content: /*
   * Copyright (C) 2016-2023 ActionTech.
   * License: http://www.gnu.org/licenses/gpl.html GPL version 2 or higher.
   */
  
  package com.actiontech.dble.alarm;
  
  import org.slf4j.Logger;
  import org.slf4j.LoggerFactory;
  
  import java.util.*;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.locks.Condition;
  import java.util.concurrent.locks.ReentrantLock;
  import java.util.function.Consumer;
  
  /**
   * copy from jre LinkedBlockingQueue change the offer when the queue is full
   *
   * @param <E>
   */
  public class AlertBlockQueue<E> extends AbstractQueue<E>
          implements BlockingQueue<E>, java.io.Serializable {
      private static final long serialVersionUID = -6903933977591709194L;
      private static final Logger LOGGER = LoggerFactory.getLogger(AlertBlockQueue.class);
  
      /**
       * Linked list node class
       */
      static class Node<E> {
          E item;
  
          /**
           * One of:
           * - the real successor Node
           * - this Node, meaning the successor is head.next
           * - null, meaning there is no successor (this is the last node)
           */
          Node<E> next;
  
          Node(E x) {
              item = x;
          }
      }
  
      /**
       * The capacity bound, or Integer.MAX_VALUE if none
       */
      private final int capacity;
  
      /**
       * Current number of elements
       */
      private final AtomicInteger count = new AtomicInteger();
  
      /**
       * Head of linked list.
       * Invariant: head.item == null
       */
      transient Node<E> head;
  
      /**
       * Tail of linked list.
       * Invariant: last.next == null
       */
      private transient Node<E> last;
  
      /**
       * Lock held by take, poll, etc
       */
      private final ReentrantLock takeLock = new ReentrantLock();
  
      /**
       * Wait queue for waiting takes
       */
      private final Condition notEmpty = takeLock.newCondition();
  
      /**
       * Lock held by put, offer, etc
       */
      private final ReentrantLock putLock = new ReentrantLock();
  
      /**
       * Wait queue for waiting puts
       */
      private final Condition notFull = putLock.newCondition();
  
      /**
       * Signals a waiting take. Called only from put/offer (which do not
       * otherwise ordinarily lock takeLock.)
       */
      private void signalNotEmpty() {
          final ReentrantLock takeLock = this.takeLock;
          takeLock.lock();
          try {
              notEmpty.signal();
          } finally {
              takeLock.unlock();
          }
      }
  
      /**
       * Signals a waiting put. Called only from take/poll.
       */
      private void signalNotFull() {
          final ReentrantLock putLock = this.putLock;
          putLock.lock();
          try {
              notFull.signal();
          } finally {
              putLock.unlock();
          }
      }
  
      /**
       * Links node at end of queue.
       *
       * @param node the node
       */
      private void enqueue(Node<E> node) {
          // assert putLock.isHeldByCurrentThread();
          // assert last.next == null;
          last = last.next = node;
      }
  
      /**
       * Removes a node from head of queue.
       *
       * @return the node
       */
      private E dequeue() {
          // assert takeLock.isHeldByCurrentThread();
          // assert head.item == null;
          Node<E> h = head;
          Node<E> first = h.next;
          h.next = h; // help GC
          head = first;
          E x = first.item;
          first.item = null;
          return x;
      }
  
      /**
       * Locks to prevent both puts and takes.
       */
      void fullyLock() {
          putLock.lock();
          takeLock.lock();
      }
  
      /**
       * Unlocks to allow both puts and takes.
       */
      void fullyUnlock() {
          takeLock.unlock();
          putLock.unlock();
      }
  
  
      public AlertBlockQueue() {
          this(Integer.MAX_VALUE);
      }
  
  
      public AlertBlockQueue(int capacity) {
          if (capacity <= 0) throw new IllegalArgumentException();
          this.capacity = capacity;
          last = head = new Node<E>(null);
      }
  
      /**
       * Creates a {@code LinkedBlockingQueue} with a capacity of
       * {@link Integer#MAX_VALUE}, initially containing the elements of the
       * given collection,
       * added in traversal order of the collection's iterator.
       *
       * @param c the collection of elements to initially contain
       * @throws NullPointerException if the specified collection or any
       *                              of its elements are null
       */
      public AlertBlockQueue(Collection<? extends E> c) {
          this(Integer.MAX_VALUE);
          final ReentrantLock putLock = this.putLock;
          putLock.lock(); // Never contended, but necessary for visibility
          try {
              int n = 0;
              for (E e : c) {
                  if (e == null)
                      throw new NullPointerException();
                  if (n == capacity)
                      throw new IllegalStateException("Queue full");
                  enqueue(new Node<E>(e));
                  ++n;
              }
              count.set(n);
          } finally {
              putLock.unlock();
          }
      }
  
      // this doc comment is overridden to remove the reference to collections
      // greater in size than Integer.MAX_VALUE
  
      /**
       * Returns the number of elements in this queue.
       *
       * @return the number of elements in this queue
       */
      public int size() {
          return count.get();
      }
  
      // this doc comment is a modified copy of the inherited doc comment,
      // without the reference to unlimited queues.
  
      /**
       * Returns the number of additional elements that this queue can ideally
       * (in the absence of memory or resource constraints) accept without
       * blocking. This is always equal to the initial capacity of this queue
       * less the current {@code size} of this queue.
       * <p>
       * <p>Note that you <em>cannot</em> always tell if an attempt to insert
       * an element will succeed by inspecting {@code remainingCapacity}
       * because it may be the case that another thread is about to
       * insert or remove an element.
       */
      public int remainingCapacity() {
          return capacity - count.get();
      }
  
      /**
       * Inserts the specified element at the tail of this queue, waiting if
       * necessary for space to become available.
       *
       * @throws InterruptedException {@inheritDoc}
       * @throws NullPointerException {@inheritDoc}
       */
      public void put(E e) throws InterruptedException {
          if (e == null) throw new NullPointerException();
          // Note: convention in all put/take/etc is to preset local var
          // holding count negative to indicate failure unless set.
          int c = -1;
          Node<E> node = new Node<E>(e);
          final ReentrantLock putLock = this.putLock;
          final AtomicInteger count = this.count;
          putLock.lockInterruptibly();
          try {
              /*
               * Note that count is used in wait guard even though it is
               * not protected by lock. This works because count can
               * only decrease at this point (all other puts are shut
               * out by lock), and we (or some other waiting put) are
               * signalled if it ever changes from capacity. Similarly
               * for all other uses of count in other wait guards.
               */
              while (count.get() == capacity) {
                  notFull.await();
              }
              enqueue(node);
              c = count.getAndIncrement();
              if (c + 1 < capacity)
                  notFull.signal();
          } finally {
              putLock.unlock();
          }
          if (c == 0)
              signalNotEmpty();
      }
  
      /**
       * Inserts the specified element at the tail of this queue, waiting if
       * necessary up to the specified wait time for space to become available.
       *
       * @return {@code true} if successful, or {@code false} if
       * the specified waiting time elapses before space is available
       * @throws InterruptedException {@inheritDoc}
       * @throws NullPointerException {@inheritDoc}
       */
      public boolean offer(E e, long timeout, TimeUnit unit)
              throws InterruptedException {
  
          if (e == null) throw new NullPointerException();
          long nanos = unit.toNanos(timeout);
          int c = -1;
          final ReentrantLock putLock = this.putLock;
          final AtomicInteger count = this.count;
          putLock.lockInterruptibly();
          try {
              while (count.get() == capacity) {
                  if (nanos <= 0) {
                      break;
                  }
                  nanos = notFull.awaitNanos(nanos);
              }
              enqueue(new Node<E>(e));
              c = count.getAndIncrement();
              if (c + 1 < capacity)
                  notFull.signal();
          } finally {
              putLock.unlock();
          }
          if (c == 0)
              signalNotEmpty();
          return true;
      }
  
      /**
       * Inserts the specified element at the tail of this queue if it is
       * possible to do so immediately without exceeding the queue's capacity,
       * returning {@code true} upon success and {@code false} if this queue
       * is full.
       * When using a capacity-restricted queue, this method is generally
       * preferable to method {@link BlockingQueue#add add}, which can fail to
       * insert an element only by throwing an exception.
       *
       * @throws NullPointerException if the specified element is null
       */
      public boolean offer(E e) {
          if (e == null) throw new NullPointerException();
          final AtomicInteger count = this.count;
          int c = -1;
          Node<E> node = new Node<E>(e);
          final ReentrantLock putLock = this.putLock;
          putLock.lock();
          try {
              if (count.get() >= capacity) {
                  takeLock.lock();
                  try {
                      if (count.get() > 0) {
                          E dropOne = dequeue();
                          LOGGER.warn("Drop AlertTask:" + dropOne.toString());
                          c = count.getAndDecrement();
                      }
                  } finally {
                      takeLock.unlock();
                  }
              }
              enqueue(node);
              c = count.getAndIncrement();
              if (c + 1 < capacity)
                  notFull.signal();
          } finally {
              putLock.unlock();
          }
          if (c == 0)
              signalNotEmpty();
          return c >= 0;
      }
  
      public E take() throws InterruptedException {
          E x;
          int c = -1;
          final AtomicInteger count = this.count;
          final ReentrantLock takeLock = this.takeLock;
          takeLock.lockInterruptibly();
          try {
              while (count.get() == 0) {
                  notEmpty.await();
              }
              x = dequeue();
              c = count.getAndDecrement();
              if (c > 1)
                  notEmpty.signal();
          } finally {
              takeLock.unlock();
          }
          if (c == capacity)
              signalNotFull();
          return x;
      }
  
      public E poll(long timeout, TimeUnit unit) throws InterruptedException {
          E x = null;
          int c = -1;
          long nanos = unit.toNanos(timeout);
          final AtomicInteger count = this.count;
          final ReentrantLock takeLock = this.takeLock;
          takeLock.lockInterruptibly();
          try {
              while (count.get() == 0) {
                  if (nanos <= 0)
                      return null;
                  nanos = notEmpty.awaitNanos(nanos);
              }
              x = dequeue();
              c = count.getAndDecrement();
              if (c > 1)
                  notEmpty.signal();
          } finally {
              takeLock.unlock();
          }
          if (c == capacity)
              signalNotFull();
          return x;
      }
  
      public E poll() {
          final AtomicInteger count = this.count;
          if (count.get() == 0)
              return null;
          E x = null;
          int c = -1;
          final ReentrantLock takeLock = this.takeLock;
          takeLock.lock();
          try {
              if (count.get() > 0) {
                  x = dequeue();
                  c = count.getAndDecrement();
                  if (c > 1)
                      notEmpty.signal();
              }
          } finally {
              takeLock.unlock();
          }
          if (c == capacity)
              signalNotFull();
          return x;
      }
  
      public E peek() {
          if (count.get() == 0)
              return null;
          final ReentrantLock takeLock = this.takeLock;
          takeLock.lock();
          try {
              Node<E> first = head.next;
              if (first == null)
                  return null;
              else
                  return first.item;
          } finally {
              takeLock.unlock();
          }
      }
  
      /**
       * Unlinks interior Node p with predecessor trail.
       */
      void unlink(Node<E> p, Node<E> trail) {
          // assert isFullyLocked();
          // p.next is not changed, to allow iterators that are
          // traversing p to maintain their weak-consistency guarantee.
          p.item = null;
          trail.next = p.next;
          if (last == p)
              last = trail;
          if (count.getAndDecrement() == capacity)
              notFull.signal();
      }
  
      /**
       * Removes a single instance of the specified element from this queue,
       * if it is present.  More formally, removes an element {@code e} such
       * that {@code o.equals(e)}, if this queue contains one or more such
       * elements.
       * Returns {@code true} if this queue contained the specified element
       * (or equivalently, if this queue changed as a result of the call).
       *
       * @param o element to be removed from this queue, if present
       * @return {@code true} if this queue changed as a result of the call
       */
      public boolean remove(Object o) {
          if (o == null) return false;
          fullyLock();
          try {
              for (Node<E> trail = head, p = trail.next;
                   p != null;
                   trail = p, p = p.next) {
                  if (o.equals(p.item)) {
                      unlink(p, trail);
                      return true;
                  }
              }
              return false;
          } finally {
              fullyUnlock();
          }
      }
  
      /**
       * Returns {@code true} if this queue contains the specified element.
       * More formally, returns {@code true} if and only if this queue contains
       * at least one element {@code e} such that {@code o.equals(e)}.
       *
       * @param o object to be checked for containment in this queue
       * @return {@code true} if this queue contains the specified element
       */
      public boolean contains(Object o) {
          if (o == null) return false;
          fullyLock();
          try {
              for (Node<E> p = head.next; p != null; p = p.next)
                  if (o.equals(p.item))
                      return true;
              return false;
          } finally {
              fullyUnlock();
          }
      }
  
      /**
       * Returns an array containing all of the elements in this queue, in
       * proper sequence.
       * <p>
       * <p>The returned array will be "safe" in that no references to it are
       * maintained by this queue.  (In other words, this method must allocate
       * a new array).  The caller is thus free to modify the returned array.
       * <p>
       * <p>This method acts as bridge between array-based and collection-based
       * APIs.
       *
       * @return an array containing all of the elements in this queue
       */
      public Object[] toArray() {
          fullyLock();
          try {
              int size = count.get();
              Object[] a = new Object[size];
              int k = 0;
              for (Node<E> p = head.next; p != null; p = p.next)
                  a[k++] = p.item;
              return a;
          } finally {
              fullyUnlock();
          }
      }
  
      /**
       * Returns an array containing all of the elements in this queue, in
       * proper sequence; the runtime type of the returned array is that of
       * the specified array.  If the queue fits in the specified array, it
       * is returned therein.  Otherwise, a new array is allocated with the
       * runtime type of the specified array and the size of this queue.
       * <p>
       * <p>If this queue fits in the specified array with room to spare
       * (i.e., the array has more elements than this queue), the element in
       * the array immediately following the end of the queue is set to
       * {@code null}.
       * <p>
       * <p>Like the {@link #toArray()} method, this method acts as bridge between
       * array-based and collection-based APIs.  Further, this method allows
       * precise control over the runtime type of the output array, and may,
       * under certain circumstances, be used to save allocation costs.
       * <p>
       * <p>Suppose {@code x} is a queue known to contain only strings.
       * The following code can be used to dump the queue into a newly
       * allocated array of {@code String}:
       * <p>
       * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
       * <p>
       * Note that {@code toArray(new Object[0])} is identical in function to
       * {@code toArray()}.
       *
       * @param a the array into which the elements of the queue are to
       *          be stored, if it is big enough; otherwise, a new array of the
       *          same runtime type is allocated for this purpose
       * @return an array containing all of the elements in this queue
       * @throws ArrayStoreException  if the runtime type of the specified array
       *                              is not a supertype of the runtime type of every element in
       *                              this queue
       * @throws NullPointerException if the specified array is null
       */
      @SuppressWarnings("unchecked")
      public <T> T[] toArray(T[] a) {
          fullyLock();
          try {
              int size = count.get();
              if (a.length < size)
                  a = (T[]) java.lang.reflect.Array.newInstance
                          (a.getClass().getComponentType(), size);
  
              int k = 0;
              for (Node<E> p = head.next; p != null; p = p.next)
                  a[k++] = (T) p.item;
              if (a.length > k)
                  a[k] = null;
              return a;
          } finally {
              fullyUnlock();
          }
      }
  
      public String toString() {
          fullyLock();
          try {
              Node<E> p = head.next;
              if (p == null)
                  return "[]";
  
              StringBuilder sb = new StringBuilder();
              sb.append('[');
              for (; ; ) {
                  E e = p.item;
                  sb.append(e == this ? "(this Collection)" : e);
                  p = p.next;
                  if (p == null)
                      return sb.append(']').toString();
                  sb.append(',').append(' ');
              }
          } finally {
              fullyUnlock();
          }
      }
  
      /**
       * Atomically removes all of the elements from this queue.
       * The queue will be empty after this call returns.
       */
      public void clear() {
          fullyLock();
          try {
              for (Node<E> p, h = head; (p = h.next) != null; h = p) {
                  h.next = h;
                  p.item = null;
              }
              head = last;
              // assert head.item == null && head.next == null;
              if (count.getAndSet(0) == capacity)
                  notFull.signal();
          } finally {
              fullyUnlock();
          }
      }
  
      /**
       * @throws UnsupportedOperationException {@inheritDoc}
       * @throws ClassCastException            {@inheritDoc}
       * @throws NullPointerException          {@inheritDoc}
       * @throws IllegalArgumentException      {@inheritDoc}
       */
      public int drainTo(Collection<? super E> c) {
          return drainTo(c, Integer.MAX_VALUE);
      }
  
      /**
       * @throws UnsupportedOperationException {@inheritDoc}
       * @throws ClassCastException            {@inheritDoc}
       * @throws NullPointerException          {@inheritDoc}
       * @throws IllegalArgumentException      {@inheritDoc}
       */
      public int drainTo(Collection<? super E> c, int maxElements) {
          if (c == null)
              throw new NullPointerException();
          if (c == this)
              throw new IllegalArgumentException();
          if (maxElements <= 0)
              return 0;
          boolean signalNotFull = false;
          final ReentrantLock takeLock = this.takeLock;
          takeLock.lock();
          try {
              int n = Math.min(maxElements, count.get());
              // count.get provides visibility to first n Nodes
              Node<E> h = head;
              int i = 0;
              try {
                  while (i < n) {
                      Node<E> p = h.next;
                      c.add(p.item);
                      p.item = null;
                      h.next = h;
                      h = p;
                      ++i;
                  }
                  return n;
              } finally {
                  // Restore invariants even if c.add() threw
                  if (i > 0) {
                      // assert h.item == null;
                      head = h;
                      signalNotFull = (count.getAndAdd(-i) == capacity);
                  }
              }
          } finally {
              takeLock.unlock();
              if (signalNotFull)
                  signalNotFull();
          }
      }
  
      /**
       * Returns an iterator over the elements in this queue in proper sequence.
       * The elements will be returned in order from first (head) to last (tail).
       * <p>
       * <p>The returned iterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       *
       * @return an iterator over the elements in this queue in proper sequence
       */
      public Iterator<E> iterator() {
          return new Itr();
      }
  
      private class Itr implements Iterator<E> {
          /*
           * Basic weakly-consistent iterator.  At all times hold the next
           * item to hand out so that if hasNext() reports true, we will
           * still have it to return even if lost race with a take etc.
           */
  
          private Node<E> current;
          private Node<E> lastRet;
          private E currentElement;
  
          Itr() {
              fullyLock();
              try {
                  current = head.next;
                  if (current != null)
                      currentElement = current.item;
              } finally {
                  fullyUnlock();
              }
          }
  
          public boolean hasNext() {
              return current != null;
          }
  
          /**
           * Returns the next live successor of p, or null if no such.
           * <p>
           * Unlike other traversal methods, iterators need to handle both:
           * - dequeued nodes (p.next == p)
           * - (possibly multiple) interior removed nodes (p.item == null)
           */
          private Node<E> nextNode(Node<E> p) {
              for (; ; ) {
                  Node<E> s = p.next;
                  if (s == p)
                      return head.next;
                  if (s == null || s.item != null)
                      return s;
                  p = s;
              }
          }
  
          public E next() {
              fullyLock();
              try {
                  if (current == null)
                      throw new NoSuchElementException();
                  E x = currentElement;
                  lastRet = current;
                  current = nextNode(current);
                  currentElement = (current == null) ? null : current.item;
                  return x;
              } finally {
                  fullyUnlock();
              }
          }
  
          public void remove() {
              if (lastRet == null)
                  throw new IllegalStateException();
              fullyLock();
              try {
                  Node<E> node = lastRet;
                  lastRet = null;
                  for (Node<E> trail = head, p = trail.next;
                       p != null;
                       trail = p, p = p.next) {
                      if (p == node) {
                          unlink(p, trail);
                          break;
                      }
                  }
              } finally {
                  fullyUnlock();
              }
          }
      }
  
      /**
       * A customized variant of Spliterators.IteratorSpliterator
       */
      static final class LBQSpliterator<E> implements Spliterator<E> {
          static final int MAX_BATCH = 1 << 25;  // max batch array size;
          final AlertBlockQueue<E> queue;
          Node<E> current;    // current node; null until initialized
          int batch;          // batch size for splits
          boolean exhausted;  // true when no more nodes
          long est;           // size estimate
  
          LBQSpliterator(AlertBlockQueue<E> queue) {
              this.queue = queue;
              this.est = queue.size();
          }
  
          public long estimateSize() {
              return est;
          }
  
          public Spliterator<E> trySplit() {
              Node<E> h;
              final AlertBlockQueue<E> q = this.queue;
              int b = batch;
              int n = (b <= 0) ? 1 : (b >= MAX_BATCH) ? MAX_BATCH : b + 1;
              if (!exhausted &&
                      ((h = current) != null || (h = q.head.next) != null) &&
                      h.next != null) {
                  Object[] a = new Object[n];
                  int i = 0;
                  Node<E> p = current;
                  q.fullyLock();
                  try {
                      if (p != null || (p = q.head.next) != null) {
                          do {
                              if ((a[i] = p.item) != null)
                                  ++i;
                          } while ((p = p.next) != null && i < n);
                      }
                  } finally {
                      q.fullyUnlock();
                  }
                  if ((current = p) == null) {
                      est = 0L;
                      exhausted = true;
                  } else if ((est -= i) < 0L)
                      est = 0L;
                  if (i > 0) {
                      batch = i;
                      return Spliterators.spliterator
                              (a, 0, i, Spliterator.ORDERED | Spliterator.NONNULL |
                                      Spliterator.CONCURRENT);
                  }
              }
              return null;
          }
  
          public void forEachRemaining(Consumer<? super E> action) {
              if (action == null) throw new NullPointerException();
              final AlertBlockQueue<E> q = this.queue;
              if (!exhausted) {
                  exhausted = true;
                  Node<E> p = current;
                  do {
                      E e = null;
                      q.fullyLock();
                      try {
                          if (p == null)
                              p = q.head.next;
                          while (p != null) {
                              e = p.item;
                              p = p.next;
                              if (e != null)
                                  break;
                          }
                      } finally {
                          q.fullyUnlock();
                      }
                      if (e != null)
                          action.accept(e);
                  } while (p != null);
              }
          }
  
          public boolean tryAdvance(Consumer<? super E> action) {
              if (action == null) throw new NullPointerException();
              final AlertBlockQueue<E> q = this.queue;
              if (!exhausted) {
                  E e = null;
                  q.fullyLock();
                  try {
                      if (current == null)
                          current = q.head.next;
                      while (current != null) {
                          e = current.item;
                          current = current.next;
                          if (e != null)
                              break;
                      }
                  } finally {
                      q.fullyUnlock();
                  }
                  if (current == null)
                      exhausted = true;
                  if (e != null) {
                      action.accept(e);
                      return true;
                  }
              }
              return false;
          }
  
          public int characteristics() {
              return Spliterator.ORDERED | Spliterator.NONNULL |
                      Spliterator.CONCURRENT;
          }
      }
  
      /**
       * Returns a {@link Spliterator} over the elements in this queue.
       * <p>
       * <p>The returned spliterator is
       * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
       * <p>
       * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
       * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
       *
       * @return a {@code Spliterator} over the elements in this queue
       * @implNote The {@code Spliterator} implements {@code trySplit} to permit limited
       * parallelism.
       * @since 1.8
       */
      public Spliterator<E> spliterator() {
          return new LBQSpliterator<E>(this);
      }
  
      /**
       * Saves this queue to a stream (that is, serializes it).
       *
       * @param s the stream
       * @throws java.io.IOException if an I/O error occurs
       * @serialData The capacity is emitted (int), followed by all of
       * its elements (each an {@code Object}) in the proper order,
       * followed by a null
       */
      private void writeObject(java.io.ObjectOutputStream s)
              throws java.io.IOException {
  
          fullyLock();
          try {
              // Write out any hidden stuff, plus capacity
              s.defaultWriteObject();
  
              // Write out all elements in the proper order.
              for (Node<E> p = head.next; p != null; p = p.next)
                  s.writeObject(p.item);
  
              // Use trailing null as sentinel
              s.writeObject(null);
          } finally {
              fullyUnlock();
          }
      }
  
      /**
       * Reconstitutes this queue from a stream (that is, deserializes it).
       *
       * @param s the stream
       * @throws ClassNotFoundException if the class of a serialized object
       *                                could not be found
       * @throws java.io.IOException    if an I/O error occurs
       */
      private void readObject(java.io.ObjectInputStream s)
              throws java.io.IOException, ClassNotFoundException {
          // Read in capacity, and any hidden stuff
          s.defaultReadObject();
  
          count.set(0);
          last = head = new Node<E>(null);
  
          // Read in all elements and place in queue
          for (; ; ) {
              @SuppressWarnings("unchecked")
              E item = (E) s.readObject();
              if (item == null)
                  break;
              add(item);
          }
      }
  }