Custom Generic Blocking Queue Implementation

One of best ways to keep polishing your java skills is to pick a java collection and try implementing it yourself. Its also a good interview questions as it involves data structures, algorithms multi-threading, generics and core java, all rolled into one implementation.

A blocking queue as the name suggests is a fixed size queue that waits when its full or empty. Its frequently used to solve the producer consumer problem. Java has a implementation of it in its java.util.concurrent package. Below I have implemented my own custom blocking queue with arrays as the data structure to represent the queue. Its different from some other implementations as I did not use any existing java collection also its generic (can store any kind of object).

Here is the implementation class below. Also its available as a public gist here: https://gist.github.com/keith0591/c20704c32270e5ec546e412db6d61034

import java.util.Arrays;

/**
 * @author MalkeithSingh on 01-12-2019
 */
public class BlockingQ<T> {

    private Object[] elements;

    private int count = 0;
    private final int size;

    public BlockingQ(int size) {
        this.size = size;
        this.elements = new Object[this.size];
    }

    public synchronized boolean add(T element) throws InterruptedException {
        while (count == size) {
            System.out.println("Q full waiting.....");
            wait();
        }
        elements[count++] = element;
        Thread.sleep(1000);
        System.out.println("inserted " + element + " into Q by " + Thread.currentThread().getName());
        notify();
        return true;
    }

    public synchronized T poll() throws InterruptedException {
        while (count == 0) {
            System.out.println("Q empty waiting.....");
            wait();
        }
        T temp = elementData(0);
        reorderArray();
        count--;
        Thread.sleep(1000);
        System.out.println("popped " + temp + " from the Q by " + Thread.currentThread().getName());
        notify();
        return temp;
    }

    @SuppressWarnings("unchecked")
    private  T elementData(int index) {
        return (T) elements[index];
    }

    private  void reorderArray() {
        elements = Arrays.copyOfRange(elements, 1, size+1);
    }

    public void printQ() {
        System.out.println(Arrays.toString(elements));
    }
}