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Tuesday, November 6, 2012

Core Java OOPS Interview Questions and Answers

1.What are the principle concepts of OOPS?
There are four principle concepts upon which object oriented design and programming rest. They are:
  • Abstraction
  • Polymorphism
  • Inheritance
  • Encapsulation
    • (i.e. easily remembered as A-PIE).

2.What is Abstraction?
Abstraction refers to the act of representing essential features without including the background details or explanations.

3.What is Encapsulation?
Encapsulation is a technique used for hiding the properties and behaviors of an object and allowing outside access only as appropriate. It prevents other objects from directly altering or accessing the properties or methods of the encapsulated object.

4.What is the difference between abstraction and encapsulation?
  • Abstraction focuses on the outside view of an object (i.e. the interface) Encapsulation (information hiding) prevents clients from seeing it’s inside view, where the behavior of the abstraction is implemented.
  • Abstraction solves the problem in the design side while Encapsulation is the Implementation.
  • Encapsulation is the deliverables of Abstraction. Encapsulation barely talks about grouping up your abstraction to suit the developer needs.

5.What is Inheritance?
  • Inheritance is the process by which objects of one class acquire the properties of objects of another class.
  • A class that is inherited is called a superclass.
  • The class that does the inheriting is called a subclass.
  • Inheritance is done by using the keyword extends.
  • The two most common reasons to use inheritance are:
    • To promote code reuse
    • To use polymorphism

6.What is Polymorphism?
Polymorphism is briefly described as "one interface, many implementations." Polymorphism is a characteristic of being able to assign a different meaning or usage to something in different contexts - specifically, to allow an entity such as a variable, a function, or an object to have more than one form.

7.How does Java implement polymorphism?
(Inheritance, Overloading and Overriding are used to achieve Polymorphism in java).
Polymorphism manifests itself in Java in the form of multiple methods having the same name.
  • In some cases, multiple methods have the same name, but different formal argument lists (overloaded methods).
  • In other cases, multiple methods have the same name, same return type, and same formal argument list (overridden methods).

8.Explain the different forms of Polymorphism.
There are two types of polymorphism one is Compile time polymorphism and the other is run time polymorphism. Compile time polymorphism is method overloading. Runtime time polymorphism is done using inheritance and interface.
Note: From a practical programming viewpoint, polymorphism manifests itself in three distinct forms in Java:
  • Method overloading
  • Method overriding through inheritance
  • Method overriding through the Java interface

9.What is runtime polymorphism or dynamic method dispatch?
In Java, runtime polymorphism or dynamic method dispatch is a process in which a call to an overridden method is resolved at runtime rather than at compile-time. In this process, an overridden method is called through the reference variable of a superclass. The determination of the method to be called is based on the object being referred to by the reference variable.

10.What is Dynamic Binding?
Binding refers to the linking of a procedure call to the code to be executed in response to the call. Dynamic binding (also known as late binding) means that the code associated with a given procedure call is not known until the time of the call at run-time. It is associated with polymorphism and inheritance.

11.What is method overloading?
Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
Note:
  • Overloaded methods MUST change the argument list
  • Overloaded methods CAN change the return type
  • Overloaded methods CAN change the access modifier
  • Overloaded methods CAN declare new or broader checked exceptions
  • A method can be overloaded in the same class or in a subclass

12.What is method overriding?
Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its superclass. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
Note:
  • The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
  • You cannot override a method marked final
  • You cannot override a method marked static

13.What are the differences between method overloading and method overriding?
Overloaded Method Overridden Method
Arguments Must change Must not change
Return type Can change Can’t change except for covariant returns
Exceptions Can change Can reduce or eliminate. Must not throw new or broader checked exceptions
Access Can change Must not make more restrictive (can be less restrictive)
Invocation Reference type determines which overloaded version is selected. Happens at compile time. Object type determines which method is selected. Happens at runtime.

14.Can overloaded methods be override too?
Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.

15.Is it possible to override the main method?
NO, because main is a static method. A static method can't be overridden in Java.

16.How to invoke a superclass version of an Overridden method?
To invoke a superclass method that has been overridden in a subclass, you must either call the method directly through a superclass instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the superclass' implementation of the method. 
  // From subclass
  super.overriddenMethod();

17.What is super?
super is a keyword which is used to access the method or member variables from the superclass. If a method hides one of the member variables in its superclass, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its superclass, the method can invoke the overridden method through the use of the super keyword.
Note:
  • You can only go back one level.
  • In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.

18.How do you prevent a method from being overridden?
To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy. 
                        public final void exampleMethod() {
                          //  Method statements
                          }

19.What is an Interface?
An interface is a description of a set of methods that conforming implementing classes must have.
Note:
  • You can’t mark an interface as final.
  • Interface variables must be static.
  • An Interface cannot extend anything but another interfaces.

20.Can we instantiate an interface?
You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.

21.Can we create an object for an interface?
Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.

22.Do interfaces have member variables?
Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.

23.What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.

24.What is a marker interface?
Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.

25.What is an abstract class?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
Note:
  • If even a single method is abstract, the whole class must be declared abstract.
  • Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
  • You can’t mark a class as both abstract and final.

26.Can we instantiate an abstract class?
An abstract class can never be instantiated. Its sole purpose is to be extended (subclassed).

27.What are the differences between Interface and Abstract class?
Abstract Class Interfaces
An abstract class can provide complete, default code and/or just the details that have to be overridden. An interface cannot provide any code at all,just the signature.
In case of abstract class, a class may extend only one abstract class. A Class may implement several interfaces.
An abstract class can have non-abstract methods. All methods of an Interface are abstract.
An abstract class can have instance variables. An Interface cannot have instance variables.
An abstract class can have any visibility: public, private, protected. An Interface visibility must be public (or) none.
If we add a new method to an abstract class then we have the option of providing default implementation and therefore all the existing code might work properly. If we add a new method to an Interface then we have to track down all the implementations of the interface and define implementation for the new method.
An abstract class can contain constructors . An Interface cannot contain constructors .
Abstract classes are fast. Interfaces are slow as it requires extra indirection to find corresponding method in the actual class.

28.When should I use abstract classes and when should I use interfaces?
Use Interfaces when…
  • You see that something in your design will change frequently.
  • If various implementations only share method signatures then it is better to use Interfaces.
  • you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.
Use Abstract Class when…
  • If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
  • When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
  • Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for nonleaf classes in class hierarchies.

29.When you declare a method as abstract, can other nonabstract methods access it?
Yes, other nonabstract methods can access a method that you declare as abstract.

30.Can there be an abstract class with no abstract methods in it?
Yes, there can be an abstract class without abstract methods.
  31.What is Constructor?
  • A constructor is a special method whose task is to initialize the object of its class.
  • It is special because its name is the same as the class name.
  • They do not have return types, not even void and therefore they cannot return values.
  • They cannot be inherited, though a derived class can call the base class constructor.
  • Constructor is invoked whenever an object of its associated class is created.

32.How does the Java default constructor be provided?
If a class defined by the code does not have any constructor, compiler will automatically provide one no-parameter-constructor (default-constructor) for the class in the byte code. The access modifier (public/private/etc.) of the default constructor is the same as the class itself.

33.Can constructor be inherited?
No, constructor cannot be inherited, though a derived class can call the base class constructor.

34.What are the differences between Contructors and Methods?
Constructors Methods
Purpose Create an instance of a class Group Java statements
Modifiers Cannot be abstract, final, native, static, or synchronized Can be abstract, final, native, static, or synchronized
Return Type No return type, not even void void or a valid return type
Name Same name as the class (first letter is capitalized by convention) -- usually a noun Any name except the class. Method names begin with a lowercase letter by convention -- usually the name of an action
this Refers to another constructor in the same class. If used, it must be the first line of the constructor Refers to an instance of the owning class. Cannot be used by static methods.
super Calls the constructor of the parent class. If used, must be the first line of the constructor Calls an overridden method in the parent class
Inheritance Constructors are not inherited Methods are inherited

35.How are this() and super() used with constructors?
  • Constructors use this to refer to another constructor in the same class with a different parameter list.
  • Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

36.What are the differences between Class Methods and Instance Methods?
Class Methods Instance Methods
Class methods are methods which are declared as static. The method can be called without creating an instance of the class Instance methods on the other hand require an instance of the class to exist before they can be called, so an instance of a class needs to be created by using the new keyword.
Instance methods operate on specific instances of classes.
Class methods can only operate on class members and not on instance members as class methods are unaware of instance members. Instance methods of the class can also not be called from within a class method unless they are being called on an instance of that class.
Class methods are methods which are declared as static. The method can be called without creating an  instance of the class. Instance methods are not declared as static.

37.How are this() and super() used with constructors?
  • Constructors use this to refer to another constructor in the same class with a different parameter list.
  • Constructors use super to invoke the superclass's constructor. If a constructor uses super, it must use it in the first line; otherwise, the compiler will complain.

38.What are Access Specifiers?
One of the techniques in object-oriented programming is encapsulation. It concerns the hiding of data in a class and making this class available only through methods. Java allows you to control access to classes, methods, and fields via so-called access specifiers..

39.What are Access Specifiers available in Java?
Java offers four access specifiers, listed below in decreasing accessibility:
  • Public- public classes, methods, and fields can be accessed from everywhere.
  • Protected- protected methods and fields can only be accessed within the same class to which the methods and fields belong, within its subclasses, and within classes of the same package.
  • Default(no specifier)- If you do not set access to specific level, then such a class, method, or field will be accessible from inside the same package to which the class, method, or field belongs, but not from outside this package.
  • Private- private methods and fields can only be accessed within the same class to which the methods and fields belong. private methods and fields are not visible within subclasses and are not inherited by subclasses.
 Situation   public   protected   default   private 
 Accessible to class
 from same package?  		yes yes yes no
 Accessible to class
 from different package?  		yes  no, unless it is a subclass  no no

40.What is final modifier?
The final modifier keyword makes that the programmer cannot change the value anymore. The actual meaning depends on whether it is applied to a class, a variable, or a method.
  • final Classes- A final class cannot have subclasses.
  • final Variables- A final variable cannot be changed once it is initialized.
  • final Methods- A final method cannot be overridden by subclasses.

41.What are the uses of final method?
There are two reasons for marking a method as final:
  • Disallowing subclasses to change the meaning of the method.
  • Increasing efficiency by allowing the compiler to turn calls to the method into inline Java code.

42.What is static block?
Static block which exactly executed exactly once when the class is first loaded into JVM. Before going to the main method the static block will execute.

43.What are static variables?
Variables that have only one copy per class are known as static variables. They are not attached to a particular instance of a class but rather belong to a class as a whole. They are declared by using the static keyword as a modifier.
   static type  varIdentifier;
where, the name of the variable is varIdentifier and its data type is specified by type.
Note: Static variables that are not explicitly initialized in the code are automatically initialized with a default value. The default value depends on the data type of the variables.

44.What is the difference between static and non-static variables?
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.

45.What are static methods?
Methods declared with the keyword static as modifier are called static methods or class methods. They are so called because they affect a class as a whole, not a particular instance of the class. Static methods are always invoked without reference to a particular instance of a class.
Note:The use of a static method suffers from the following restrictions:
  • A static method can only call other static methods.
  • A static method must only access static data.
  • A static method cannot reference to the current object using keywords super or this.                           46.What is an Iterator ?
  • The Iterator interface is used to step through the elements of a Collection.
  • Iterators let you process each element of a Collection.
  • Iterators are a generic way to go through all the elements of a Collection no matter how it is organized.
  • Iterator is an Interface implemented a different way for every Collection.

47.How do you traverse through a collection using its Iterator?
To use an iterator to traverse through the contents of a collection, follow these steps:
  • Obtain an iterator to the start of the collection by calling the collection’s iterator() method.
  • Set up a loop that makes a call to hasNext(). Have the loop iterate as long as hasNext() returns true.
  • Within the loop, obtain each element by calling next().

48.How do you remove elements during Iteration?
Iterator also has a method remove() when remove is called, the current element in the iteration is deleted.

49.What is the difference between Enumeration and Iterator?
Enumeration Iterator
Enumeration doesn't have a remove() method Iterator has a remove() method
Enumeration acts as Read-only interface, because it has the methods only to traverse and fetch the objects Can be abstract, final, native, static, or synchronized
Note: So Enumeration is used whenever we want to make Collection objects as Read-only.

50.How is ListIterator?
ListIterator is just like Iterator, except it allows us to access the collection in either the forward or backward direction and lets us modify an element

51.What is the List interface?
  • The List interface provides support for ordered collections of objects.
  • Lists may contain duplicate elements.

52.What are the main implementations of the List interface ?
The main implementations of the List interface are as follows :
  • ArrayList : Resizable-array implementation of the List interface. The best all-around implementation of the List interface.
  • Vector : Synchronized resizable-array implementation of the List interface with additional "legacy methods."
  • LinkedList : Doubly-linked list implementation of the List interface. May provide better performance than the ArrayList implementation if elements are frequently inserted or deleted within the list. Useful for queues and double-ended queues (deques).

53.What are the advantages of ArrayList over arrays ?
Some of the advantages ArrayList has over arrays are:
  • It can grow dynamically
  • It provides more powerful insertion and search mechanisms than arrays.

54.Difference between ArrayList and Vector ?
ArrayList Vector
ArrayList is NOT synchronized by default. Vector List is synchronized by default.
ArrayList can use only Iterator to access the elements. Vector list can use Iterator and Enumeration Interface to access the elements.
The ArrayList increases its array size by 50 percent if it runs out of room. A Vector defaults to doubling the size of its array if it runs out of room
ArrayList has no default size. While vector has a default size of 10.

55.How to obtain Array from an ArrayList ?
Array can be obtained from an ArrayList using toArray() method on ArrayList.
 List arrayList = new ArrayList();
   arrayList.add(…

 Object  a[] = arrayList.toArray();

56.Why insertion and deletion in ArrayList is slow compared to LinkedList ?
  • ArrayList internally uses and array to store the elements, when that array gets filled by inserting elements a new array of roughly 1.5 times the size of the original array is created and all the data of old array is copied to new array.
  • During deletion, all elements present in the array after the deleted elements have to be moved one step back to fill the space created by deletion. In linked list data is stored in nodes that have reference to the previous node and the next node so adding element is simple as creating the node an updating the next pointer on the last node and the previous pointer on the new node. Deletion in linked list is fast because it involves only updating the next pointer in the node before the deleted node and updating the previous pointer in the node after the deleted node.

57.Why are Iterators returned by ArrayList called Fail Fast ?
Because, if list is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove or add methods, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

58.How do you decide when to use ArrayList and When to use LinkedList?
If you need to support random access, without inserting or removing elements from any place other than the end, then ArrayList offers the optimal collection. If, however, you need to frequently add and remove elements from the middle of the list and only access the list elements sequentially, then LinkedList offers the better implementation.

59.What is the Set interface ?

    The Set interface provides methods for accessing the elements of a finite mathematical set
    Sets do not allow duplicate elements
    Contains no methods other than those inherited from Collection
    It adds the restriction that duplicate elements are prohibited
    Two Set objects are equal if they contain the same elements

60.What are the main Implementations of the Set interface ?

The main implementations of the List interface are as follows:

    HashSet
    TreeSet
    LinkedHashSet
    EnumSet

61.What is a HashSet ?

    A HashSet is an unsorted, unordered Set.
    It uses the hashcode of the object being inserted (so the more efficient your hashcode() implementation the better access performance you’ll get).
    Use this class when you want a collection with no duplicates and you don’t care about order when you iterate through it.

62.What is a TreeSet ?

TreeSet is a Set implementation that keeps the elements in sorted order. The elements are sorted according to the natural order of elements or by the comparator provided at creation time.

63.What is an EnumSet ?

An EnumSet is a specialized set for use with enum types, all of the elements in the EnumSet type that is specified, explicitly or implicitly, when the set is created.

64.Difference between HashSet and TreeSet ?

HashSet     TreeSet
HashSet is under set interface i.e. it  does not guarantee for either sorted order or sequence order.     TreeSet is under set i.e. it provides elements in a sorted  order (acceding order).
We can add any type of elements to hash set.     We can add only similar types
of elements to tree set.


65.What is a Map ?

    A map is an object that stores associations between keys and values (key/value pairs).
    Given a key, you can find its value. Both keys  and  values are objects.
    The keys must be unique, but the values may be duplicated.
    Some maps can accept a null key and null values, others cannot.


66.What are the main Implementations of the Map interface ?

The main implementations of the List interface are as follows:

    HashMap
    HashTable
    TreeMap
    EnumMap


67.What is a TreeMap ?

TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

68.How do you decide when to use HashMap and when to use TreeMap ?

For inserting, deleting, and locating elements in a Map, the HashMap offers the best alternative. If, however, you need to traverse the keys in a sorted order, then TreeMap is your better alternative. Depending upon the size of your collection, it may be faster to add elements to a HashMap, then convert the map to a TreeMap for sorted key traversal.

69.Difference between HashMap and Hashtable ?

HashMap     Hashtable
HashMap lets you have null values as well as one null key.     HashTable  does not allows null values as key and value.
The iterator in the HashMap is fail-safe (If you change the map while iterating, you’ll know).     The enumerator for the Hashtable is not fail-safe.
HashMap is unsynchronized.     Hashtable is synchronized.

Note: Only one NULL is allowed as a key in HashMap. HashMap does not allow multiple keys to be NULL. Nevertheless, it can have multiple NULL values.


70.How does a Hashtable internally maintain the key-value pairs?

TreeMap actually implements the SortedMap interface which extends the Map interface. In a TreeMap the data will be sorted in ascending order of keys according to the natural order for the key's class, or by the comparator provided at creation time. TreeMap is based on the Red-Black tree data structure.

71.What Are the different Collection Views That Maps Provide?

Maps Provide Three Collection Views.

    Key Set - allow a map's contents to be viewed as a set of keys.
    Values Collection - allow a map's contents to be viewed as a set of values.
    Entry Set - allow a map's contents to be viewed as a set of key-value mappings.


72.What is a KeySet View ?

KeySet is a set returned by the keySet() method of the Map Interface, It is a set that contains all the keys present in the Map.

73.What is a Values Collection View ?

Values Collection View is a collection returned by the values() method of the Map Interface, It contains all the objects present as values in the map.

74.What is an EntrySet View ?

Entry Set view is a set that is returned by the entrySet() method in the map and contains Objects of type Map. Entry each of which has both Key and Value.

75.How do you sort an ArrayList (or any list) of user-defined objects ?

Create an implementation of the java.lang.Comparable interface that knows how to order your objects and pass it to java.util.Collections.sort(List, Comparator).

76.What is the Comparable interface ?

The Comparable interface is used to sort collections and arrays of objects using the Collections.sort() and java.utils.Arrays.sort() methods respectively. The objects of the class implementing the Comparable interface can be ordered.

The Comparable interface in the generic form is written as follows:

    interface Comparable<T>

where T is the name of the type parameter.

All classes implementing the Comparable interface must implement the compareTo() method that has the return type as an integer. The signature of the compareTo() method is as follows:

      int i = object1.compareTo(object2)

    If object1 < object2: The value of i returned will be negative.
    If object1 > object2: The value of i returned will be positive.
    If object1 = object2: The value of i returned will be zero.


77.What are the differences between the Comparable and Comparator interfaces ?

Comparable     Comparato

It uses the compareTo() method.

int objectOne.compareTo(objectTwo).
    t uses the compare() method.

int compare(ObjOne, ObjTwo)
It is necessary to modify the class whose instance is going to be sorted.     A separate class can be created in order to sort the instances.
Only one sort sequence can be created.     Many sort sequences can be created.
It is frequently used by the API classes.     It used by third-party classes to sort instances.











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Sunday, November 4, 2012

Best jQuery Tips & Tricks for jQuery Programmers

1. Optimize performance of complex selectors

Query a subset of the DOM when using complex selectors drastically improves performance:
var subset = $("");
$("input[value^='']", subset);

2. Set Context and improve the performance

On the core jQuery function, specify the context parameter when. Specifying the context parameter allows jQuery to start from a deeper branch in the DOM, rather than from the DOM root. Given a large enough DOM, specifying the context parameter should translate to performance gains.
$("input:radio", document.forms[0]);

3. Live Event Handlers

Set an event handler for any element that matches a selector, even if it gets added to the DOM after the initial page load:
$('button.someClass').live('click', someFunction);
This allows you to load content via ajax, or add them via javascript and have the event handlers get set up properly for those elements automatically.
Likewise, to stop the live event handling:
$('button.someClass').die('click', someFunction);
These live event handlers have a few limitations compared to regular events, but they work great for the majority of cases. Live event will work starting from jQuery 1.3

4. Checking the Index

jQuery has .index but it is a pain to use as you need the list of elements and pass in the element you want the index of
var index = e.g $('#ul>li').index( liDomObject );
The following is easier:
if you want to know the index of an element within a set, e.g. list items within a unordered list:
$("ul > li").click(function ()
{
    var index = $(this).prevAll().length;
});

5. Use jQuery data method

jQuery’s data() method is useful and not well known. It allows you to bind data to DOM elements without modifying the DOM.

6. Fadeout Slideup effect to remove an element

Combine more than one effects in jQuery to animate and remove an element from DOM.
$("#myButton").click(function() {
         $("#myDiv").fadeTo("slow", 0.01, function(){ //fade
             $(this).slideUp("slow", function() { //slide up
                 $(this).remove(); //then remove from the DOM
             });
         });
});

7. Checking if an element exists

Use following snippet to check whether an element exists or not.
if ($("#someDiv").length) {
    //hooray!!! it exists...
}

8. Add dynamically created elements into the DOM

Use following code snippet to create a DIV dynamically and add it into the DOM.
Further Reading: Dynamically Add/Remove rows in HTML table using JavaScript
var newDiv = $('<div></div>');
newDiv.attr("id","myNewDiv").appendTo("body");

9. Line breaks and chainability

Instead of doing:
$("a").hide().addClass().fadeIn().hide();
You can increase readability like so:
$("a")
  .hide()
  .addClass()
  .fadeIn()
  .hide();

10. Creating custom selectors

$.extend($.expr[':'], {
    over100pixels: function(a) {
        return $(a).height() > 100;
    }
});
$('.box:over100pixels').click(function() {
    alert('The element you clicked is over 100 pixels high');
});

11. Cloning an object in jQuery

Use .clone() method of jQuery to clone any DOM object in JavaScript.
// Clone the DIV
var cloned = $('#somediv').clone();
jQuery’s clone() method does not clone a JavaScript object. To clone JavaScript object, use following code.
// Shallow copy
var newObject = jQuery.extend({}, oldObject);
// Deep copy
var newObject = jQuery.extend(true, {}, oldObject);

12. Test if something is hidden using jQuery

We use .hide(), .show() methods in jquery to change the visibility of an element. Use following code to check the whether an element is visible or not.
if($(element).is(":visible") == "true") {
       //The element is Visible
}

13. Alternate way of Document Ready

//Instead of
$(document).ready(function() {
    //document ready
});
//Use
$(function(){
    //document ready
});

14. Selecting an element with . (period) in its ID

Use backslash in the selector to select the element having period in its ID.
$("#Address\\.Street").text("Enter this field");

15. Counting immediate child elements

If you want to count all the DIVs present in the element #foo
<div id="foo">
  <div id="bar"></div>
  <div id="baz">
    <div id="biz">
  </div>
  <span><span>
</div>
//jQuery code to count child elements
$("#foo > div").size()

16. Make an element to “FLASH”

jQuery.fn.flash = function( color, duration )
{
    var current = this.css( 'color' );
    this.animate( { color: 'rgb(' + color + ')' }, duration / 2 );
    this.animate( { color: current }, duration / 2 );
}
//Then use the above function as:
$( '#importantElement' ).flash( '255,0,0', 1000 );

17. Center an element on the Screen

jQuery.fn.center = function () {
    this.css("position","absolute");
    this.css("top", ( $(window).height() - this.height() ) / 2+$(window).scrollTop() + "px");
    this.css("left", ( $(window).width() - this.width() ) / 2+$(window).scrollLeft() + "px");
    return this;
}
//Use the above function as:
$(element).center();

18. Getting Parent DIV using closest

If you want to find the wrapping DIV element (regardless of the ID on that DIV) then you’ll want this jQuery selector:
$("#searchBox").closest("div");

19. Disable right-click contextual menu

There’s many Javascript snippets available to disable right-click contextual menu, but JQuery makes things a lot easier:
$(document).ready(function(){
    $(document).bind("contextmenu",function(e){
        return false;
    });
});

20. Get mouse cursor x and y axis

This script will display the x and y value – the coordinate of the mouse pointer.
$().mousemove(function(e){
    //display the x and y axis values inside the P element
    $('p').html("X Axis : " + e.pageX + " | Y Axis " + e.pageY);
});
<p></p> 
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Friday, November 2, 2012

Best JSP/Servlets interview questions Answers

1. What is the difference between JSP and Servlets ?
JSP is used mainly for presentation only. A JSP can only be HttpServlet that means the only supported protocol in JSP is HTTP. But a servlet can support any protocol like HTTP, FTP, SMTP etc.
2. What is difference between custom JSP tags and beans?
Custom JSP tag is a tag you defined. You define how a tag, its attributes and its body are interpreted, and then group your tags into collections called tag libraries that can be used in any number of JSP files. To use custom JSP tags, you need to define three separate components: the tag handler class that defines the tag’s behavior ,the tag library descriptor file that maps the XML element names to the tag implementations and the JSP file that uses the tag library
JavaBeans are Java utility classes you defined. Beans have a standard format for Java classes. You use tags
Custom tags and beans accomplish the same goals — encapsulating complex behavior into simple and accessible forms. There are several differences:
Custom tags can manipulate JSP content; beans cannot. Complex operations can be reduced to a significantly simpler form with custom tags than with beans. Custom tags require quite a bit more work to set up than do beans. Custom tags usually define relatively self-contained behavior, whereas beans are often defined in one servlet and used in a different servlet or JSP page. Custom tags are available only in JSP 1.1 and later, but beans can be used in all JSP 1.x versions.
3. What are the different ways for session tracking?
Cookies, URL rewriting, HttpSession, Hidden form fields
4. What mechanisms are used by a Servlet Container to maintain session information?
Cookies, URL rewriting, and HTTPS protocol information are used to maintain session information
5. Difference between GET and POST
In GET your entire form submission can be encapsulated in one URL, like a hyperlink. query length is limited to 255 characters, not secure, faster, quick and easy. The data is submitted as part of URL.
In POST data is submitted inside body of the HTTP request. The data is not visible on the URL and it is more secure.
6. What is session?
The session is an object used by a servlet to track a user’s interaction with a Web application across multiple HTTP requests. The session is stored on the server.
7. What is servlet mapping?
The servlet mapping defines an association between a URL pattern and a servlet. The mapping is used to map requests to Servlets.
8. What is servlet context ?
The servlet context is an object that contains a information about the Web application and container. Using the context, a servlet can log events, obtain URL references to resources, and set and store attributes that other servlets in the context can use.
9. What is a servlet ?
servlet is a java program that runs inside a web container.
10. Can we use the constructor, instead of init(), to initialize servlet?
Yes. But you will not get the servlet specific things from constructor. The original reason for init() was that ancient versions of Java couldn’t dynamically invoke constructors with arguments, so there was no way to give the constructor a ServletConfig. That no longer applies, but servlet containers still will only call your no-arg constructor. So you won’t have access to a ServletConfig or ServletContext.
12. How many JSP scripting elements are there and what are they?
There are three scripting language elements: declarations, scriptlets, expressions.
13. How do I include static files within a JSP page?
Static resources should always be included using the JSP include directive. This way, the inclusion is performed just once during the translation phase.
14. How can I implement a thread-safe JSP page?
You can make your JSPs thread-safe adding the directive <%@ page isThreadSafe="false" % > within your JSP page.
15. What is the difference in using request.getRequestDispatcher() and context.getRequestDispatcher()?
In request.getRequestDispatcher(path) in order to create it we need to give the relative path of the resource. But in resourcecontext.getRequestDispatcher(path) in order to create it we need to give the absolute path of the resource.
16. What are the lifecycle of JSP?
When presented with JSP page the JSP engine does the following 7 phases.
Page translation: -page is parsed, and a java file which is a servlet is created.
Page compilation: page is compiled into a class file
Page loading : This class file is loaded.
Create an instance :- Instance of servlet is created
jspInit() method is called
_jspService is called to handle service calls
_jspDestroy is called to destroy it when the servlet is not required.
17. What are context initialization parameters?
Context initialization parameters are specified by the in the web.xml file, these are initialization parameter for the whole application.
18. What is a Expression?
Expressions are act as place holders for language expression, expression is evaluated each time the page is accessed. This will be included in the service method of the generated servlet.
19. What is a Declaration?
It declares one or more variables or methods for use later in the JSP source file. A declaration must contain at least one complete declarative statement. You can declare any number of variables or methods within one declaration tag, as long as semicolons separate them. The declaration must be valid in the scripting language used in the JSP file. This will be included in the declaration section of the generated servlet.
20. What is a Scriptlet?
A scriptlet can contain any number of language statements, variable or expressions that are valid in the page scripting language. Within scriptlet tags, you can declare variables to use later in the file, write expressions valid in the page scripting language, use any of the JSP implicit objects or any object declared with a . Generally a scriptlet can contain any java code that are valid inside a normal java method. This will become the part of generated servlet’s service method.

21. What are the implicit objects?
Certain objects that are available for the use in JSP documents without being declared first. These objects are parsed by the JSP engine and inserted into the generated servlet. The implicit objects are: request, response, pageContext, session, application, out, config, page, exception
22. What’s the difference between forward and sendRedirect?
forward is server side redirect and sendRedirect is client side redirect. When you invoke a forward request, the request is sent to another resource on the server, without the client being informed that a different resource is going to process the request. This process occurs completely with in the web container And then returns to the calling method. When a sendRedirect method is invoked, it causes the web container to return to the browser indicating that a new URL should be requested. Because the browser issues a completely new request any object that are stored as request attributes before the redirect occurs will be lost. This extra round trip a redirect is slower than forward. Client can disable sendRedirect.
23. What are the different scopes available ?
page, request, session, application
24. Is JSP extensible ?
Yes, it is. JSP technology is extensible through the development of custom actions, or tags, which are encapsulated in tag libraries.
25. What’s the Servlet Interface?
The central abstraction in the Servlet API is the Servlet interface. All servlets implement this interface, either directly or, more commonly, by extending a class that implements it such as HttpServlet.
26. What are two different types of Servlets ?
GenericServlet and HttpServlet. HttpServlet is used to implement HTTP protocol, where as Generic servlet can implement any protocol.
27. What is the life cycle of servlet?
Each servlet has the same life cycle: first, the server loads and initializes the servlet by calling the init method. This init() method will be executed only once during the life time of a servlet. Then when a client makes a request, it executes the service method. finally it executes the destroy() method when server removes the servlet.
28. Can we call destroy() method on servlets from service method ?
Yes.
29. What is the need of super.init (config) in servlets ?
Then only we will be able to access the ServletConfig from our servlet. If there is no ServletConfig our servlet will not have any servlet nature.
30. What is the difference between GenericServlet and HttpServlet?
GenericServlet supports any protocol. HttpServlet supports only HTTP protocol. By extending GenericServlet we can write a servlet that supports our own custom protocol or any other protocol.
32. Can we write a constructor for servlet ?
Yes. But the container will always call the default constructor only. If default constructor is not present , the container will throw an exception.
33. What is the difference between <%@ include ...> (directive include) and ?
@ include is static include. It is inline inclusion. The contents of the file will get included on Translation phase. It is something like inline inclusion. We cannot have a dynamic filename for directive include. is dynamic include. Here the included file will be processed as a separate file and the response will be included. We can have a dynamic filename for . We can also pass parameters to
34. Can I just abort processing a JSP?
Yes. You can put a return statement to abort JSP processing.
35. What are the parameters for service method ?
ServletRequest and ServletResponse
36. What are cookies ?
Cookies are small textual information that are stored on client computer. Cookies are used for session tracking.
37. How do I prevent the output of my JSP or Servlet pages from being cached by the browser?
By setting appropriate HTTP header attributes we can prevent caching by the browser
<%
response.setHeader("Cache-Control","no-store"); //HTTP 1.1
response.setHeader("Pragma","no-cache"); //HTTP 1.0
response.setDateHeader ("Expires", 0); //prevents caching at the proxy server
%>
38. How to refer the “this” variable within a JSP page?
Under JSP 1.0, the page implicit object page is equivalent to “this”, and returns a reference to the servlet generated by the JSP page.
39. How many JSP scripting elements and what are they?
There are three scripting elements in JSP . They are declarations, scriptlets, expressions.
40. Can we implement an interface in JSP ?
No.

41. What is the meaning of response has already been committed error?
You will get this error only when you try to redirect a page after you already have flushed the output buffer. This happens because HTTP specification force the header to be set up before the lay out of the page can be shown. When you try to send a redirect status, your HTTP server cannot send it right now if it hasn’t finished to set up the header. Simply it is giving the error due to the specification of HTTP 1.0 and 1.1
42. How do I use a scriptlet to initialize a newly instantiated bean?
A jsp:useBean action may optionally have a body. If the body is specified, its contents will be automatically invoked when the specified bean is instantiated (Only at the time of instantiation.) Typically, the body will contain scriptlets or jsp:setProperty tags to initialize the newly instantiated bean, although you are not restricted to using those alone.
43. What is JSP?
JSP is a server side scripting technology. JSP allows Java as well as a few special tags to be embedded into a web file (HTML/XML, etc). The suffix must ends with .jsp.
44. What are JSP Actions?
JSP actions use constructs in XML syntax to control the behavior of the servlet engine. You can dynamically insert a file, reuse JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Available actions include: jsp:include, jsp:useBean, jsp:setProperty, jsp:getProperty, jsp:forward and Jsp: plugin
45. What is the difference between ServletContext and ServletConfig?
The ServletConfig gives the information about the servlet initialization parameters. The servlet engine implements the ServletConfig interface in order to pass configuration information to a servlet. The server passes an object that implements the ServletConfig interface to the servlet’s init() method. The ServletContext gives information about the container. The ServletContext interface provides information to servlets regarding the environment in which they are running. It also provides standard way for servlets to write events to a log file.
46. How can a servlet refresh automatically?
We can use a client-side Refresh or Server Push
47. What is Server side push?
Server Side push is useful when data needs to change regularly on the clients application or browser, without intervention from client. The mechanism used is, when client first connects to Server, then Server keeps the TCP/IP connection open.
48. What is client side refresh?
The standard HTTP protocols ways of refreshing the page, which is normally supported by all browsers.
<META HTTP-EQUIV="Refresh" CONTENT="5; URL=/servlet/MyServlet/">
This will refresh the page in the browser automatically and loads the new data every 5 seconds.
49. What is the Max amount of information that can be saved in a Session Object ?
There is no such limit on the amount of information that can be saved in a Session Object. The only limit is the Session ID length , which should not exceed more than 4K.
50. Why should we go for inter servlet communication?
The three major reasons to use inter servlet communication are: a) Direct servlet manipulation – allows to gain access to the other currently loaded servlets and perform certain tasks (through the ServletContext object) b) Servlet reuse – allows the servlet to reuse the public methods of another servlet. c) Servlet collaboration – requires to communicate with each other by sharing specific information (through method invocation)
51. What is a output comment?
A comment that is sent to the client in the viewable page source. The JSP engine handles an output comment as un interpreted HTML text, returning the comment in the HTML output sent to the client. You can see the comment by viewing the page source from your Web browser.
52. What is a Hidden Comment
Hidden Comments are JSP comments. A comments that documents the JSP page but is not sent to the client. The JSP engine ignores a hidden comment, and does not process any code within hidden comment tags.
53. What are the differences between a session and a cookie?
Session is stored in server but cookie stored in client. Session should work regardless of the settings on the client browser. There is no limit on the amount of data that can be stored on session. But it is limited in cookie. Session can store objects and cookies can store only strings. Cookies are faster than session.
54. What is HttpTunneling?
HTTP tunneling is used to encapsulate other protocols within the HTTP or HTTPS protocols. Normally the intranet is blocked by a firewall and the network is exposed to the outer world only through a specific web server port, that listens for only HTTP requests. To use any other protocol, that by passes the firewall, the protocol is embedded in HTTP and send as HttpRequest.
55. How to pass information from JSP to included JSP?
By using tag.
56. What is the better way to enable thread-safe servlets and JSPs? SingleThreadModel Synchronization?
The better approach is to use synchronization. Because SingleThreadModel is not scalable. SingleThreadModel is pretty resource intensive from web server’s perspective. The most serious issue is when the number of concurrent requests exhaust the servlet instance pool. In that case, all the un serviced requests are queued until something becomes free – which results in poor performance.
57. What is the difference between ServletContext and PageContext?
ServletContext gives the information about the container and PageContext gives the information about the Request
58. Why in Servlet 2.4 specification SingleThreadModel has been deprecated?
SingleThreadModel is pretty resource intensive from web server’s perspective. When the number of concurrent requests exhaust the servlet instance pool, all the un serviced requests are queued until something becomes free – which results in poor performance.
59. How do you pass data (including JavaBeans) to a JSP from a servlet?
By forwarding the request to the servlet ( the data must be there in the request scope) we can pass the data from a JSP to servlet. Also we can use a session to pass the data.
60. How can I set a cookie? 
 Cookie c = new Cookie("name","value");
    response.addCookie(c);
 

61. How will you delete a cookie?

  1. Cookie c = new Cookie ("name", null);  
  2. c.setMaxAge(0);  
  3. response.addCookie(killCookie);  
62. What is the difference between Context init parameter and Servlet init parameter?
Servlet init parameters are for a single servlet only. No body out side that servlet can access that. It is declared inside the tag inside Deployment Descriptor, where as context init parameter is for the entire web application. Any servlet or JSP in that web application can access context init parameter. Context parameters are declared in a tag directly inside the tag. The methods for accessing context init parameter is getServletContext ().getInitParamter (“name”) where as method for accessing servlet init parameter is getServletConfig ().getInitParamter (“name”);
63. What are the different ways for getting a servlet context?
We will get ServletContext by calling getServletConfig ().getServletContext (). This is because a ServletConfig always hold a reference to ServletContext. By calling this.getServletContext () also we will get a ServletContext object.
64. What is the difference between an attribute and a parameter?
The return type of attribute is object, where the return type of parameter is String. The method to retrieve attribute is getAttribute () where as for parameter is getParameter (). We have a method setAttribute to set an attribute. But there is no setters available for setting a parameter.
65. How to make a context thread safe?
Synchronizing the ServletContext is the only solution to make a ServletContext thread safe.
Eg:

  1. synchronized (getServletContext ()) {  
  2.       // do whatever you want with thread safe context.  
  3. }  
66. What is the difference between setting the session time out in deployment descriptor and setting the time out programmatically?
In DD time out is specified in terms of minutes only. But in programmatically it is specified in seconds. A session time out value of zero or less in DD means that the session will never expire. To specify session will never expire programmatically it must be negative value.
67. What JSP lifecycle methods we can override?
You cannot override the _jspService() method within a JSP page. You can however, override the jspInit() and jspDestroy() methods within a JSP page.
68. How will you include a static file in a JSP page?
You can include a static resource to a JSP using or <%@ inlcude >.
69. How you can perform browser redirection?
We can use the method sendRedirect of HttpServletResponse or forward method of RequestDispatcher.
70. Can we use ServletOutputStream object from a JSP page?
No. You are supposed to use JSPWriter object (given to you in the form of the implicit object out) only for replying to clients.
71. How can you stop JSP execution in the middle of processing a request?
We can use the return statement to stop the processing of JSP. Because JSP is compiled to servlet and all the statements will go inside service method, any time you can stop the processing using return statement.
72. How can I invoke a JSP error page from a servlet?
You can invoke the JSP error page and pass the exception object to it from within a servlet. For that you need to create a request dispatcher for the JSP error page, and pass the exception object as a javax.servlet.jsp.jspException request attribute.
73. How will you pass information from JSP to included JSP?
By using <%jsp:param> tag.
74. How does JSP handle runtime exceptions?
Using errorPage attribute of page directive JSP handles runtime exceptions. We need to specify isErrorPage=true if the current page is intended to use as a JSP error page.
75. How can I enable session tracking for JSP pages if the browser has disabled cookies?
By default session tracking uses cookies to associate a session identifier with a user. If the browser does not support cookies, or if cookies are disabled, you can still use session tracking using URL rewriting. For URL rewriting to be effective, you need to append the session ID for each and every link that is part of your servlet response. By using the methods response.encodeURL() and response.encodeRedirectURL() we can achieve this.
76. How can you declare methods in your JSP page?
You can declare methods as declarations in your JSP page. The methods can then be invoked within any other methods you declare, or within JSP scriptlets and expressions.
77. How can we set the inactivity period on a per-session basis?
We can set the session time out programmatically by using the method setMaxInactiveInterval() of HttpSession.
78. How do you pass an init parameter to a JSP?
You need to configure the DD for passing init parameter to a JSP. You can configure the DD as follows.

  1.     <servlet>  
  2.           <servlet-name>test.jsp</servlet-name>  
  3.           <jsp-file>test.jsp  
  4.                <init-param>  
  5. <param-name>Abc</param-name>  
  6. <param-value>Xyz</param-value>  
  7.            </init-param>  
  8.     </jsp-file></servlet>  
  9.       
79. How can my application get to know when a HttpSession is removed?
You can define a class which implements HttpSessionBindingListener and override the valueUnbound() method.
80. How many cookies can one set in the response object of the servlet? Also, are there any restrictions on the size of cookies?
If the client is using Netscape, the browser can receive and store 300 total cookies and 4 kilobytes per cookie. And the no of cookie is restricted to 20 cookies per server or domain

81. When a session object gets added or removed to the session, which event will get notified ?
HttpSessionBindingListener will get notified When an object is added and/or removed from the session object, or when the session is invalidated, in which case the objects are first removed from the session, whether the session is invalidated manually or automatically (timeout).
82. What is URL Encoding and URL Decoding ?
URL encoding is the method of replacing all the spaces and other extra characters into their corresponding Hex Characters and Decoding is the process of converting all Hex Characters back to their normal form.
83. What is the difference between an applet and a servlet?
Servlets run inside a web server and applets run inside web browsers. Applets must have graphical user interfaces whereas servlets have no graphical user interfaces.
84. What are the different web servers available name few of them?
Tomcat, Java Web Server, JRun,
85. What is a Java Bean?
A Java Bean is a software component that has been designed to be reusable in a variety of different environments.
86. What are the different types of ServletEngines?
The different types of ServletEngines available are:- Standalone ServletEngine: This is a server that includes built-in support for servlets. Add-on ServletEngine: It is a plug-in to an existing server. It adds servlet support to a server that was not originally designed with servlets in mind.
87. What is a Session Id?
It is a unique id assigned by the server to the user when a user starts a session.
88. What is use of parseQueryString ?
It parses a query string and builds a Hashtable of key-value pairs, where the values are arrays of strings. The query string should have the form of a string packaged by the GET or POST method.
89. When init() and Destroy() will be called.
init() is called whenever the servlet is loaded for the first time into the web server. Destroy will be called whenever the servlet is removed from the web server.
90. What is the use of setComment and getComment methods in Cookies ?
setComment: If a web browser presents this cookie to a user, the cookie’s purpose will be described using this comment. This is not supported by version zero cookies.
getComment: Returns the comment describing the purpose of this cookie, or null if no such comment has been defined.
91. Why we are used setMaxAge() and getMaxAge() in Cookies ?
setMaxAge : Sets the maximum age of the cookie. The cookie will expire after that many seconds have passed. Negative values indicate the default behavior, and will be deleted when the browser closes.
getMaxAge : Returns the maximum specified age of the cookie. If none was specified, a negative value is returned, indicating the default behavior described with setMaxAge.
92. What is the use of setSecure() and getSecure() in Cookies ?
setSecure method indicates to the web browser that the cookie should only be sent using a secure protocol (https). getSecure method returns the value of the ‘secure’ flag.
93. How do you communicate between the servlets?
We can communicate between servlets by using RequestDespatcher interface and servlet chaining.
94. What is Servlet chaining?
Servlet chaining is a technique in which two or more servlets can cooperate in servicing a single request. In servlet chaining, one servlet’s output is the input of next servlet. This process continues until the last servlet is reached. Its output is then sent back to the client. We are achieving Servlet Chaining with the help of RequestDispatcher.
95. How will you communicate from an applet to servlet?
There are three ways to communicate from an applet to servlet and they are: HTTP Communication (Text-based and object-based) , Socket Communication and RMI Communication
96. Can we call a servlet with parameters in the URL?
Yes. You can call a servlet with parameters in the URL like ?param=value
97. How do servlets handle multiple simultaneous requests?
When a request comes in, the web server will start a new thread and the request is assigned to a thread, which calls a service method of the servlet.
98. Explain the directory structure of a web application?
The directory structure of a web application consists of two parts. A private directory called WEB-INF and a public resource directory which contains files server to public. WEB-INF folder consists of web.xml (the deployment descriptor), classes directory (where we keeps all our classes and servlets) and lib directory (where we keeps all our jar files). The public folder contains the static resources of the web application.
99. What is pre initialization of a servlet?
A container doesn’t initialize the servlets when it starts up. It initializes a servlet when it receives a request for that servlet first time. This is called lazy loading. The servlet specification defines the element, which can be specified in the deployment descriptor to make the servlet container load and initialize the servlet as soon as it starts up. The process of loading a servlet before any request comes in is called preloading or pre initializing a servlet.
100. What are the uses of ServletRequest?
The ServletRequest gives information such as the names of the parameters passed by the client, the protocol (scheme) being used by the client, and the names of the remote host that made the request and the server that received it. The input stream, ServletInputStream.


101. What are the uses of ServletResponse interface?
ServletResponse allows the servlet to set the content length and MIME type of that response. It provides an output stream, ServletOutputStream and a Writer through which the servlet can send data.
102. How HTTP Servlet handles client requests?
An HTTP Servlet handles client requests through its service method. The service method supports standard HTTP client requests by dispatching each request to a method designed to handle that request.
103. Is HTML page a web component?
No. Static HTML pages and applets are not considered as web components by J2EE specification. Even the server-side utility classes are not considered web components.
104. What is the container?
A container is a runtime support of a system-level entity. Containers provide components with services such as lifecycle management, security, deployment, and threading.
105. What is the web container?
Servlet and JSP containers are collectively referred to as Web containers.
106. What is deployment descriptor?
A deployment descriptor is an XML based file which describes a web application’s deployment settings. The name of deployment descriptor of a web application is web.xml.
107. How can I print the stack trace of an exception from a JSP page?
By creating an object of PrintWriter we can print the stack trace in JSP page. Eg:
PrintWriter pw = response.getWriter();
exception.printStackTrace(pw);
108. Do objects stored in a HTTP Session need to be serializable? Or can it store any object?
No, the objects need not to be serializable. We can store any type of objects in session.
109. What is the difference between JspWriter and PrintWriter?
JspWriter is buffered.
110. How can you implement singleton pattern in servlets ?
All servlets are singleton only , if the servlet is not implementing SingleThreadModel.
111. What is the difference between an application server and a web server?
Everything in web server must be achieved programmatically. But application server provides a lot of services like security, transaction , scalability. Application server will be having EJB support where web server don’t have an EJB support.
112. What is servlet exception?
It indicates that there is an exception occurred in the servlet.
113. Can we implement Runnable interface from within our servlet?
Our servlet can implement the Runnable interface.
114. What is a WAR file?
WAR stands for Web Archive. It is a compressed version of your web application. You can use this WAR file to deploy your web application.
115. What is a servlet filter?
Servlet filters are pluggable Web components that allow us to implement pre-processing and post-processing logic in our Web applications.
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Thursday, November 1, 2012

Long Term Solution for java.lang.OutOfMemoryError:

This Error is thrown when the Java Virtual Machine cannot allocate an object because it is out of memory, and no more memory could be made available by the garbage collector. Note: Its an Error (extends java.lang.Error) not Exception. Two important types of OutOfMemoryError are often encountered

  1. java.lang.OutOfMemoryError: Java heap space

    2. The quick solution is to add these flags to JVM command line when Java runtime is started:

    1. -Xms1024m -Xmx1024m   

  2. java.lang.OutOfMemoryError: PermGen space

    3. The solution is to add these flags to JVM command line when Java runtime is started:

    1. -XX:+CMSClassUnloadingEnabled-XX:+CMSPermGenSweepingEnabled  
Long Term Solution: Increasing the Start/Max Heap size or changing Garbage Collection options may not always be a long term solution for your Out Of Memory Error problem. Best approach is to understand the memory needs of your program and ensure it uses memory wisely and does not have leaks. You can use a Java memory profiler to determine what methods in your program are allocating large number of objects and then determine if there is a way to make sure they are no longer referenced, or to not allocate them in the first place.
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