+/*
+ * Copyright (c) 2015 Cisco Systems, Inc. and others. All rights reserved.
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v1.0 which accompanies this distribution,
+ * and is available at http://www.eclipse.org/legal/epl-v10.html
+ */
+package org.opendaylight.yangtools.util;
+
+import com.google.common.annotations.Beta;
+import com.google.common.base.Preconditions;
+import com.google.common.collect.Iterators;
+import java.io.Serializable;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.Set;
+import javax.annotation.Nonnull;
+import org.opendaylight.yangtools.concepts.Immutable;
+
+/**
+ * A {@link Set} containing a single value. For some reason neither Java nor Guava provide direct access to the retained
+ * element -- which is desirable in some situations, as is the case in {@link SharedSingletonMap#entrySet()}.
+ */
+@Beta
+public abstract class SingletonSet<E> implements Set<E>, Immutable, Serializable {
+ private static final long serialVersionUID = 1L;
+
+ private static final SingletonSet<?> NULL_SINGLETON = new SingletonSet<Object>() {
+ private static final long serialVersionUID = 1L;
+
+ @Override
+ public boolean contains(final Object o) {
+ return o == null;
+ }
+
+ @Override
+ public int hashCode() {
+ return 0;
+ }
+
+ @Override
+ public Object getElement() {
+ return null;
+ }
+
+ @Override
+ public String toString() {
+ return "[null]";
+ }
+
+ private Object readResolve() {
+ return NULL_SINGLETON;
+ }
+ };
+
+ @SuppressWarnings("unchecked")
+ public static <E> SingletonSet<E> of(@Nonnull final E element) {
+ if (element == null) {
+ return (SingletonSet<E>) NULL_SINGLETON;
+ }
+ return new RegularSingletonSet<E>(element);
+ }
+
+ public abstract E getElement();
+
+ @Override
+ public final int size() {
+ return 1;
+ }
+
+ @Override
+ public final boolean isEmpty() {
+ return false;
+ }
+
+ @Override
+ public final Iterator<E> iterator() {
+ return Iterators.singletonIterator(getElement());
+ }
+
+ @Override
+ public final Object[] toArray() {
+ return new Object[] { getElement() };
+ }
+
+ @SuppressWarnings("unchecked")
+ @Override
+ public final <T> T[] toArray(final T[] a) {
+ if (a.length > 0) {
+ a[0] = (T)getElement();
+ return a;
+ }
+
+ return (T[]) new Object[] { (T) getElement() };
+ }
+
+ @Override
+ public final boolean add(final E e) {
+ throw new UnsupportedOperationException();
+ }
+
+ @Override
+ public final boolean remove(final Object o) {
+ throw new UnsupportedOperationException();
+ }
+
+ @Override
+ public final boolean containsAll(final Collection<?> c) {
+ if (c.isEmpty()) {
+ return true;
+ }
+ if (c.size() != 1) {
+ return false;
+ }
+
+ return otherContains(c);
+ }
+
+ @Override
+ public final boolean addAll(final Collection<? extends E> c) {
+ throw new UnsupportedOperationException();
+ }
+
+ @Override
+ public final boolean retainAll(final Collection<?> c) {
+ throw new UnsupportedOperationException();
+ }
+
+ @Override
+ public final boolean removeAll(final Collection<?> c) {
+ throw new UnsupportedOperationException();
+ }
+
+ @Override
+ public final void clear() {
+ throw new UnsupportedOperationException();
+ }
+
+ @Override
+ public abstract int hashCode();
+
+ @Override
+ public final boolean equals(final Object obj) {
+ if (obj == this) {
+ return true;
+ }
+ if (!(obj instanceof Set)) {
+ return false;
+ }
+
+ final Set<?> s = (Set<?>)obj;
+ return s.size() == 1 && otherContains(s);
+ }
+
+ private boolean otherContains(final Collection<?> other) {
+ try {
+ return other.contains(getElement());
+ } catch (ClassCastException | NullPointerException e) {
+ return false;
+ }
+ }
+
+ private static final class RegularSingletonSet<E> extends SingletonSet<E> {
+ private static final long serialVersionUID = 1L;
+ private final E element;
+
+ RegularSingletonSet(final E element) {
+ this.element = Preconditions.checkNotNull(element);
+ }
+
+ @Override
+ public boolean contains(final Object o) {
+ return element.equals(o);
+ }
+
+ @Override
+ public E getElement() {
+ return element;
+ }
+
+ @Override
+ public int hashCode() {
+ return getElement().hashCode();
+ }
+
+ @Override
+ public String toString() {
+ return "[" + element + ']';
+ }
+ }
+}