Introduction
In this article we discuss how various Java Collection classes work.
HashSet class
HashSet extends AbstractSet and implements the Set interface. It creates a collection that uses a hash table (a hash table stores information by using a mechanism called hashing) for storage. It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.
This class supports four constructors that are the following:
- HashSet( )
- HashSet(Collection cln)
- HashSet(int capacity, float fillRatio)
- HashSet(int capacity)
Example
import java.util.*;
class HashSetEx
{
public static void main(String args[])
{
HashSet arylst=new HashSet();
arylst.add("Paul");
arylst.add("John");
arylst.add("Paul");
arylst.add("San");
Iterator itrtr=arylst.iterator();
while(itrtr.hasNext())
{
System.out.println(itrtr.next());
}
}
}
Output
![Fig-1.jpg]()
LinkedHashSet class
- It contains only unique elements like HashSet.
- It extends HashSet class and implements Set interface.
- Maintains insertion order.
Example
import java.util.*;
class LinkedHashSetEx
{
public static void main(String args[])
{
LinkedHashSet arylst=new LinkedHashSet();
arylst.add("Paul");
arylst.add("John");
arylst.add("Paul");
arylst.add("San");
Iterator itrtr=arylst.iterator();
while(itrtr.hasNext())
{
System.out.println(itrtr.next());
}
}
}
Output
![Fig-2.jpg]()
TreeSet class
- This class implements NavigableSet interface that extends the SortedSet interface.
- It contains only unique elements like HashSet.
- Maintains ascending order.
Example
import java.util.*;
class TreeSetEx
{
public static void main(String args[])
{
TreeSet arylst=new TreeSet();
arylst.add("Paul");
arylst.add("John");
arylst.add("Paul");
arylst.add("San");
Iterator itrtr=arylst.iterator();
while(itrtr.hasNext())
{
System.out.println(itrtr.next());
}
}
}
Output
![fig-3.jpg]()
Queue Interface
It is basically used to order the elements in the FIFO (first in first out) manner.
Public Methods
Some public methods used in Queue Interface are:
- remove()
- poll()
- elements()
- peek()
- boolean add (object)
- boolean offer (object)
Priority Queue classes
The class provides the ability to use a queue, but it does not order the elements in a FIFO manner.
Example
import java.util.*;
class PriorityQueueEx
{
public static void main(String args[])
{
PriorityQueue que=new PriorityQueue();
que.add("Paul");
que.add("John");
que.add("Dinesh");
que.add("San");
que.add("Rahul");
System.out.println("head:" +que.element());
System.out.println("head:" +que.peek());
System.out.println("iterating the que elements:");
Iterator itrtr=que.iterator();
while(itrtr.hasNext())
{
System.out.println(itrtr.next());
}
que.remove();
que.poll();
System.out.println("after removing two elements of queue:");
Iterator itrtr2=que.iterator();
while(itrtr2.hasNext())
{
System.out.println(itrtr2.next());
}
}
}
Output
![fig-4.jpg]()
HashMap class
- may have one null key and multiple null values.
- contains values based on the key.
- implements the Map interface and extends the AbstractMap class.
- contains only unique elements.
- maintains no order.
Example
import java.util.*;
class HashMapEx
{
public static void main(String args[])
{
HashMap hsmp=new HashMap();
hsmp.put(10, "Paul");
hsmp.put(11, "John");
hsmp.put(12, "San");
Set s=hsmp.entrySet();
Iterator itrtr=s.iterator();
while(itrtr.hasNext())
{
Map.Entry me=(Map.Entry)itrtr.next();
System.out.println(me.getKey()+" " +me.getValue());
}
}
}
Output
![fig-5.jpg]()