Group subsystem Northbound and Southbound API definition

Change-Id: I1843562ff7fdf0dfdf82a8757382d494698ded3f
diff --git a/core/api/src/main/java/org/onosproject/net/group/GroupBuckets.java b/core/api/src/main/java/org/onosproject/net/group/GroupBuckets.java
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+/*
+ * Copyright 2015 Open Networking Laboratory
+ *
+ * Licensed 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 org.onosproject.net.group;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import java.util.List;
+
+
+import com.google.common.collect.ImmutableList;
+
+public final class GroupBuckets {
+    private final List<GroupBucket> buckets;
+
+    /**
+     * Creates a immutable list of group bucket.
+     *
+     * @param buckets list of group bucket
+     */
+    public GroupBuckets(List<GroupBucket> buckets) {
+        this.buckets = ImmutableList.copyOf(checkNotNull(buckets));
+    }
+
+    /**
+     * Immutable list of group buckets.
+     *
+     * @return list of group bucket
+     */
+    public List<GroupBucket> buckets() {
+        return buckets;
+    }
+
+}
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