Collections

Java Collections Framework is a collection of interfaces and classes which help in storing and processing the data efficiently. Collection Framework provides an architecture to store and manipulates the group of objects.

All collections frameworks contain the following:

·         Interfaces: These are abstract data types that represent collections.

·         Classes: These are the concrete implementations of the collection interfaces.

Advantages of Collections:

Reduces effort of implementing our own collection classes rather we can use core collection classes 

Code quality is enhanced with the use of well tested collections framework classes.

Reusability and Interoperability

Set: A Set is a Collection that cannot contain duplicate elements.The Set interface contains only methods inherited from Collection .

Since Set is an interface you need to instantiate a concrete implementation of the interface in order to use it. You can choose between the following Set implementations in the Java Collections API:

java.util.EnumSet

java.util.HashSet

java.util.LinkedHashSet

java.util.TreeSet

examples of how to create a Set instance:

Set setA = new EnumSet();

Set setB = new HashSet();

Set setC = new LinkedHashSet();

Set setD = new TreeSet();

Methods decleared in Set:

add( ):Adds an object to the collection

clear( ):Removes all objects from the collection

contains( ):Returns true if a specified object is an element within the collection

isEmpty( ):Returns true if the collection has no elements

iterator( ):Returns an Iterator object for the collection which may be used to retrieve an object

remove( ):Removes a specified object from the collection

size( ):Returns the number of elements in the collection

Sample program:

import java.util.HashSet;

import java.util.Iterator;

import java.util.Set;

public class SetExample {

public static void main(String args[]) {

// Creating new Set

Set Set1 = new HashSet();

// Adding elements

Set1.add("A");

Set1.add("B");

Set1.add("C");

//Removing elements:

Set1.remove("A");

// Use of Iterator

Iterator iterator = Set1.iterator();

while (iterator.hasNext())

{

System.out.println("abc " + iterator.next());

}

// Use of for-each

for (Object object : Set1)

System.out.println("for-each loop: " + object);

// Clearing all the elements

Set1.clear();

System.out.println("Set1 is Empty: " + Set1.isEmpty());

// Checking the number of elements

System.out.println("Set1 has: " + Set1.size() + " Elements");

}

}

ArrayList:

import java.util.ArrayList;

import java.util.Iterator;

public class SetExample {

    public static void main(String args[]) {

        ArrayList<String> alist=new ArrayList<String>(); //creating arraylist

        alist.add("Ajoy");//adding object in arraylist

        alist.add("Rahul");

        alist.add("Rakesh");

        alist.add("Sudhir");

        Iterator itr=alist.iterator();

        while(itr.hasNext()){

            System.out.println(itr.next());

        }

        ArrayList<String> alist2=new ArrayList<String>();

        alist2.add("Sudhir1");

        alist2.add("Ajoy1");

        alist.addAll(alist2);

        Iterator itr1=alist.iterator();

        while(itr1.hasNext()){

            System.out.println(itr1.next());

        }

        ArrayList<String> alist3=new ArrayList<String>();

        alist3.add("Sudhir1");

        alist3.add("Ajoy1");

        alist.removeAll(alist3);

        Iterator itr2=alist.iterator();

        while(itr2.hasNext()){

            System.out.println(itr2.next());

        }

    }

}

HashTable:

import java.util.Hashtable;

import java.util.Enumeration;

public class SetExample {

 public static void main(String[] args) {

   Enumeration names;

   String key;

   // Creating a Hashtable

   Hashtable<String, String> hashtable = new Hashtable<String, String>();

   // Adding Key and Value pairs to Hashtable

   hashtable.put("Key1","Ajoy");

   hashtable.put("Key2","Rakesh");

   hashtable.put("Key3","Rahul");

   hashtable.put("Key4","Smith");

   hashtable.put("Key5","Mayank");

   names = hashtable.keys();

   while(names.hasMoreElements())

   {

      key = (String) names.nextElement();

      System.out.println("The key: " +key+ " has Value: " + hashtable.get(key));

   }

 }

}

HashMap:

import java.util.HashMap;

import java.util.Map;

import java.util.Iterator;

import java.util.Set;

public class TestHashMap {

    public static void main(String args[]) {

        HashMap<Integer, String> h = new HashMap<Integer, String>();

        h.put(1, "Ajoy");

        h.put(2, "Rakesh");

        h.put(3, "Rahul");

        h.put(4, "Smith");

        h.put(5, "Mayank");

        Set set = h.entrySet();

        Iterator i = set.iterator();

        while(i.hasNext()) {

            Map.Entry m = (Map.Entry)i.next();

            System.out.println("The key "+ m.getKey() + " has Value "+ m.getValue());

        }

        /* Get values based on key*/

        String val= h.get(1);

        System.out.println("Value at index 1 is: "+val);

        /* Remove values based on key*/

        h.remove(2);

        System.out.println("New Set after reomoval");

        Set set2 = h.entrySet();

        Iterator i1 = set2.iterator();

        while(i1.hasNext()) {

            Map.Entry m1 = (Map.Entry)i1.next();

            System.out.println("The key "+ m1.getKey() + " has Value "+ m1.getValue());

        }

    }

}