Group subsystem Northbound and Southbound API definition

Change-Id: I1843562ff7fdf0dfdf82a8757382d494698ded3f
diff --git a/core/api/src/main/java/org/onosproject/net/group/Group.java b/core/api/src/main/java/org/onosproject/net/group/Group.java
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+++ b/core/api/src/main/java/org/onosproject/net/group/Group.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 org.onosproject.core.GroupId;
+
+/**
+ * ONOS representation of group that is stored in the system.
+ */
+public interface Group extends GroupDescription {
+    /**
+     * State of the group object in ONOS.
+     * PENDING_ADD: group create request is processed by ONOS and
+     * not yet received the confirmation from data plane
+     * ADDED: group is created in the data plane
+     * PENDING_UPDATE: group update request is processed by ONOS and
+     * not received the confirmation from data plane post which state
+     * moves to ADDED state
+     * PENDING_DELETE: group delete request is processed by ONOS and
+     * not received the confirmation from data plane
+     */
+    public enum GroupState {
+        PENDING_ADD,
+        ADDED,
+        PENDING_UPDATE,
+        PENDING_DELETE
+    }
+
+    /**
+     * Return group identifier associated with a group object.
+     *
+     * @return GroupId Group Identifier
+     */
+    public GroupId id();
+
+    /**
+     * Return current state of a group object.
+     *
+     * @return GroupState Group State
+     */
+    public GroupState state();
+
+    /**
+     * Returns the number of milliseconds this group has been alive.
+     *
+     * @return number of millis
+     */
+    long life();
+
+    /**
+     * Returns the number of packets processed by this group.
+     *
+     * @return number of packets
+     */
+    long packets();
+
+    /**
+     * Returns the number of bytes processed by this group.
+     *
+     * @return number of bytes
+     */
+    long bytes();
+}