Collections in Java

Collections:

• Collections in java is a framework that provides an architecture to store and display the data.
• Collections API provides interfaces and classes.
• Using Collection we can perform operations like searching,sorting,insertion,deletion,manipulation.
• Collections is nothing but group of different type of objects together as a single entity is also known as Collections.
• Collections are growable Nature.

• Collection Framework having Interfaces like Set,List,Map.Every Interface having classes.

Why Collection Came?

• Arrays are fixed in size.we can't increase size of Arrays.that's y only collections came.collections having growable nature.we can increase size.

Set Interface:

• A set is a collection that having only unique elements.Duplicate won't be there.
• Set having no index.
• Set allows only one null value.
• Set having classes like :

• HashSet
• LinkedHashMap
• TreeSet

HashSet:

HashSet having only unique elements.

HashSet having no Order.

HashSet allows only one null value.

Program for HashSet:

package.com.instanceofjava;

import.java.util.HashSet;

public class A{

public static void main(String args[]){

HashSet hashset=new HashSet();

hashset.add("Indhu");

hashset.add("Indhu");

hashset.add("Sindhu");

hashset.add("swathi");

hashset.add(null);

hashset.add(null);

hashset.add("Lavs");

Iterator it=hashset.iterator();

while(it.hasNext()){

System.out.println(it.next);

}

}

}

Output:

Indhu

null

Lavs

Sindhu

swathi

LinkedHashSet :

• LinkedHashSet allows only unique elements.
• LinkedHashSet allows Insertion order.
• LinkedHashSet allows only one null value.

Program for LinkedHashSet;

package.com.instanceofjava;

import.java.util.LinkedHashSet;

public class testSet{

public static void main(String args[]){

LinkedHashSet linkedHashSet=new LinkedHashSet();

linkedHashSet.add("Indhu');

linkedHashSet.add("Indhu");

linkedHashSet.add("Sindhu");

linkedHashSet.add("Bindhu");

linkedHashSet.add(null);

linkedHashSet.add(null);

Iterator it=linkedHashSet.iterator();

while(it.hasNext()){

System.out.println(it.next);

}

}

}

Output:

Indhu

Sindhu

Bindhu

null

TreeSet:

• TreeSet Allows only Unique elements.
• Treeset having sorted order
• TreeSet doesn't allow  any null value.

Program for TreeSet:

package.com.instanceofjava;

import.java.util.TreeSet;

Public class testSet2{

public static void main(String args[]){

TreeSet treeSet=new treeSet();

treeSet.add("Sindhu");

treeSet.add("Sindhu");

treeSet.add("Indhu");

treeSet.add("Bindhu');

Iterator it=treeSet.iterator();

while(it.hasNext()){

System.out.println(it.next);

}

}

}

Output:

Bindhu

Indhu

Sindhu

• in Set interface if you want to add elements you can use add() method.
• If you want to remove you can use remove() method.