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:

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

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