Introduce UnmodifiableCollection

This is a utility class which recognizes itself and Guava's
ImmutableCollection subclasses during contruction, preventing unneeded
encapsulation.

Change-Id: I99deeb570a74ecee5f0bb8a89c25b363284e5f6e
Signed-off-by: Robert Varga <rovarga@cisco.com>

diff --git a/common/util/src/main/java/org/opendaylight/yangtools/util/UnmodifiableCollection.java b/common/util/src/main/java/org/opendaylight/yangtools/util/UnmodifiableCollection.java
new file mode 100644 (file)
index 0000000..3e9d414
--- /dev/null
+++ b/common/util/src/main/java/org/opendaylight/yangtools/util/UnmodifiableCollection.java
@@ -0,0 +1,135 @@
+/*
+ * Copyright (c) 2013 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.base.Preconditions;
+import com.google.common.collect.ImmutableCollection;
+import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.ImmutableSet.Builder;
+import com.google.common.collect.Iterators;
+import java.io.Serializable;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Iterator;
+import javax.annotation.Nonnull;
+
+/**
+ * An unmodifiable view over a {@link Collection}. Unlike the view returned via
+ * {@link Collections#unmodifiableCollection(Collection)}, this class checks its
+ * argument to ensure multiple encapsulation does not occur.
+ *
+ * this class checks
+ * the argument so it prevents multiple encapsulation. Subclasses of
+ * {@link ImmutableCollection} are also recognized and not encapsulated.
+ * An attempt is also made to identi
+ *
+ * @param <E> the type of elements in this collection
+ */
+public final class UnmodifiableCollection<E> implements Collection<E>, Serializable {
+    private static final long serialVersionUID = 1L;
+    private static final Class<?> UNMODIFIABLE_COLLECTION_CLASS;
+    private static final Collection<Class<?>> SINGLETON_CLASSES;
+
+    static {
+        UNMODIFIABLE_COLLECTION_CLASS = Collections.unmodifiableCollection(new ArrayList<Object>()).getClass();
+
+        final Builder<Class<?>> b = ImmutableSet.builder();
+        b.add(Collections.singleton(null).getClass());
+        b.add(Collections.singletonList(null).getClass());
+        SINGLETON_CLASSES = b.build();
+    }
+
+    private final Collection<E> delegate;
+
+    private UnmodifiableCollection(final Collection<E> delegate) {
+        this.delegate = Preconditions.checkNotNull(delegate);
+    }
+
+    /**
+     * Create an unmodifiable view of the target collection. If the instance is known
+     * to be unmodifiable, that instance is returned.
+     *
+     * @param collection Target collection
+     * @return An unmodifiable view of the collection
+     */
+    public static <T> Collection<T> create(final @Nonnull Collection<T> collection) {
+        if (collection instanceof UnmodifiableCollection || collection instanceof ImmutableCollection ||
+                Collections.EMPTY_LIST == collection || Collections.EMPTY_SET == collection ||
+                UNMODIFIABLE_COLLECTION_CLASS.isInstance(collection) || SINGLETON_CLASSES.contains(collection.getClass())) {
+            return collection;
+        }
+
+        return new UnmodifiableCollection<>(collection);
+    }
+
+    @Override
+    public Iterator<E> iterator() {
+        return Iterators.unmodifiableIterator(delegate.iterator());
+    }
+
+    @Override
+    public int size() {
+        return delegate.size();
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return delegate.isEmpty();
+    }
+
+    @Override
+    public boolean contains(final Object o) {
+        return delegate.contains(o);
+    }
+
+    @Override
+    public Object[] toArray() {
+        return delegate.toArray();
+    }
+
+    @Override
+    public <T> T[] toArray(final T[] a) {
+        return delegate.toArray(a);
+    }
+
+    @Override
+    public boolean containsAll(final Collection<?> c) {
+        return delegate.containsAll(c);
+    }
+
+    @Override
+    public boolean add(final E e) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean addAll(final Collection<? extends E> c) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean remove(final Object o) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean removeAll(final Collection<?> c) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public boolean retainAll(final Collection<?> c) {
+        throw new UnsupportedOperationException();
+    }
+
+    @Override
+    public void clear() {
+        throw new UnsupportedOperationException();
+    }
+}