Add an empty Deque implementation

A utility simpleton java.util.Deque which is always empty.
This can be used as a java.uil.Queue, too.

Change-Id: I6435e36bf36b9baf0983cf2fcc13d261c6fa9248
Signed-off-by: Robert Varga <rovarga@cisco.com>

diff --git a/common/util/src/main/java/org/opendaylight/yangtools/util/EmptyDeque.java b/common/util/src/main/java/org/opendaylight/yangtools/util/EmptyDeque.java
new file mode 100644 (file)
index 0000000..566b2c3
--- /dev/null
+++ b/common/util/src/main/java/org/opendaylight/yangtools/util/EmptyDeque.java
@@ -0,0 +1,166 @@
+/*
+ * Copyright (c) 2016 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 java.util.AbstractQueue;
+import java.util.Collections;
+import java.util.Deque;
+import java.util.Iterator;
+import java.util.Spliterator;
+import java.util.Spliterators;
+import org.opendaylight.yangtools.concepts.Immutable;
+
+/**
+ * A specialized always-empty implementation of {@link java.util.Deque}. This implementation will always refuse new
+ * elements in its {@link #offer(Object)} method.
+
+ * @author Robert Varga
+ *
+ * @param <E> the type of elements held in this collection
+ */
+@Beta
+public final class EmptyDeque<E> extends AbstractQueue<E> implements Deque<E>, Immutable {
+    private static final EmptyDeque<?> INSTANCE = new EmptyDeque<>();
+    private static final Object[] EMPTY_ARRAY = new Object[0];
+
+    private EmptyDeque() {
+        // No instances
+    }
+
+    @SuppressWarnings("unchecked")
+    public static <T> EmptyDeque<T> instance() {
+        return (EmptyDeque<T>) INSTANCE;
+    }
+
+    @Override
+    public boolean offer(final E entry) {
+        return false;
+    }
+
+    @Override
+    public boolean offerFirst(final E e) {
+        return false;
+    }
+
+    @Override
+    public boolean offerLast(final E e) {
+        return false;
+    }
+
+    @Override
+    public E poll() {
+        return null;
+    }
+
+    @Override
+    public E pollFirst() {
+        return null;
+    }
+
+    @Override
+    public E pollLast() {
+        return null;
+    }
+
+    @Override
+    public E peek() {
+        return null;
+    }
+
+    @Override
+    public E peekFirst() {
+        return null;
+    }
+
+    @Override
+    public E peekLast() {
+        return null;
+    }
+
+    @Override
+    public Iterator<E> iterator() {
+        return Collections.emptyIterator();
+    }
+
+    @Override
+    public Spliterator<E> spliterator() {
+        return Spliterators.emptySpliterator();
+    }
+
+    @Override
+    public int size() {
+        return 0;
+    }
+
+    @Override
+    public Object[] toArray() {
+        return EMPTY_ARRAY;
+    }
+
+    @Override
+    public <T> T[] toArray(final T[] a) {
+        return Preconditions.checkNotNull(a);
+    }
+
+    @Override
+    public void addFirst(final E e) {
+        add(e);
+    }
+
+    @Override
+    public void addLast(final E e) {
+        add(e);
+    }
+
+    @Override
+    public E removeFirst() {
+        return remove();
+    }
+
+    @Override
+    public E removeLast() {
+        return remove();
+    }
+
+    @Override
+    public E getFirst() {
+        return element();
+    }
+
+    @Override
+    public E getLast() {
+        return element();
+    }
+
+    @Override
+    public boolean removeFirstOccurrence(final Object o) {
+        return false;
+    }
+
+    @Override
+    public boolean removeLastOccurrence(final Object o) {
+        return false;
+    }
+
+    @Override
+    public void push(final E e) {
+        addFirst(e);
+    }
+
+    @Override
+    public E pop() {
+        return removeFirst();
+    }
+
+    @Override
+    public Iterator<E> descendingIterator() {
+        return Collections.emptyIterator();
+    }
+}