What is collection in java

Collection:-

• Collection is a group of object which could be homogeneous or heterogeneous with some size.
• Collection in java are data structures primary defined through a set of classes and interfaces.
• It also refer to as a container which contain the group of multiple element into a single unit.

Collections are used for:-

• To generalized the array concept.
• To store multiple type of object.
• To define set of interfaces for storing java objects.
• Collection is group of classes which gives the functionality of handling vectors,arraylist,hashmap,hashtable,stack etc.

Collection interfaces uses following methods:-

1.add():-To add element in collection.

2.remove():-To remove element from collection.

3.toarray():-Convert collection into array of element.

4.contains():-To check if specified element is in the collection respectively.     

Types Of Collections:-

Set:-

• Extends collection interface
• Does't allow duplicate element.
• Support all methods present in collection interface.

Types of set:-

1. HashSet class:-

• Implemented using a hash table.
• No ordering of element.

Example:-

 import java.util.*;
class TestHashSet
{
        public static void main(String arg[])
        {
                  HashSet obj=new HashSet();
                  obj.add("Apple");
                  obj.add("Orange");
                  obj.add("Apple");
                  obj.add("Banana");
System.out.println("elements are"+obj);
System.out.println("Size is:"+obj.size());
        }
}

Output:- elements are Apple,Orange,Banana
              Size is:- 3


 

   2.TreeSet class:-

• In this values are sorted in asending order.
• It implement tree structure.
• Guarantee ordering of element.

 Example:-

 import java.util.*;
class TestTreeSet
{
        public static void main(String arg[])
        {
                  TreeSet obj=new TreeSet();
                  obj.add("Z");
                  obj.add("A");
                  obj.add("F");
                  obj.add("E");
System.out.println("elements are"+obj);
System.out.println("Size is:"+obj.size());
        }
}

Output:- elements are A E F Z
              Size is:- 4

3.LinkHashSet class:-

• It is an subclass of hashset.
• LinkHashSet are doubly linked lists.
• They are in fix order but not in sorted manner.

 Example:-

 import java.util.*;

class TestLinkHashSet
{
        public static void main(String arg[])
        {
                 LinkHashSet obj=new LinkHashSet();
                  obj.add("One");
                  obj.add("Two");
                  obj.add("Three");
                  obj.add("Four");
System.out.println("elements are"+obj);
System.out.println("Size is:"+obj.size());
        }
}

Output:- elements are One Two Three Four
              Size is:- 4

List:-

• List is order collection called sequence.
• Array list and linked list are implementation classes of listinterface.
• It extends collection interface.
• It may have duplicate element.
• In list elements are added at end of list.

 Types of  list:-

1. ArrayList Class:-

• Array list is an array based implementation where elements can be access directly via get and set menthod.
• It can dynamically increase or decrease.

2.LinkList Class:-

• It will give better performance onadd or remove compare to arraylist.
• It will give poor performance on get and set method compare to arraylist.

Queue:-

• Queue is a collection used to hold multiple elements prior to processing.
• Beside basic collection operation a queue provide additional insertion,extraction and innspection operation.
• In queue ordering is First Come First Out basis(FIFO).