2 * Copyright (c) 2016 Cisco Systems, Inc. and others. All rights reserved.
4 * This program and the accompanying materials are made available under the
5 * terms of the Eclipse Public License v1.0 which accompanies this distribution,
6 * and is available at http://www.eclipse.org/legal/epl-v10.html
8 package org.opendaylight.yangtools.util;
10 import static java.util.Objects.requireNonNull;
12 import com.google.common.annotations.Beta;
13 import java.util.AbstractQueue;
14 import java.util.Collections;
15 import java.util.Deque;
16 import java.util.Iterator;
17 import java.util.Spliterator;
18 import java.util.Spliterators;
19 import org.opendaylight.yangtools.concepts.Immutable;
22 * A specialized always-empty implementation of {@link java.util.Deque}. This implementation will always refuse new
23 * elements in its {@link #offer(Object)} method.
25 * @author Robert Varga
27 * @param <E> the type of elements held in this collection
30 public final class EmptyDeque<E> extends AbstractQueue<E> implements Deque<E>, Immutable {
31 private static final EmptyDeque<?> INSTANCE = new EmptyDeque<>();
32 private static final Object[] EMPTY_ARRAY = new Object[0];
34 private EmptyDeque() {
38 @SuppressWarnings("unchecked")
39 public static <T> EmptyDeque<T> instance() {
40 return (EmptyDeque<T>) INSTANCE;
44 @SuppressWarnings("checkstyle:parameterName")
45 public boolean offer(final E e) {
50 @SuppressWarnings("checkstyle:parameterName")
51 public boolean offerFirst(final E e) {
56 @SuppressWarnings("checkstyle:parameterName")
57 public boolean offerLast(final E e) {
67 public E pollFirst() {
82 public E peekFirst() {
92 public Iterator<E> iterator() {
93 return Collections.emptyIterator();
97 public Spliterator<E> spliterator() {
98 return Spliterators.emptySpliterator();
107 public Object[] toArray() {
112 @SuppressWarnings("checkstyle:parameterName")
113 public <T> T[] toArray(final T[] a) {
114 return requireNonNull(a);
118 @SuppressWarnings("checkstyle:parameterName")
119 public void addFirst(final E e) {
124 @SuppressWarnings("checkstyle:parameterName")
125 public void addLast(final E e) {
130 public E removeFirst() {
135 public E removeLast() {
140 public E getFirst() {
150 @SuppressWarnings("checkstyle:parameterName")
151 public boolean removeFirstOccurrence(final Object o) {
156 @SuppressWarnings("checkstyle:parameterName")
157 public boolean removeLastOccurrence(final Object o) {
162 @SuppressWarnings("checkstyle:parameterName")
163 public void push(final E e) {
169 return removeFirst();
173 public Iterator<E> descendingIterator() {
174 return Collections.emptyIterator();