LRU Cache Implementation

import java.util.Deque;
import java.util.HashSet; 
import java.util.LinkedList; 
import java.util.Iterator;
  
public class LRUCache {

    private Deque<Integer> doublyQueue;
  
    
    private HashSet<Integer> hashSet;
  
   
    private final int CACHE_SIZE;
  
    LRUCache(int capacity) {
        doublyQueue = new LinkedList<>();
        hashSet = new HashSet<>();
        CACHE_SIZE = capacity;
    }
 
    public void refer(int page) {
        if (!hashSet.contains(page)) {
            if (doublyQueue.size() == CACHE_SIZE) {
                int last = doublyQueue.removeLast();
                hashSet.remove(last);
            } 
        } 
        else {
            doublyQueue.remove(page);
        } 
        doublyQueue.push(page);
        hashSet.add(page);
    }
  
    
    public void display() {
        Iterator<Integer> itr = doublyQueue.iterator();
        while (itr.hasNext()) { 
            System.out.print(itr.next() + " "); 
        } 
    }
  
    public static void main(String[] args) {
        LRUCache cache = new LRUCache(4);
        cache.refer(1);
        cache.refer(2);
        cache.refer(3);
        cache.refer(1);
        cache.refer(4);
        cache.refer(5);
        cache.display();
    }
}