public Entry<E> firstEntry() {
    Iterator<Entry<E>> entryIterator = entryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  @CheckForNull
  public Entry<E> lastEntry() {
    Iterator<Entry<E>> entryIterator = descendingEntryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  @CheckForNull
  public Entry<E> pollFirstEntry() {

  public Entry<E> firstEntry() {
    Iterator<Entry<E>> entryIterator = entryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  @CheckForNull
  public Entry<E> lastEntry() {
    Iterator<Entry<E>> entryIterator = descendingEntryIterator();
    return entryIterator.hasNext() ? entryIterator.next() : null;
  }

  @Override
  @CheckForNull
  public Entry<E> pollFirstEntry() {

     */
    @Test
    public void testHasNext() {
        final ArrayIterator<String> itr = new ArrayIterator<String>("A", "B");
        assertThat(itr.hasNext(), is(true));
        itr.next();
        assertThat(itr.hasNext(), is(true));
        itr.next();
        assertThat(itr.hasNext(), is(not(true)));
    }

    /**
     * @throws Exception
     */
    @Test
    public void testRemove() throws Exception {

        assertNotNull(iterator);
        assertTrue(iterator.hasNext());
        assertTrue(iterator.hasNext());
        Entry<String, String> entry = iterator.next();
        assertNotNull(entry);
        entry = iterator.next();
        assertNotNull(entry);
        assertThrows(NoSuchElementException.class, () -> iterator.next());
        assertFalse(iterator.hasNext());
    }

    /**
     *
     */
    @Test
    public void test() {
        final SingleValueIterator<String> it = new SingleValueIterator<String>("Foo");
        assertThat(it.hasNext(), is(true));
        assertThat(it.next(), is("Foo"));
        assertThat(it.hasNext(), is(not(true)));
    }

            Iterator<Exclusion> i1 = ce1.iterator();
            Iterator<Exclusion> i2 = ce2.iterator();
            while (i1.hasNext() && i2.hasNext()) {
                if (!equals(i1.next(), i2.next())) {
                    return false;
                }
            }
            return !i1.hasNext() && !i2.hasNext();
        }
        return false;
    }

    private boolean equals(Exclusion e1, Exclusion e2) {

            Iterator<Exclusion> i1 = ce1.iterator();
            Iterator<Exclusion> i2 = ce2.iterator();
            while (i1.hasNext() && i2.hasNext()) {
                if (!equals(i1.next(), i2.next())) {
                    return false;
                }
            }
            return !i1.hasNext() && !i2.hasNext();
        }
        return false;
    }

    private boolean equals(Exclusion e1, Exclusion e2) {

                in.nextNull();
                return null;
            }

            in.beginObject();
            String message = null;
            Throwable cause = null;
            while (in.hasNext()) {
                String name = in.nextName();
                switch (name) {
                    case "message": {
                        message = in.nextString();
                        break;

                    for (PGPPublicKeyRing keyring : keyrings) {
                        Iterator<PGPPublicKey> it = keyring.getPublicKeys();
                        while (it.hasNext()) {
                            PGPPublicKey key = it.next();
                            Fingerprint fingerprint = Fingerprint.of(key);
                            keyToKeyringBuilder.put(fingerprint, keyring);

        vector.add("a");
        final EnumerationIterator<String> itr = new EnumerationIterator<String>(vector.elements());
        assertThat(itr.hasNext(), is(true));
        assertThat(itr.next(), is("a"));
        assertThat(itr.hasNext(), is(not(true)));
    }

    /**
     * @throws Exception
     */
    @Test
    public void testRemove() throws Exception {