New Application.
g. Click OK.
Packaging the Enterprise Bean
In this section you will run the New Enterprise Bean Wizard of the Application Deployment Tool to perform these tasks:
Create the bean's deployment descriptor.
Package the deployment descriptor and the bean's classes in an EJB .jar file.
Insert the EJB .jar file into the application's ConverterApp.ear file.
To start the New Enterprise Bean Wizard, from the File menu choose New Enterprise Bean. The wizard displays the following dialog boxes.
Introduction Dialog Box:
a. Read this explanatory text for an overview of the wizard's features.
b. Click Next.
EJB JAR Dialog Box:
a. In the combo box labelled "Enterprise Bean will go in," select ConverterApp.
b. In the JAR Display Name field enter ConverterJAR. Representing the EJB .jar file that contains the bean, this name will be displayed in the tree view.
c. Click the Add button next to the Contents text area.
d. In the Add Contents to JAR dialog box, choose the directory containing the .class files. (This directory is $J2EE_HOME/doc/guides/ejb/examples/converter, where $J2EE_HOME is the location of your J2EE SDK installation.) You may either type the directory name in the Root Directory field or locate it by clicking Browse.
e. Select each of the following classes from the text area and click Add: Converter.class, ConverterEJB.class, and ConverterHome.class.
f. Click OK.
g. Click Next.
General Dialog Box:
a. In the Enterprise Bean Class combo box, select ConverterEJB.
b. In the Home Interface combo box, select ConverterHome.
c. In the Remote Interface combo box, select Converter.
d. Select the Session radio button.
e. Select the Stateless radio button.
f. In the Enterprise Bean Display Name field, enter ConverterBean. This name will represent the enterprise bean in the tree view.
g. Click Next.
Environment Entries Dialog Box:
Because you may skip the remaining dialog boxes, click Finish.
Deploying the J2EE Application
Now that the J2EE application contains an enterprise bean, it is ready for deployment.
1. Specify the JNDI name of the enterprise bean.
a. In the Application Deployment Tool, select ConverterApp in the tree view.
b. Select the JNDI Names tab.
c. In the JNDI Names field, enter MyConverter and press Return. The client will use this name to locate the home interface. (See a later section, "Locate the Home Interface".)
2. Deploy the J2EE application.
a. From the Tools menu, choose Deploy Application.
b. In the first dialog box, verify that the Target Server selection is either "localhost" or the name of the host running the J2EE server.
c. Select the checkbox labelled "Return Client Jar."
d. In the text field that appears, enter the full path name for the file ConverterAppClient.jar. This file will reside in the doc/guides/ejb/examples/converter subdirectory of your J2EE SDK installation.
e. Click Next.
f. In the second dialog box, verify that the JNDI name for the ConverterBean is MyConverter.
g. Click Next.
h. In the third dialog box, click Finish.
i. In the Deployment Progress dialog box, click OK when the deployment completes
Building the Client
The ConverterClient program is a stand-alone Java application. To create ConverterClient you perform these steps:
The full source code for the ConverterClient program follows.
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.rmi.PortableRemoteObject;
import Converter;
import ConverterHome;
public class ConverterClient {
public static void main(String[] args) {
try {
Context initial = new InitialContext();
Object objref = initial.lookup("MyConverter");
ConverterHome home =
(ConverterHome)PortableRemoteObject.narrow(objref,
ConverterHome.class);
Converter currencyConverter = home.create();
double amount = currencyConverter.dollarToYen(100.00);
System.out.println(String.valueOf(amount));
amount = currencyConverter.yenToEuro(100.00);
System.out.println(String.valueOf(amount));
currencyConverter.remove();
} catch (Exception ex) {
System.err.println("Caught an unexpected exception!");
ex.printStackTrace();
}
}
}
javac -classpath %CPATH% ConverterClient.java
Running the Client
java -classpath "$CPATH" ConverterClient Converter.jar
Any information mail to selestin@aol.in
immediate response or call 9942291292
All the best
First you install j2ee server.
download from sun.com
Coding the Enterprise Bean
Every enterprise bean requires the following code:
Remote interface
Home interface
Enterprise bean class
Coding the Remote Interface
A remote interface defines the business methods that a client may call. The business methods are implemented in the enterprise bean code. The source code for the Converter remote interface follows.
import javax.ejb.EJBObject;
import java.rmi.RemoteException;
public interface Converter extends EJBObject {
public double dollarToYen(double dollars) throws RemoteException;
public double yenToEuro(double yen) throws RemoteException;
}
Coding the Home Interface
A home interface defines the methods that allow a client to create, find, or remove an enterprise bean. The ConverterHome interface contains a single create method, which returns an object of the remote interface type. Here is the source code for the ConverterHome interface:
import java.io.Serializable;
import java.rmi.RemoteException;
import javax.ejb.CreateException;
import javax.ejb.EJBHome;
public interface ConverterHome extends EJBHome {
Converter create() throws RemoteException, CreateException;
}
Coding the Enterprise Bean Class
The enterprise bean in our example is a stateless session bean called ConverterEJB. This bean implements the two business methods, dollarToYen and yenToEuro, that the Converter remote interface defines. The source code for the ConverterEJB class follows.
import java.rmi.RemoteException;
import javax.ejb.SessionBean;
import javax.ejb.SessionContext;
public class ConverterEJB implements SessionBean {
public double dollarToYen(double dollars) {
return dollars * 121.6000;
}
public double yenToEuro(double yen) {
return yen * 0.0077;
}
public ConverterEJB() {}
public void ejbCreate() {}
public void ejbRemove() {}
public void ejbActivate() {}
public void ejbPassivate() {}
public void setSessionContext(SessionContext sc) {}
}
Compiling the Enterprise Bean's Source Code
Now you are ready to compile the remote interface (Converter.java), home interface (ConverterHome.java) , and the enterprise bean class (ConverterEJB.java).
Creating the J2EE Application
You cannot deploy an enterprise bean directly into an J2EE server. Instead, you add the bean to a J2EE application, which you then deploy. In this section, you will create a new J2EE application called ConverterApp and store it in the file named ConverterApp.ear.
1. At the command line prompt, start the J2EE server:
j2ee -verbose
(To stop the server, type j2ee -stop.)
2. In another terminal window, run the Application Deployment Tool:
deploytool
(To access the tool's context-sensitive help, press f1.)
3. Create a new J2EE application.
a. In the Application Deployment Tool, select the File menu.
b. From the File menu choose New Application.
c. Click Browse.
d. In the file chooser, locate the directory where you want to place the .ear file that contains the J2EE application.
e. In the File name field enter ConverterApp.ear.
f. Click New Application.
g. Click OK.
Packaging the Enterprise Bean
In this section you will run the New Enterprise Bean Wizard of the Application Deployment Tool to perform these tasks:
Create the bean's deployment descriptor.
Package the deployment descriptor and the bean's classes in an EJB .jar file.
Insert the EJB .jar file into the application's ConverterApp.ear file.
To start the New Enterprise Bean Wizard, from the File menu choose New Enterprise Bean. The wizard displays the following dialog boxes.
Introduction Dialog Box:
a. Read this explanatory text for an overview of the wizard's features.
b. Click Next.
EJB JAR Dialog Box:
a. In the combo box labelled "Enterprise Bean will go in," select ConverterApp.
b. In the JAR Display Name field enter ConverterJAR. Representing the EJB .jar file that contains the bean, this name will be displayed in the tree view.
c. Click the Add button next to the Contents text area.
d. In the Add Contents to JAR dialog box, choose the directory containing the .class files. (This directory is $J2EE_HOME/doc/guides/ejb/examples/converter, where $J2EE_HOME is the location of your J2EE SDK installation.) You may either type the directory name in the Root Directory field or locate it by clicking Browse.
e. Select each of the following classes from the text area and click Add: Converter.class, ConverterEJB.class, and ConverterHome.class.
f. Click OK.
g. Click Next.
General Dialog Box:
a. In the Enterprise Bean Class combo box, select ConverterEJB.
b. In the Home Interface combo box, select ConverterHome.
c. In the Remote Interface combo box, select Converter.
d. Select the Session radio button.
e. Select the Stateless radio button.
f. In the Enterprise Bean Display Name field, enter ConverterBean. This name will represent the enterprise bean in the tree view.
g. Click Next.
Environment Entries Dialog Box:
Because you may skip the remaining dialog boxes, click Finish.
Deploying the J2EE Application
Now that the J2EE application contains an enterprise bean, it is ready for deployment.
1. Specify the JNDI name of the enterprise bean.
a. In the Application Deployment Tool, select ConverterApp in the tree view.
b. Select the JNDI Names tab.
c. In the JNDI Names field, enter MyConverter and press Return. The client will use this name to locate the home interface. (See a later section, "Locate the Home Interface".)
2. Deploy the J2EE application.
a. From the Tools menu, choose Deploy Application.
b. In the first dialog box, verify that the Target Server selection is either "localhost" or the name of the host running the J2EE server.
c. Select the checkbox labelled "Return Client Jar."
d. In the text field that appears, enter the full path name for the file ConverterAppClient.jar. This file will reside in the doc/guides/ejb/examples/converter subdirectory of your J2EE SDK installation.
e. Click Next.
f. In the second dialog box, verify that the JNDI name for the ConverterBean is MyConverter.
g. Click Next.
h. In the third dialog box, click Finish.
i. In the Deployment Progress dialog box, click OK when the deployment completes
Building the Client
The ConverterClient program is a stand-alone Java application. To create ConverterClient you perform these steps:
The full source code for the ConverterClient program follows.
import javax.naming.Context;
import javax.naming.InitialContext;
import javax.rmi.PortableRemoteObject;
import Converter;
import ConverterHome;
public class ConverterClient {
public static void main(String[] args) {
try {
Context initial = new InitialContext();
Object objref = initial.lookup("MyConverter");
ConverterHome home =
(ConverterHome)PortableRemoteObject.narrow(objref,
ConverterHome.class);
Converter currencyConverter = home.create();
double amount = currencyConverter.dollarToYen(100.00);
System.out.println(String.valueOf(amount));
amount = currencyConverter.yenToEuro(100.00);
System.out.println(String.valueOf(amount));
currencyConverter.remove();
} catch (Exception ex) {
System.err.println("Caught an unexpected exception!");
ex.printStackTrace();
}
}
}
javac -classpath %CPATH% ConverterClient.java
Running the Client
java -classpath "$CPATH" ConverterClient Converter.jar
Any information mail to selestin@aol.in
immediate response or call 9942291292
All the best
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