Saturday 24 December 2016

HTML introduction and its basics ?

 Introduction of HTML

      HTML is a hypertext markup language.
      HTML is the most commonly used markup language for creating Web pages.
     A markup language provides a way to describe the structure of text-based information on a Web pages.Hypertext is similar to regular text.
     HTML is designed by w3c (world wide web consortium) is an organization.

  USES OF HTML IN MARKET
                                                    
      In  google
      Online shop ex: flipkart,amazon,.snapdeal,etc...
      Business
      organization 
      Education 
      Research center

Creating and Saving HTML Document 

     We can create a HTML Document using a text editor, like Notepad, adding our html code to the document, and saving it with  filename.html extension.

 The following step to create and save an html document:

  Step1:  Start->All program->Accessories->Notepad.
  Step2: The Notepad window opens with blank document.
  Step3:  Add the html code in the document.
  Step4:  click File->Save As
  Step5:   Save As dialog box appear and saving the document inside folder filename.html and click ok.

Opening the html document in a web Browser

 The following step to open an html document:

Step1:  Open the folder contain our html document.
Step2:  Double click the html document.html document will open in the default web browser in our computer.

 HTML ADVANTAGE

   Don't require additional software to run HTML.
   It can be run in normal web browser. for example: Internet Explorer,Mozilla Firefox,Opera etc...

First Look  at HTML

  In case of HTML is more important of show the text in web browser using basic tags.
  An HTML tag is format of a pair of angle brackets (<tag>) text place in this bracket.
  That angle brackets defines an HTML element .
                    
HTML element have two properties
             1. Attributes(ex: <img align='left'>) : align is the attribute
             2. Content
syntax:    <element-name> content  </element-name>
for example:
                        <strong> hello </strong>
                      In above example hello is a called of content.

Basic structure of an HTML document

 The basic structure  of HTML 
syntax:
          <!DOCTYPE>
           <html>
           <body>
           Content of the web page
            </body>
            </html>

example:
          <!DOCTYPE>
          <html>
           <body>
           welcome
           </body>
           </html>

 Result:
          The result of web page welcome


Title of web page

syntax:
          <!DOCTYPE>
           <html>
           <head>
            <title>
            Title of the web page
           </title>
           </head>
           <body>
           Content of the web page
            </body>
            </html>

example:
          <!DOCTYPE>
          <html>
          <head>
           <title> html </title>
           </head>
           <body>               
           <h1> welcome </h1>
            <p> The <b><i><u> html<u><i><b> is hypertext markup language. It is used to create a web page.A markup language provides a way to describe the structure of text-based information on a web page.</p>
             <hr>
           </body>
           </html>
Result:
 

The preceding example uses some of the most common element found in HTML document:
  •  <!DOCTYPE> indicates the particular version of html being used in the document.but that tag not used mostly.
  • <html>,<body>,<head>tag pair are used to specify the general structure of the document.
  • <title> and </title> tag pair specifies the title of the document.It appear in the title bar of the web browser.
  •  <h1> and </h1> is called header tag pair.It creates a headline to indicate some important information.
  •  <hr> tag is used to only inserts a horizontal rule, or bar,across the screen.
  • <p> and </p> is called paragraph tag.It is used to create a paragraph of text. 
  • <u></u>  is used for underline the text within opening and closing tags.
  • <i></i> is used to italic style for the text within opening and closing tags.
  • <b></b> is used to bold style for the text within opening and closing tags.

<HTML> Tag

        The <html> tag place on the beginning and the end of an HTML document.It is also called the root element.
        The <html> tag, directly contains only the <head> tag, the <body> tag, and <frameset> tag instead of <body>tag.
        The <html>tag's used in a document, as a container for all other elements.

<HTML>ELEMENT

Attributes:

class
  1. dir
  2. id
  3. lang
  4. version
  5. xmlns
1. class Attribute 
       The class attribute is used to indicate the class.  It belongs to id, class is used to associate a tag with a name.
 2. dir Attribute
       Dir is a gives the direction of the text.
       we can set this attribute of text direction
  •  "ltr"-left to right
  •  "rtl"-right to left 

3. id Attribute
         The id attribute is used to set the unique name for the element & used for referring to it.

4.lang Attribute
          The lang attribute is used for base language for the element.

5.version Attribute
          Indicate  the  version of the language used.

6.xmlns Attribute
           It is used for declare a namespace for custom tags in an html document.

The <HEAD> Element    

         The information in the head of an HTML document is very important because it is used to describe or augment the content of the document.
          The information contained with in the <head> tag is information about information of the page, which generally of referred to as meta-information.   

Attributes of the< head> element:

  1. class
  2. id 
  3. dir
  4. lang
  5. profile
  6. style
  7. title 
1. class 
       The class attribute is used to indicate the class.  It belongs to id, class is used to associate a tag with a name.
 2. dir
       Dir is a gives the direction of the text.
       we can set this attribute of text direction
  •  "ltr"-left to right
  •  "rtl"-right to left 

3. id 
         The id attribute is used to set the unique name for the element & used for referring to it.

4. lang
          The lang attribute is used for base language for the element.

5. profile
         Gives the location of one or more white-space separated metadata profile URLs for the current document.

6. style
         The style attribute is used to add style sheet information directly to a tag.

7. title
         The title is used to provide advisory text about an element or its contents.

Elements to be added to the <head> Element:
  1. <base>
  2. <basefont>
  3. <bgsound>
  4. <isindex>
  5. <link>
  6. <meta>
  7. <nextid>
  8. <noscript>
  9. <script>
  10. <style>
  11. <title>         
Description: 
1. <base>
       Base URL for the document.

2. <Basefont>
       Basefont for the document.

3.<bgsound>
       Background sound.

4. <isindex>
        Rudimentary input control.

 5. <link>
      Link are use to connect one page to another.

 6. <meta>
      It gives the header information.

 7. <nextid>
      Hint for the name value to use when creating a new hyperlink element.

 8. <noscript>
      Holds text that only appears if the browser does not support the <script> element.

9 .<script>
       Holds programming script statements, like  javascript.

10. <style>
         Includes style information for rendering.

11 .<title>
       The title of the web page that appears in the web browser.  

The <title> Element

        The <title> element contains the title of the HTML document. It will appear in the title bar of the web browser.
         Each <head> element should contain a <title> element.

Attributes of the <title> Element:
  1. class
  2. dir
  3. lang
  4. style     
Description: 

1. class 
       The class attribute is used to indicate the class.  It belongs to id, class is used to associate a tag with a name.
 2. dir
       Dir is a gives the direction of the text.
       we can set this attribute of text direction
  •  "ltr"-left to right
  •  "rtl"-right to left 
3. lang
          The lang attribute is used for base language for the element.

3. style
         The style attribute is used to add style sheet information directly to a tag.

The <BODY> Element  

          The <body> element  contains the body of the HTML document. The element  include the entire content that will appear in the web browser.
          It can include text, images, and multimedia element.

Attributes of the <body> Element: 
  1. alink
  2. background
  3. bgcolor
  4. bgproperties
  5. bottommargin
  6. class
  7. id
  8. dir
  9. lang
  10. language
  11. leftmargin
  12. link
  13. marginheight
  14. marginwidth
  15. rigthmargin
  16. scroll
  17. style 
  18. text
  19. title
  20. topmargin
  21. vlink
Description: 
 1. alink
       It is used to color of the  hyperlinks when you are clicked; 
       You can set this attribute to a predefined color or a color value.
2. background
      The element used to set the background image.
3. bgcolor
     The color of the background of the browser; you can set this attribute to a predefined color or a color value.
4.bgproperties
      Indicate if the background should scroll when the text scrolls;
      If you set this attribute to fixed; It is only allowed value, the background will not scroll when the text scrolls.
5. bottommargin
         It  specifies the bottom margin, which is the empty space at the bottom of the document.
   6. class 
       The class attribute is used to indicate the class.  It belongs to id, class is used to associate a tag with a name.

7. id 
         The id attribute is used to set the unique name for the element & used for referring to it.


 8. dir
       Dir is a gives the direction of the text.
       we can set this attribute of text direction
  •  "ltr"-left to right
  •  "rtl"-right to left 

9. lang
          The lang attribute is used for base language for the element.

10. language
         Scripting language used for the element.

11. leftmargin
         Specifies the leftmargin, which is the empty space to the left of the document.

12. link
         Color of the hyperlinks that have not yet been visited.

13. marginheight
         Gives the height of the top and bottom margins, in pixels.

14. marginwidth
         Gives the width of the left and right margins, in pixels.

15. rightmargin
          Specifies the rightmargin, which is the empty space to right of the document, in pixel.

16. scroll
       Specifies the vertical scroll bar appears to the right of the document.

17. style
        Specifies whether a vertical scrollbar appears to the right of the document. Default value for the attribute "yes" or "no".

18. text
        Set the color of the text in the document.

19. title
        Hold the additional information for the element.

20. topmargin
         Specifies the topmargin, which is the empty space to top of the document, in pixel.

21. vlink
        Color of the hyperlinks that have been visited.
        


       
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Thursday 15 December 2016

Why TreeSet does not accept null value ?

In TreeSet, all elements are sorted according to the natural ordering of the element. Also it does not contain duplicate and null value. If you want to modify the natural ordering of the element, you can use your own Comparator at the time initialization, to sort its value based on your own logic.

 If you try to store null value , it throws NullPointerExeption .

The reason is, if you look at the internal implementation of the TreeSet, it uses natural ordering, that means TreeSet uses Comparable interface by default to sort its value by comparing other value. 

 Just think programmatically, how will you compare the null value to other. Is it make sense to compare the value with other(NULL) which doesn't have any value in it.  That's why java developer made it TreeSet such a way that, it will throw NullPointerExeption when you try to store null value.
And also if you look at the definition of add method in TreeSet

 add Method in TreeSet

 
  public boolean add(E e) throws NullPointerExeption {  
     return m.put(e, PRESENT)==null;  
  }

Hope you clear with the explaination.


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Tuesday 6 December 2016

Why ConcurrentHashMap does not allow null keys and null values ?

We all know that ConcurrentHashMap (optimized version of  Hashtable) doesn't allow null key or null values. To know the reason behind this implementation, we need to know the internal implementation and its uses.

 I am separating the above explanation in two steps.

  1. not allowing null key
  2. not allowing null values
1) Not allowing null key
The reason behind this implementation is due to ambiguity nature of the ConcurrentMap (ConcurrentHashMap & ConcurrentSkipListMaps) when it comes to threading environment. The main reason is that if map.get(key) returns null, we can't detect whether the key explicitly maps to null vs the key isn't mapped. In ConcurrentHashMap, It might be possible that key 'K' might be deleted in between the get(k) and containsKey(k) calls.

 For example in below code of ConcurrentHashMap
Line 1: if (concurrentHashMap.containsKey(k)) {     
Line 2:      return concurrentHashMap.get(k);  
Line 3: } else {
Line 4:      throw new KeyNotPresentException();
Line 5: }

 The above example i am  explaining with two thread(1 & 2). In Line 2, after checking thread 1 with the key "K" in concurrentHashMap(Line 1 - contains some value for the key "K"),  it will returns null (if thread 2 removed the key 'K' from concurrentHashMap). Instead of throwing KeyNotPresentException , it actually returns null.

 The above scenario will not happen in case of non-concurrent map (HashMap), because there is no Concurrent access. So null key are allowed in HashMap by eliminating the ambiguity.

 2) Not allowing null values
And also null values are not allowed in the ConcurrentMap (ConcurrentHashMap & ConcurrentSkipListMaps). If you look at the below code in ConcurrentHashMap.

 Line 1. public V put(K key, V value) {
Line 2.   if (value == null)
Line 3.     throw new NullPointerException();
Line 4.   int hash = hash(key.hashCode());
Line 5.   return segmentFor(hash).put(key, hash, value, false);
Line 6. }

 In line 2 & 3, throws NullPointerException by checking the value, this means, passed value could be used through the ConcurrentHashMap without expecting it to be null. I checked the usage of value in ConcurrentHashMap, there is no evidence or piece of code throwing NullPointerException.  Then i started investigating the internal implementation.

 So wrote a simple program with modified ConcurrentHashMap by removing those Line 2 and Line 3 in the above code and created two thread (1 and 2), one take cares of inserting null values and other with reading it. Surprisingly i did not get any error. So i decided to check performance of get and put operation by measuring the time of both modified and unmodified version of ConcurrentHashMap. The get method of modified ConcurrentHashMap takes 5-8 times more time than unmodified one and put method of modified ConcurrentHashMap takes 3-5 times more than unmodified one.

 The Reason is segment of ConcurrentHashMap takes a lock if the value of HashEntry becomes null.

 GET METHOD
1 get(Object key, int hash) {
2.    if (count != 0) { 
3.      HashEntry e = getFirst(hash);
4.      while (e != null) {
5.        if (e.hash == hash && key.equals(e.key)) {
6.          V v = e.value;
7.            if (v != null)
8.              return v;
9.          return readValueUnderLock(e); //  takes lock if the value is null
10.       }
11.       e = e.next;
12.     }

 PUT METHOD
1.  V put(K key, int hash, V value, boolean onlyIfAbsent) {
2.    lock();
3.    try {
4.      int c = count;
5.      if (c++ > threshold) 
6.        rehash();
7.      HashEntry[] tab = table;
8.      int index = hash & (tab.length - 1);
9.      HashEntry first = tab[index];
10.     HashEntry e = first;
11.     while (e != null && (e.hash != hash || !key.equals(e.key)))
12.       e = e.next;
13.     V oldValue;
14.     if (e != null) {
15.       oldValue = e.value;
16.       if (!onlyIfAbsent)
17.         e.value = value;
18.       } else {
19.         oldValue = null;
20.         ++modCount;
21.         tab[index] = new HashEntry(key, hash, first, value);  // New value for HashEntry
22.         count = c;
23.       }
24.     return oldValue;
25.   } finally {
26.   unlock();
27. }
28.  }

 Now the questions is, if ConcurrentHashMap  is not allowing null value, then why the null check is implemented for HashEntry value in Line 7 of get method.  Reason is, consider a situation that writer thread is trying to create a new HashEntry in Line 21 of put method but not yet initialized, so actual value is not reflecting in value attribute of HashEntry, in the same time reader thread reads it value as null, so this recheck is done in Line 7 in the get method. 

 And extra check in the Line 9 in get method is very costly in terms of performance and it is avoided 
for not null value. so modified ConcurrentHashMap takes long time to read and write null values than unmodified ConcurrentHashMap with non-null values.


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Friday 2 December 2016

Explain the JAVA process when you run main class ?


When  you invoke a main class with the command "java MainClass", your application will start executing the main method, then it continue its execution depends on your programming logic. But before start executing the main method, there is a sequence of JAVA process involve to successfully carry its execution.

 JAVA PROCESS:-

  1. Creation of  new JVM 
  2. Memory allocation to JVM.
  3. JVM loads MainClass into Memory with the help of class loader.
  4. Execution of Main Method by JVM.


Creation of  new JVM
After invoking the application with the help of command "java MainClass". The first step is java process will create new jvm with unique id to manage the process. This unique id is used to identify your process from other running process.

 Memory allocation to JVM
After creating the JVM, java process will allocate the memory to JVM based on the JVM parameter set by the java user, if not it takes default value.  Default values depends on the actual environment (32 bit  or 64 bit operating system). Allocation of memory for heap, stack, class area, native area , etc are done in this step.

 JVM loads MainClass into Memory with the help of class loader
Once the memory is allocated, next step is, JVM loads the MainClass into memory with the help of ClassLoader (click to read detailed explaination class loader and its execution life cycle). 

 Execution of Main Method by JVM
After loading of MainClass into memory, JVM call the static main method by using class name MainClass.main(args) with the help of parameter passed by the user. JVM construct the String array based on the input given by the user while invoking the java command. In this above case its empty, So JVM calls main method with empty String array argument. After that your execution will continue depends on your business logic.

 In case if JVM doesn't find main method , it throws 
"java.lang.NoSuchMethodError: main Exception in thread 'main'"
That's all about Java Process.


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Thursday 1 December 2016

what is class loader and its execution life cycle ?

Class loader is part of JRE, which is responsible to load all the required classes dynamically in memory , it helps JVM initialize and execute the java application without knowing the files located in the system. With the help of class loader, JVM execute the byte code of .class files which is compiled from .java files by java compiler (javac).

 In java, there are 3 types or hierarchy in the class loader.
  1. Bootstrap loader, parent class of all loader.
  2. Extension loader, extends bootstrap loader.
  3. System or Application loader.

1) Bootstrap class loader

Parent class of all loader and core part of JVM, which loads the java core libraries or classes located from <JAVA_HOME>/jre/lib/rt.jar.It is written in native code. 

2) Extension loader

Extension class loader extends bootstrap loader , which loads the java classes from the <JAVA_HOME>/jre/ext directory or any other directory specified by java.ext.dirs system property. Extension loader is implemented by the sun.misc.Launcher$ExtClassLoader class.

 3) Sytem or Application class loader

 System class loader extends Extension loader and loads all the classes from java.class.path which  is CLASSPATH environment variable,-classpath or -cp option, Class-Path attribute of Manifest inside JAR file.  System loader is implemented by the sun.misc.Launcher$AppClassLoader class.
Life cycle of class loader




When JVM is requesting the object of Employee.class with the help of System class loader, it delegates control to its super class Extension loader and in turn its delegates to its super class Bootstrap class loader. Bootstrap try to looks from <JAVA_HOME>/jre/lib. if present it returns the Employee class definition otherwise, Extension loader take care of the control to load from <JAVA_HOME>/jre/ext directory or any other directory specified by java.ext.dirs system property.if present, it returns the Employee class definition, otherwise its subclass System loader looks from CLASSPATH. If class is not present in any of the class loader, then application throws ClassNotFoundException, otherwise it loads the Employee.class in the memory , which helps JVM to execute the application.  The entire life cycle works in the concept of delegation principle.

 That's all about what is class loader and its execution life cycle.


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