How Various Java Collection Classes Work

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

 

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 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

 

TreeSet class

• This class implements a 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

 

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:

1. remove()
2. poll()
3. elements()
4. peek()
5. boolean add (object)
6. 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

 

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