J2EE Interview Question.......

                                  Basic Java interview questions

1. What is a Marker Interface? An interface with no methods. Example: Serializable, Remote, Cloneable
2. What interface do you implement to do the sorting? - Comparable
3. What is the eligibility for a object to get cloned? - It must implement the Cloneable interface
4. What is the purpose of abstract class? - It is not an instantiable class. It provides the concrete implementation for some/all the methods. So that they can reuse the concrete functionality by inheriting the abstract class.
5. What is the difference between interface and abstract class? - Abstract class defined with methods. Interface will declare only the methods. Abstract classes are very much useful when there is a some functionality across various classes. Interfaces are well suited for the classes which varies in functionality but with the same method signatures.
6. What do you mean by RMI and how it is useful? - RMI is a remote method invocation. Using RMI, you can work with remote object. The function calls are as though you are invoking a local variable. So it gives you a impression that you are working really with a object that resides within your own JVM though it is somewhere.
7. What is the protocol used by RMI? - RMI-IIOP
8. What is a hashCode? - hash code value for this object which is unique for every object.
9. What is a thread? - Thread is a block of code which can execute concurrently with other threads in the JVM.
10. What is the algorithm used in Thread scheduling? - Fixed priority scheduling.
11. What is hash-collision in Hashtable and how it is handled in Java? - Two different keys with the same hash value. Two different entries will be kept in a single hash bucket to avoid the collision.
12. What are the different driver types available in JDBC? - 1. A JDBC-ODBC bridge 2. A native-API partly Java technology-enabled driver 3. A net-protocol fully Java technology-enabled driver 4. A native-protocol fully Java technology-enabled driver For more information: Driver Description
13. Is JDBC-ODBC bridge multi-threaded? - No
14. Does the JDBC-ODBC Bridge support multiple concurrent open statements per connection? - No
15. What is the use of serializable? - To persist the state of an object into any perminant storage device.
16. What is the use of transient? - It is an indicator to the JVM that those variables should not be persisted. It is the users responsibility to initialize the value when read back from the storage.
17. What are the different level lockings using the synchronization keyword? Class level lock Object level lock Method level lock Block level lock
18. What is the use of preparedstatement? - Preparedstatements are precompiled statements. It is mainly used to speed up the process of inserting/updating/deleting especially when there is a bulk processing.
19. What is callable statement? Tell me the way to get the callable statement?  Callablestatements are used to invoke the stored procedures. You can obtain the callablestatement from Connection using the following methods prepareCall(String sql) prepareCall(String sql, int resultSetType, int resultSetConcurrency)
20. In a statement, I am executing a batch. What is the result of the execution? It returns the int array. The array contains the affected row count in the corresponding index of the SQL.
21. Can a abstract method have the static qualifier? - No
22. What are the different types of qualifier and what is the default qualifier? - public, protected, private, package (default)
23. What is the super class of Hashtable? - Dictionary
24. What is a lightweight component? - Lightweight components are the one which doesn’t go with the native call to obtain the graphical units. They share their parent component graphical units to render them. Example, Swing components
25. What is a heavyweight component? - For every paint call, there will be a native call to get the graphical units. Example, AWT.
26. What is an applet? - Applet is a program which can get downloaded into a client environment and start executing there.
27. What do you mean by a Classloader? - Classloader is the one which loads the classes into the JVM.
28. What are the implicit packages that need not get imported into a class file? - java.lang
29. What is the difference between lightweight and heavyweight component? - Lightweight components reuses its parents graphical units. Heavyweight components goes with the native graphical unit for every component. Lightweight components are faster than the heavyweight components.
30. What are the ways in which you can instantiate a thread? - Using Thread class By implementing the Runnable interface and giving that handle to the Thread class.
31. What are the states of a thread? - 1. New 2. Runnable 3. Not Runnable 4. Dead
32. What is a socket? - A socket is an endpoint for communication between two machines.
33. How will you establish the connection between the servlet and an applet? - Using the URL, I will create the connection URL. Then by openConnection method of the URL, I will establish the connection, through which I can be able to exchange data.
34. What are the threads will start, when you start the java program? - Finalizer, Main, Reference Handler, Signal Dispatcher

1. How can you achieve Multiple Inheritance in Java?

Java's interface mechanism can be used to implement multiple inheritance, with one important difference from c++ way of doing MI: the inherited interfaces must be abstract. This obviates the need to choose between different implementations, as with interfaces there are no implementations.

2. Replacing Characters in a String

// Replace all occurrences of 'a' with 'o'

String newString = string.replace('a', 'o');

Replacing Substrings in a String

static String replace(String str,

String pattern, String replace) {

int s = 0;

int e = 0;

StringBuffer result = new StringBuffer();

while ((e = str.indexOf(pattern, s)) >= 0) {

result.append(str.substring(s, e));

result.append(replace);

s = e+pattern.length();

}

result.append(str.substring(s));

return result.toString();

}

Converting a String to Upper or Lower Case

// Convert to upper case

String upper = string.toUpperCase();

// Convert to lower case

String lower = string.toLowerCase();

Converting a String to a Number

int i = Integer.parseInt("123");

long l = Long.parseLong("123");

float f = Float.parseFloat("123.4");

double d = Double.parseDouble("123.4e10");

Breaking a String into Words

String aString = "word1 word2 word3";

StringTokenizer parser =

new StringTokenizer(aString);

while (parser.hasMoreTokens()) {

processWord(parser.nextToken());

3. Searching a String

String string = "aString";

// First occurrence.

int index = string.indexOf('S'); // 1

// Last occurrence.

index = string.lastIndexOf('i'); // 4

// Not found.

index = string.lastIndexOf('z');

4. Connecting to a Database and Strings Handling

Constructing a String

If you are constructing a string with several appends, it may be more efficient to construct it using a

StringBuffer and then convert it to an immutable String object.

StringBuffer buf = new StringBuffer("Initial Text");

// Modify

int index = 1;

buf.insert(index, "abc");

buf.append("def");

// Convert to string

String s = buf.toString();

Getting a Substring from a String

int start = 1;

int end = 4;

String substr = "aString".substring(start, end); // Str

5. What is a transient variable?

A transient variable is a variable that may not be serialized. If you don't want some field not to be serialized, you can mark that field transient or static.

6. What is the difference between Serializalble and Externalizable interface?

When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject()two methods to control more complex object serailization process. When you use Externalizable interface, you have a complete control over your class's serialization process.

7. How many methods in the Externalizable interface?

There are two methods in the Externalizable interface. You have to implement these two methods in order to make your class externalizable. These two methods are readExternal() and writeExternal().

8. How many methods in the Serializable interface?

There is no method in the Serializable interface. The Serializable interface acts as a marker, telling the object serialization tools that your class is serializable.

9. How to make a class or a bean serializable?

By implementing either the java.io.Serializable interface, or the java.io.Externalizable interface. As long as one class in a class's inheritance hierarchy implements Serializable or Externalizable, that class is serializable.

10. What is the serialization?

The serialization is a kind of mechanism that makes a class or a bean persistence by having its properties or fields and state information saved and restored to and from storage.

11. What are synchronized methods and synchronized statements?

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.

12. What is synchronization and why is it important?

With respect to multithreading, synchronization is the capability to control the 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 causes dirty data and leads to significant errors.

13. What is the purpose of finalization?

The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected.

14. What classes of exceptions may be caught by a catch clause?

A catch clause can catch any exception that may be assigned to the Throwable type. This includes the Error and Exception types.

15. What is the difference between the Reader/Writer class hierarchy and the InputStream/OutputStream

class hierarchy?

The Reader/Writer class hierarchy is character-oriented, and the InputStream/OutputStream class hierarchy is byte-oriented.

16. What happens when a thread cannot acquire a lock on an object?

If a thread attempts to execute a synchronized method or synchronized statement and is unable to acquire an object's lock, it enters the waiting state until the lock becomes available.

17. What restrictions are placed on method overriding?

Overridden methods must have the same name, argument list, and return type. The overriding method may not limit the access of the method it overrides. The overriding method may not throw any exceptions that may not be thrown by the overridden method.

18. What restrictions are placed on method overloading?

Two methods may not have the same name and argument list but different return types.

19. How does multithreading take place on a computer with a single CPU?

The operating system's task scheduler allocates execution time to multiple tasks. By quickly switching between executing tasks, it creates the impression that tasks execute sequentially.

20. How is it possible for two String objects with identical values not to be equal under the == operator?

The == operator compares two objects to determine if they are the same object in memory. It is possible for two String objects to have the same value, but located indifferent areas of memory.

21. How are this() and super() used with constructors?

this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

22. What class allows you to read objects directly from a stream?

The ObjectInputStream class supports the reading of objects from input streams.

23. What is the ResourceBundle class?

The ResourceBundle class is used to store locale-specific resources that can be loaded by a program to tailor the program's appearance to the particular locale in which it is being run.

24. What interface must an object implement before it can be written to a stream as an object?

An object must implement the Serializable or Externalizable interface before it can be written to a stream as an object.

25. What is Serialization and deserialization?

Serialization is the process of writing the state of an object to a byte stream. Deserialization is the process of restoring these objects.

26. What are the Object and Class classes used for?

The Object class is the highest-level class in the Java class hierarchy. The Class class is used to represent the classes and interfaces that are loaded by a Java program.

27. Can you write Java code for declaration of multiple inheritance in Java ?

Class C extends A implements B{}

28. What do you mean by multiple inheritance in C++ ?

Multiple inheritance is a feature in C++ by which one class can be of different types. Say class

teachingAssistant is inherited from two classes say teacher and Student.

29. Write the Java code to declare any constant (say gravitational constant) and to get its value.

Class ABC{

static final float GRAVITATIONAL_CONSTANT = 9.8;

    public void getConstant(){

       system.out.println("Gravitational_Constant: " + GRAVITATIONAL_CONSTANT);

    }

}

30. What are the disadvantages of using threads?

DeadLock.

31. Given two tables Student(SID, Name, Course) and Level(SID, level) write the SQL statement to get the name and SID of the student who are taking course = 3

and at freshman level.

SELECT Student.name, Student.SID

FROM Student, Level

WHERE Student.SID = Level.SID

AND Level.Level = "freshman"

AND Student.Course = 3;

32. What do you mean by virtual methods?

virtual methods are used to use the polymorhism feature in C++. Say class A is inherited from class B. If we

declare say fuction f() as virtual in class B and override the same function in class A then at runtime

appropriate method of the class will be called depending upon the type of the object.

33. What do you mean by static methods?

By using the static method there is no need creating an object of that class to use that method. We can directly call that method on that class. For example, say class A has static function f(), then we can call f() function as A.f(). There is no need of creating an object of class A.

34. What do mean by polymorphism, inheritance, encapsulation?

Polymorhism: is a feature of OOPl that at run time depending upon the type of object the appropriate method is called.

Inheritance: is a feature of OOPL that represents the "is a" relationship between different objects(classes).

Say in real life a manager is a employee. So in OOPL manger class is inherited from the employee class.

Encapsulation: is a feature of OOPL that is used to hide the information.

35. What are the advantages of OOPL?

Object oriented programming languages directly represent the real life objects. The features of OOPL as inhreitance, polymorphism, encapsulation makes it powerful.

36. How many methods do u implement if implement the Serializable Interface?

The Serializable interface is just a "marker" interface, with no methods of its own to implement.

37. Are there any other 'marker' interfaces?

java.rmi.Remote

java.util.EventListener

38. What is the difference between instanceof and isInstance?

instanceof is used to check to see if an object can be cast into a specified type without throwing a cast class exception. isInstance() determines if the specified object is assignment-compatible with the object represented by this Class. This method is the dynamic equivalent of the Java language instanceof operator.The method returns true if the specified Object argument is nonnull and can be cast to the reference type represented by this Class object without raising a ClassCastException. It returns false otherwise.

39. why do you create interfaces, and when MUST you use one?

You would create interfaces when you have two or more functionalities talking to each other. Doing it this way help you in creating a protocol between the parties involved.

40. What's the difference between the == operator and the equals() method? What test does Object.equals()

use, and why?

The == operator would be used, in an object sense, to see if the two objects were actually the same object.This operator looks at the actually memory address to see if it actually the same object. The equals() method is used to compare the values of the object respectively. This is used in a higher level to see if the object values are equal.

Of course the the equals() method would be overloaded in a meaningful way for whatever object that you were working with.

41. Discuss the differences between creating a new class, extending a class and implementing an interface;and when each would be appropriate.

* Creating a new class is simply creating a class with no extensions and no implementations. The signature is as follows

public class MyClass(){

}

* Extending a class is when you want to use the functionality of another class or classes. The extended class inherits all of the functionality of the previous class. An example

of this when you create your own applet class and extend from java.applet.Applet. This gives you all of the functionality of the java.applet.Applet class. The signature would

look like this public class MyClass extends MyBaseClass{}

* Implementing an interface simply forces you to use the methods of the interface implemented. This gives you two advantages. This forces you to follow a standard(forces you to use certain methods) and in doing so gives you a channel for polymorphism. This isn’t the only way you can do polymorphism but this is one of the ways.public class Fish implements Animal{}

42. Given a text file, input.txt, provide the statement required to open this file with the appropriate I/O stream to be able to read and process this file.

43. Name four methods every Java class will have.

public String toString();

public Object clone();

public boolean equals();

public int hashCode();

44. What does the "abstract" keyword mean in front of a method? A class?

Abstract keyword declares either a method or a class. If a method has a abstract keyword in front of it, it is called abstract method.Abstract method has no body. It has only arguments and return type. Abstract methods act as placeholder methods that are implemented in the subclasses. Abstract classes can't be instantiated.If a class is declared as abstract,no objects of that class can be created.If a class contains any abstract method it must be declared as abstract.

45. Does Java have destructors?

No garbage collector does the job working in the background

46. Are constructors inherited? Can a subclass call the parent's class constructor? When?

You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of one of it's superclasses. One of the main reasons is because you probably don't want to override the superclasses constructor, which would be possible if they were inherited. By giving the developer the ability to override a superclasses constructor you would erode the encapsulation abilities of the language.

48. Does Java have "goto"?

No

49. What does the "final" keyword mean in front of a variable? A method? A class?

FINAL for a variable : value is constant

FINAL for a method : cannot be overridden

FINAL for a class : cannot be derived