GeneralQueue.java

package com.dataStructures;

import com.types.Queue;

import java.util.Iterator;
import java.util.LinkedList;
import java.util.NoSuchElementException;

/**
 * linkedList based implementation of queue.
 * This implementation is not thread safe and need exclusive thread safety measures from the client.
 *
 * @param <T>
 */
public class GeneralQueue<T> implements Queue<T> {

    private LinkedList<T> queue;
    private Iterator<T> itr;

    /**
     * Overloaded constructor to create queue of specific size
     */
    public GeneralQueue() {
        queue = new LinkedList<>();
    }

    @Override
    public boolean add(T t) {

        if (queue == null) {
            throw new IllegalStateException();
        }
        if (t == null) {
            throw new NullPointerException();
        }
        queue.add(t);
        return true;
    }

    @Override
    public boolean remove(T t) {
        if (null == queue) {
            throw new NullPointerException();
        }
        if (queue.isEmpty()) {
            throw new NoSuchElementException();
        }
        queue.remove(t);
        return true;
    }

    @Override
    public boolean isEmpty() {
        return null == queue || queue.size() == 0;
    }

    @Override
    public Iterator<T> iterator() {
        if (queue == null) {
            return null;
        }
        itr = queue.iterator();
        return itr;
    }

    @Override
    public boolean offer(T t) {
        if (null == queue) {
            return false;
        }
        if (t == null) {
            throw new NullPointerException();
        }
        queue.add(t);
        return true;
    }

    @Override
    public T poll() {
        if (queue == null || queue.isEmpty()) {
            return null;
        }

        return queue.pollFirst();
    }

    @Override
    public T element() {
        if (queue == null || queue.isEmpty()) {
            throw new NoSuchElementException();
        }

        return queue.peekFirst();
    }

    @Override
    public T peek() {
        if (null == queue || queue.size() == 0) {
            return null;
        }

        return queue.peekFirst();
    }

    @Override
    public boolean hasNext() {
        return itr.hasNext();
    }

    @Override
    public T next() {
        return itr.next();
    }

    @Override
    public Object[] toArray() {
        Object[] elements = {};
        if (null == queue || queue.isEmpty()) {
            return elements;
        }
        elements = new Object[queue.size()];
        for (int i = 0; i < queue.size(); i++) {
            elements[i] = queue.get(i);
        }

        return elements;
    }

    @Override
    public int size() {
        if (null == queue || queue.isEmpty()) {
            return 0;
        }
        return queue.size();
    }
}