Implementing Queue in Java

Queue is a First-In-First-Out (FIFO) data structure. In a FIFO data structure, the first element added to the queue will be the first to be removed. Suppose if a queue contains 4 elements and 5th element added at back and if we need to remove that element then we need to remove all front 4 elements first. As like Stack implementation in java we can implement Queue Data Structure and also we have operations in Queue like enqueue(), dequeue(), front(), isempty() etc.,

enqueue() - Insert element in front of the Queue.

dequeue() - Return front element and remove same element from the Queue.

front() - Same as dequeue() returns front element but same element won't be removed from the Queue.

clean() - cleans the Queue.



Lets see simple example to implement Queue using ArrayList and using Generics for holding any datatype.

import java.util.ArrayList;
import java.util.List;

public class MyQueue <E>{

 private List<E> list = null;
 
 public MyQueue() {
  list = new ArrayList<E>();
 }

 public void enqueue(E val){
  list.add(val);
 }
 
 public E dequeue(){
  E val = null;
  if(list.size() > 0){
   val = list.get(0);
   list.remove(0);
  }  
  return val;
 }
 
 public boolean isEmpty(){
  if(list.size() == 0)return true;
  else return false;
 }
 
 public int size(){
  return list.size();
 }
 
 public E front(){
  E val = null;
  if(list.size() > 0){
   val = list.get(0);
  }  
  return val;
 }
 
 public void clean(){
  list = new ArrayList<E>();
 }
 
 @Override
 public String toString() {
  return list.toString();
 }
}

Our Queue class is ready and we can test with below code.

public class TestMyQueue {

 public static void main(String[] args) {
  
  MyQueue<String> queue = new MyQueue<String>();
  
  int qSize = queue.size();
  System.out.println("QUEUE SIZE : "+qSize);
  
  //ENQUEUE
  queue.enqueue("one");
  queue.enqueue("two");
  queue.enqueue("three");
  queue.enqueue("four");
  queue.enqueue("five");
  System.out.println("5 ELEMENTS ADDED IN QUEUE");
  qSize = queue.size();
  System.out.println("QUEUE SIZE : "+qSize);
  
  //QUEUE
  System.out.println("QUEUE      : "+queue);
  
  //FRONT
  System.out.println("FRONT       : "+queue.front());
  
  //DEQUEUE
  System.out.println("DEQUEUE        : "+queue.dequeue());
    
  qSize = queue.size();
  System.out.println("QUEUE SIZE : "+qSize);
  
  //FRONT
  System.out.println("FRONT       : "+queue.front());
  
  //ISEMPTY
  System.out.println("EMPTY      : "+queue.isEmpty());
  
  //CLEAN
  queue.clean();
  System.out.println("QUEUE CLEANED");
  
  //ISEMPTY
  System.out.println("EMPTY      : "+queue.isEmpty());
  
 }
}

OUTPUT:

QUEUE SIZE     : 0
5 ELEMENTS ADDED IN QUEUE
QUEUE SIZE     : 5
QUEUE             : [one, two, three, four, five]
FRONT             : one
DEQUEUE         : one
QUEUE SIZE      : 4
FRONT             : two
EMPTY             : false
QUEUE CLEANED
EMPTY             : true