HashSet()
HashSet(Collection c)
HashSet(int capacity)
HashSet(int capacity, float fillRatio)

The first form constructs a default has set. The second form initializes the hash set by using the elements of c. The third form initializes the capacity of the hash set to capacity. The fourth form initializes both the capacity and the fill ration (also called load capacity) of the hash set from its arguments. The fill ratio must be between 0.0 and 1.0, and it determines how full the hash set can be before it is re sized upward. Specifically when the number of elements is greater than the capacity of the hash set multiplied by its fill ratio, the hash set is expanded. For constructors that do not take a fill ratio, 0.75 is used.

HashSet does not define any additional methods beyond those provided by its superclasses and interfaces.
Importantly, note that a hash set does not guarantee the order of its elements, because the process of hashing doesn’t usually lend itself to the creation of sorted sets. If you need sorted storage, then another collection, such as TreeSet, is a better choice.

Here is an example that demonstrates HashSet:

//Demonstrate HashSet.

import java.util.*;
class HashSet Demo {
public static void main(String args[])
{
//create a hash set

HashSet hash=new HashSet();

//add elements to the hash set

hash.add(“B”);
hash.add(“A”);
hash.add(“D”);
hash.add(“E”);
hash.add(“C”);
hash.add(“F”);
System.out.println(hash);
}
}

The following is the output from this program:

[ A,F, E, D, C, B]
As explained , the elements are not stored in sorted order, and the precise output may vary.

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Establishing Communication between a Server and a Client through a socket is called Socket Programming

• To create the Client Socket Socket Class is available in java.net package.
• To create ServerSocket, ServerSocket class is avialable in java.net package
• A Client is a machine that sends a request for some service
• A Server is machine that provides services to the client

Note:  Write a java program to create a client and server Communication between them

// A Server that sends a String to the client as a response to the client connection

import java.io.*;
import java.net.*;

class Server1
{
public static void main(String args[])throws IOException
{

//create the serversocket

ServerSocket ss=new ServerSocket(777);

//make the socket to accept the client connection

Socket s=ss.accept();
System.out.println(“connection established”);

//flow of data in the socket

OutputStream obj=s.getOutputStream();

//to send data to the client

PrintStream ps=new PrintStream(obj);

//to receive data from the client

BufferedReader br=new BufferedReader(new InputStreamReader(s.getInputStream()));

//to read data from keyboard

BufferedReader kb=new BufferedReader(new InputStreamReader(System.in));
while(true)
{
String str,str1;
while((str=br.readLine())!=null)
{
System.out.println(str);
str1=kb.readLine();
ps.println(str1);
}

//disconnet server

ss.close();
s.close();
br.close();
kb.close();
ps.close();
System.exit(0);
}
}
}

// A Client that sends a Request to the Server for the response

import java.io.*;
import java.net.*;

class Client1
{
public static void main(String args[])throws IOException
{

//create client socket

Socket s=new Socket(“localhost”,777);

//data send by the client the dos is most preferable

DataOutputStream dos=new DataOutputStream(s.getOutputStream());
BufferedReader br=new BufferedReader(new InputStreamReader(s.getInputStream()));
BufferedReader kb=new BufferedReader(new InputStreamReader(System.in));
String str,str1;
while(!(str=kb.readLine()).equals(“exit”))
{
dos.writeBytes(str+”\n”);
str1=br.readLine();
System.out.println(str1);
}

//disconnect the client

dos.close();
br.close();
kb.close();
}
}

Procedure of Compiling  and executing the program

1. First Compile the Client Program and wait for the server compilation javac Client1.java
2. Secound Compile the Server Program and wait for the client compilation javac Server1.java
3. and Run the server wait for the request from the client  java Server1.
4. But don’t run the Server program before the Compilation of the Client compilation
5. Run the Client and send the Request to the Server which is waiting for the connection to be establish by java Client1.
6. After the client sends the request the server will show the message as the Connection is established and ready for the communication.

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When the service on the same socket is changed, a new port number should be alloted.

Note :- Some alloted port numbers :

port no. Application

13                                    Date and time Services

21                                    FTP,Which transfers files

23                                   Telnet,Which provides remote login

25                                   SMTP,Which delivers mail messages

67                                   BOOTP,Which provides configuration at  boot time

80                                  HTTP, Which transfers web pages

109                          POP,Whcih enables users to access mail boxes on remote sys.

for port no Internally 2 bytes is used

NOTE:

1) To create Server Socket, ServeerSocket class is avialable in java.net.package

2) To create Client Socket Socket class is available in java.net.package

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Rep) LoadFactor is useful to determine the point where the capacity of the Hashtable (HashMap) will be doubled For Hashtable initial capacity * loadfactory–>11 * 0.75=8

This class implements a hashtable, which maps keys to values. Any non-null object can be used as a key or as a value.To successfully store and retrieve objects from a Hashtable , the objects used as keys must implement the HashCode method and the equals method.

The load factor is a measure of how full the hash table is allowed to get before its capacity is automatically increased. When the number of entries in the Hashtable exceeds the product of the load factor and the current capacity, the capacity is increased by calling the rehash method.
Therefore after starting 8th pair key values, the capacity of the Hashtable will be “Doubled” –>11*2=22.

Hashtable ht=new Hashtable(100);

|__ It is the inital capacity of the hashtable

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Res) A Collection FrameWork represents a class library to handle groups of objects

–> Collection framework is implemented in Java.Util.Package.

–> Collection does’t store copy of all the object, but it stores references of all objects.

–> Collection classes will act upon only referenced datatypes they do not act upon primitive datatypes.

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In essence, an ArrayList is a variable-length array of object references. Thjat is, an ArrayList can dynamically increase or decrease in size. Array lists are created with an initial size. When this size is exceeded, the collection is automatically enlarged. When objects are removed, the arraya may be shrunk.

It is dynamically growing array that stores objects. It is not Synchronized and Vector is a Synchronized

Note: You can’t store primitive data type. we can store only object into ArrayList.

ArrayList has the constructors shown here:

–> ArrayList();  //This constructor builds an empty array list

–> ArrayList(Collection c)  // This constructor builds an array list that is initialized with the elements of collection c.

–> ArrayList(int capacity)  //This Contructor builds an array list that has the specified initial capacity.

1) To Creat  an ArrayList

ArrayList arl=new ArrayList();  //creating an object

ArrayList Arl=new ArrayList(20);  //creating an object with the size

2) To add objects use add() method

arl.add(“APPLE”);

arl.add(2,”APPLE”);

3) To Remove objects use remove().

arl.remove(“APPLE”);  //remove an string

arl.remove(2);  //removing the string  of specified location

4) To Know Number of objects, use size()

int n= arl.size()   // It displays the size of the array list.

Example of using an ArrayList.

//Demonstrating ArrayList.

import java.util.*;

class Arraylist Demo  {
public static void main(String args[])  {

ArrayList list=new ArrayList();  //creating an object for ArrayList class
System.out.println(“Initial size of al:”+al.size());

//add  elements to the array list
al.add(“C”);
al.add(“B”);
al.add(“A”);
al.add(“D”);
al.add(“E”);
al.add(“T”);
al.add(1,”A2″);

System.out.println(“Size of al after additions: “+ al.size());

//display the array list
System.out.println(“Content of al:”+al);

//Remove elements from the array list
al.remove(“F”);
al.remove(2);

System.out.println(“size of al after deletion:” +al.size());
System.out.println(“contents of al:”+al);
}
}

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C and C++ programmers will know that generic utility functions with variable arguments can be used to pass different arguments depending upon the calling situations. Java does not support the concept of variable arguments to a function. This feature can be achieved in Java through the use of the Vector class contained in the java.util package. This class can be used to create a generic dynamic array known as Vector that can hold objects of any type and any number. The objects do not have to have to be homogenous. Array can be easily implemented as vector. Vectors are created like arrays as follows:

Vector intVect  = new Vector();   //declaring without size

Vector list        = new Vector(3);  // declaring with size.

Note that a vector can be declared without specifying any size explicitly. A vector can accommodate an unknown number of items. Even, when a size is specified, this can be overlooked and different number of items may be put into the vector. Remember, in contrast, an array must always have its size specified.

Vector possess a number of advantages over arrays.

1.  It is convenient to use vectors to store objects.

2. A vector can be used to store alist of objects that may vary in size.

3.  We can add and delete objects from the list as and when required.

A major constraint in using vectors is that we cannot directly store simple data types in a vector; we can only store objects. Therefore, we need to convert simple tyhpes to objects. This can be done using the wrapper classes.

The vector class supports a number of methods that can be used to manipulate the vectors created. Important ones are listed

Method Call and task performed

list.addElement (item) Adds the item specified to the list at the end.

list.elementAt(10) Gives the name of the 10th object.

list.size() Gives the number of objects present.

list.removeElement(item) Removes the specified item from the list.

list.removeElementAt(n) Removes the item stored in the nth position of the list.

list.removeAllElements() Removes all the item stored in the nth position of the list.

list.copyInto(array) Copies all items from list to array.

list.insertElementAt(item,n) Inserts the item at nth position.

Example of working with vectors:

import java.util.*;

class LanguageVector
{
public static void main(String args[])
{
Vector list=new Vector();  //creating an object for vector class
int length=args.length;

for(int i=0;i<length;i++)
{
list.addElement(args[i]);;
list.insertElementAt(“COBOL”,2);
int size=list.size();
String listArray[]=new String[size];
list.copyInto(listArray);
System.out.println(“List of Languages”);

for(int i=0;i<size;i++)
{
System.out.println(listArray[i]);
}
}
}

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1) Explain Java Collections Framework?

A) Java Collections Framework provides a well designed set of interfaces and classes that support operations on a collection of objects.

2) Explain Iterator Interface.

A) An Iterator is similar to the Enumeration interface. With the Iterator interface methods, you can traverse a collection from start to end and safely remove elements from the underlying collection. The iterator() method generally used in query operations.
Basic methods:
iterator.remove();
iterator.hasNext();
iterator.next();

3) Explain Enumeration Interface?

The Enumeration interface allows you to iterate through all the elements of a collection. Iterating through an Enumeration is similar to iterating through an Iterator. However, there is no removal support with Enumeration.
Basic methods:
boolean hasMoreElements();
Object nextElement();

4) What is the difference between Enumeration and Iterator interface?

The Enumeration interface allows you to iterate through all the elements of a collection. Iterating through an Enumeration is similar to iterating through an Iterator. However, there is no removal support with Enumeration.

5) Explain Set Interface.

In mathematical concept, a set is just a group of unique items, in the sense that the group contains no duplicates. The Set interface extends the Collection interface. Set does not allow duplicates in the collection. In Set implementations null is valid entry, but allowed only once.

6) What are the two types of Set implementations available in the Collections Framework?

HashSet and TreeSet are the two Set implementations available in the Collections Framework.

7) What is the difference between HashSet and TreeSet?

HashSet Class implements java.util.Set interface to eliminate the duplicate entries and uses hashing for storage. Hashing is nothing but mapping between a key value and a data item, this provides efficient searching.

The TreeSet Class implements java.util.Set interface provides an ordered set, eliminates duplicate entries and uses tree for storage.

What is a List?

List is an ordered and non duplicated collection of objects. The List interface extends the Collection interface.

9) What are the two types of List implementations available in the Collections Framework?

ArrayList and LinkedList are the two List implementations available in the Collections Framework.

10) What is the difference between ArrayList and LinkedList?

The ArrayList Class implements java.util.List interface and uses array for storage. An array storage’s are generally faster but we cannot insert and delete entries in middle of the list. To achieve this kind of addition and deletion requires a new array constructed. You can access any element at randomly.

The LinkedList Class implements java.util.List interface and uses linked list for storage. A linked list allow elements to be added, removed from the collection at any location in the container by ordering the elements. With this implementation you can only access the elements in sequentially.

11) What collection will you use to implement a queue?
LinkedList

12) Explain Map Interface?.

A map is a special kind of set with no duplicates. The key values are used to lookup, or index the stored data. The Map interface is not an extension of Collection interface, it has it’s own hierarchy. Map does not allow duplicates in the collection. In Map implementations null is valid entry, but allowed only once.

13) What are the two types of Map implementations available in the Collections Framework?

HashMap and TreeMap are two types of Map implementations available in the Collections Framework.

14) What is the difference between HashMap and TreeMap?

The HashMap Class implements java.util.Map interface and uses hashing for storage. Indirectly Map uses Set functionality so, it does not permit duplicates. The TreeMap Class implements java.util.Map interface and uses tree for storage. It provides the ordered map.

15) Explain the functionality of Vector Class?

Once array size is set you cannot change size of the array. To deal with this kind of situations we use Vector. Vector grows and shrink it’s size automatically. Vector allows to store only objects not primitives. To store primitives, convert primitives in to objects using wrapper classes before adding them into Vector. The Vector reallocates and resizes itself automatically.

16) What does the following statement convey?

Vector vt = new Vector(3, 10);
vt is an instance of Vector class with an initial capacity of 3 and grows in increment of 10 in each relocation

17) How do you store a primitive data type within a Vector or other collections class?

You need to wrap the primitive data type into one of the wrapper classes found in the java.lang package, like Integer, Float, or Double, as in:
Integer in = new Integer(5);

18) What is the difference between Vector and ArrayList?

Vector and ArrayList are very similar. Both of them represent a growable array. The main difference is that Vector is synchronized while ArrayList is not.

19) What is the difference between Hashtable and HashMap?

Both provide key-value access to data. The key differences are :
a. Hashtable is synchronised but HasMap is not synchronised.
b. HashMap permits null values but Hashtable doesn’t allow null values.
c. iterator in the HashMap is fail-safe while the enumerator for the Hashtable is not fail safe.

20) How do I make an array larger?

You cannot directly make an array larger. You must make a new (larger) array and copy the original elements into it, usually with System.arraycopy(). If you find yourself frequently doing this, the Vector class does this automatically for you, as long as your arrays are not of primitive data types.

21) Which is faster, synchronizing a HashMap or using a Hashtable for thread-safe access?

Because a synchronized HashMap requires an extra method call, a Hashtable is faster for synchronized access.

22) In which package would you find the interfaces and classes defined in the Java Collection Framework?

java.util

23) Which method in the System class allows you to copy elements from one array to another?

System.arraycopy()

24) When using the System.arraycopy() method, What exception is thrown if the destination array is smaller than the source array?

ArrayIndexOutofBoundsException

25) What is the use of Locale class?

The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region

26) What is the use of GregorianCalendar class?

The GregorianCalendar provides support for traditional Western calendars

27) What is the use of SimpleTimeZone class?

The SimpleTimeZone class provides support for a Gregorian calendar

28) What is the use of 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.

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