Backporting PrimitiveCollections class

Motivation:
PrimitiveCollections is not in the 4.1 branch.  It is needed by HTTP/2.

Modifications:
Backport this class.

Result:
PrimitiveCollections is in 4.1.

common/src/main/java/io/netty/util/collection/PrimitiveCollections.java

@@ -0,0 +1,258 @@
+/*
+ * Copyright 2014 The Netty Project
+ *
+ * The Netty Project licenses this file to you under the Apache License, version 2.0 (the
+ * "License"); you may not use this file except in compliance with the License. You may obtain a
+ * copy of the License at:
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software distributed under the License
+ * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
+ * or implied. See the License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package io.netty.util.collection;
+
+import io.netty.util.internal.EmptyArrays;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.NoSuchElementException;
+
+/**
+ * Utility methods for primitive collections.
+ */
+public final class PrimitiveCollections {
+
+    private static final IntObjectMap<Object> EMPTY_INT_OBJECT_MAP = new EmptyIntObjectMap();
+
+    private PrimitiveCollections() {
+    }
+
+    /**
+     * Returns an unmodifiable empty {@link IntObjectMap}.
+     */
+    @SuppressWarnings("unchecked")
+    public static <V> IntObjectMap<V> emptyIntObjectMap() {
+        return (IntObjectMap<V>) EMPTY_INT_OBJECT_MAP;
+    }
+
+    /**
+     * Creates an unmodifiable wrapper around the given map.
+     */
+    public static <V> IntObjectMap<V> unmodifiableIntObjectMap(final IntObjectMap<V> map) {
+        return new UnmodifiableIntObjectMap<V>(map);
+    }
+
+    /**
+     * An empty map. All operations that attempt to modify the map are unsupported.
+     */
+    private static final class EmptyIntObjectMap implements IntObjectMap<Object> {
+
+        @Override
+        public Object get(int key) {
+            return null;
+        }
+
+        @Override
+        public Object put(int key, Object value) {
+            throw new UnsupportedOperationException("put");
+        }
+
+        @Override
+        public void putAll(IntObjectMap<Object> sourceMap) {
+            throw new UnsupportedOperationException("putAll");
+        }
+
+        @Override
+        public Object remove(int key) {
+            throw new UnsupportedOperationException("remove");
+        }
+
+        @Override
+        public int size() {
+            return 0;
+        }
+
+        @Override
+        public boolean isEmpty() {
+            return true;
+        }
+
+        @Override
+        public void clear() {
+            // Do nothing.
+        }
+
+        @Override
+        public boolean containsKey(int key) {
+            return false;
+        }
+
+        @Override
+        public boolean containsValue(Object value) {
+            return false;
+        }
+
+        @Override
+        public Iterable<Entry<Object>> entries() {
+            return Collections.emptySet();
+        }
+
+        @Override
+        public int[] keys() {
+            return EmptyArrays.EMPTY_INTS;
+        }
+
+        @Override
+        public Object[] values(Class<Object> clazz) {
+            return EmptyArrays.EMPTY_OBJECTS;
+        }
+
+        @Override
+        public Collection<Object> values() {
+            return Collections.emptyList();
+        }
+    }
+
+    /**
+     * An unmodifiable wrapper around a {@link IntObjectMap}.
+     *
+     * @param <V> the value type stored in the map.
+     */
+    private static final class UnmodifiableIntObjectMap<V> implements IntObjectMap<V>,
+            Iterable<IntObjectMap.Entry<V>> {
+        final IntObjectMap<V> map;
+
+        UnmodifiableIntObjectMap(IntObjectMap<V> map) {
+            this.map = map;
+        }
+
+        @Override
+        public V get(int key) {
+            return map.get(key);
+        }
+
+        @Override
+        public V put(int key, V value) {
+            throw new UnsupportedOperationException("put");
+        }
+
+        @Override
+        public void putAll(IntObjectMap<V> sourceMap) {
+            throw new UnsupportedOperationException("putAll");
+        }
+
+        @Override
+        public V remove(int key) {
+            throw new UnsupportedOperationException("remove");
+        }
+
+        @Override
+        public int size() {
+            return map.size();
+        }
+
+        @Override
+        public boolean isEmpty() {
+            return map.isEmpty();
+        }
+
+        @Override
+        public void clear() {
+            throw new UnsupportedOperationException("clear");
+        }
+
+        @Override
+        public boolean containsKey(int key) {
+            return map.containsKey(key);
+        }
+
+        @Override
+        public boolean containsValue(V value) {
+            return map.containsValue(value);
+        }
+
+        @Override
+        public Iterable<Entry<V>> entries() {
+            return this;
+        }
+
+        @Override
+        public Iterator<Entry<V>> iterator() {
+            return new IteratorImpl(map.entries().iterator());
+        }
+
+        @Override
+        public int[] keys() {
+            return map.keys();
+        }
+
+        @Override
+        public V[] values(Class<V> clazz) {
+            return map.values(clazz);
+        }
+
+        @Override
+        public Collection<V> values() {
+            return map.values();
+        }
+
+        /**
+         * Unmodifiable wrapper for an iterator.
+         */
+        private class IteratorImpl implements Iterator<Entry<V>> {
+            final Iterator<Entry<V>> iter;
+
+            IteratorImpl(Iterator<Entry<V>> iter) {
+                this.iter = iter;
+            }
+
+            @Override
+            public boolean hasNext() {
+                return iter.hasNext();
+            }
+
+            @Override
+            public Entry<V> next() {
+                if (!hasNext()) {
+                    throw new NoSuchElementException();
+                }
+                return new EntryImpl(iter.next());
+            }
+
+            @Override
+            public void remove() {
+                throw new UnsupportedOperationException("remove");
+            }
+        }
+
+        /**
+         * Unmodifiable wrapper for an entry.
+         */
+        private class EntryImpl implements Entry<V> {
+            final Entry<V> entry;
+
+            EntryImpl(Entry<V> entry) {
+                this.entry = entry;
+            }
+
+            @Override
+            public int key() {
+                return entry.key();
+            }
+
+            @Override
+            public V value() {
+                return entry.value();
+            }
+
+            @Override
+            public void setValue(V value) {
+                throw new UnsupportedOperationException("setValue");
+            }
+        }
+    }
+}