Queue.java

// A class to represent a queue 
// The queue, front stores the front node of LL and rear stores the 
// last node of LL 
public class Queue { 
  QNode front, rear; 

  public Queue() 
  { 
      this.front = this.rear = null; 
  } 

  // Method to add an key to the queue. 
  void enqueue(int key) 
  { 

      // Create a new LL node 
      QNode temp = new QNode(key); 

      // If queue is empty, then new node is front and rear both 
      if (this.rear == null) { 
          this.front = this.rear = temp; 
          return; 
      } 

      // Add the new node at the end of queue and change rear 
      this.rear.next = temp; 
      this.rear = temp; 
  } 

  // 큐에서 원소를 삭제하는 연산
  QNode dequeue() 
  { 
      // If queue is empty, return NULL. 
      if (this.front == null) 
          return null; 

      // Store previous front and move front one node ahead 
      QNode temp = this.front; 
      this.front = this.front.next; 

      // If front becomes NULL, then change rear also as NULL 
      if (this.front == null) 
          this.rear = null; 
      return temp; 
  } 

  static class QNode { 
    int key; 
    QNode next; 
  
    // constructor to create a new linked list node 
    public QNode(int key) 
    { 
        this.key = key; 
        this.next = null; 
    } 
  } 

  public static void main(String[] args) 
  { 
      Queue q = new Queue(); 
      q.enqueue(10); 
      q.enqueue(20); 
      q.dequeue(); 
      q.dequeue(); 
      q.enqueue(30); 
      q.enqueue(40); 
      q.enqueue(50); 

      System.out.println("Dequeued item is " + q.dequeue().key); 
  } 
}