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How to implement size-limited Queue that holds last N elements in Java?

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  • Difficulty Level : Expert
  • Last Updated : 23 Nov, 2022
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What is Size Limited Queue?

Size limited queue is the same as a normal queue but it has the special property that at any time queue can store at max N element, if we try to push more than N elements in the queue then accordingly the elements added at the starting will be removed from the queue means size will remain to fix i.e. N. 

If you want to know more about queues refer to this article.

How to implement size limited queue in Java?

We can extend the LinkedList class that is available in Java and apply the necessary modifications to that class to create a size-limited queue in Java. I’ve modified the technique that adds elements to the queue in the code shown below.

Approach: Implement the idea below to solve the problem:

  • Initialize the fixed size of the Queue and keep on adding elements in Queue unless its size becomes exactly equal to the fixed size. 
  • Override the add() method of the LinkedList class to incorporate the size restriction.
    • If the Queue size gets equal to the fixed size remove the first element of the queue and then add a new one. 

Below is the implementation of the above approach.

Java




// Java code for custom Limited-Size Queue
// implementation
import java.util.LinkedList;
import java.util.Queue;
  
class GFG {
    // Class SizeLimitedQueue which extends
    // LinkedList
  
    // SizeLimitedQueue class is
    // implementation of Limited-Size Queue
    public static class SizeLimitedQueue<E>
        extends LinkedList<E> {
  
        // Variable which store the
        // SizeLimitOfQueue of the queue
        private int SizeLimitOfQueue;
  
        // Constructor method for initializing
        // the SizeLimitOfQueue variable
        public SizeLimitedQueue(int SizeLimitOfQueue)
        {
  
            this.SizeLimitOfQueue = SizeLimitOfQueue;
        }
  
        // Override the method add() available
        // in LinkedList class so that it allow
        // addition  of element in queue till
        // queue size is less than
        // SizeLimitOfQueue otherwise it remove
        // the front element of queue and add
        // new element
        @Override
        public boolean add(E o)
        {
  
            // If queue size become greater
            // than SizeLimitOfQueue then
            // front of queue will be removed
            while (this.size() == SizeLimitOfQueue) {
  
                super.remove();
            }
            super.add(o);
            return true;
        }
    }
  
    // Driver Code
    public static void main(String[] args)
    {
        // Create queue with size
        // SizeLimitOfQueue as 3
        Queue<Integer> q = new SizeLimitedQueue<>(3);
  
        // Adds elements {0, 1, 2, 3, 4} to
        // the queue
        for (int i = 0; i < 5; i++)
  
            q.add(i);
        // So now queue is [2, 3, 4] as [0, 1]
        // removed due to size
        // SizeLimitOfQueue of 3
  
        // To view Size of the queue
        int size = q.size();
        System.out.println("Size of queue-" + size);
  
        // Display contents of the queue.
        System.out.println("Elements of queue " + q);
  
        // To remove the head of queue.
        int removedele = q.remove();
        System.out.println("Removed element-" + removedele);
  
        // Add elements {5, 6} to
        // the queue
        for (int i = 5; i < 7; i++)
            q.add(i);
  
        // Display contents of the queue.
        System.out.println("Elements of queue " + q);
  
        // To view the head of queue
        int head = q.peek();
        System.out.println("Head of queue-" + head);
  
        // To view Size of the queue
        size = q.size();
  
        System.out.println("Size of queue-" + size);
    }
}


Output

Size of queue-3
Elements of queue [2, 3, 4]
Removed element-2
Elements of queue [4, 5, 6]
Head of queue-4
Size of queue-3

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