Initial implementation of XMPP Publish/Subscribe

Change-Id: I9930056b83004fa7a085f72f63ba973f9ec2d95b
diff --git a/protocols/xmpp/pubsub/api/src/main/java/org/onosproject/xmpp/pubsub/model/XmppPubSubError.java b/protocols/xmpp/pubsub/api/src/main/java/org/onosproject/xmpp/pubsub/model/XmppPubSubError.java
new file mode 100644
index 0000000..dcf60d2
--- /dev/null
+++ b/protocols/xmpp/pubsub/api/src/main/java/org/onosproject/xmpp/pubsub/model/XmppPubSubError.java
@@ -0,0 +1,51 @@
+/*
+ * Copyright 2018-present Open Networking Foundation
+ *
+ * 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.xmpp.pubsub.model;
+
+import org.xmpp.packet.PacketError;
+import static org.onosproject.xmpp.pubsub.XmppPubSubConstants.APP_BASE_CONDITION_MAP;
+import static org.onosproject.xmpp.pubsub.XmppPubSubConstants.PUBSUB_ERROR_NS;
+import static org.onosproject.xmpp.pubsub.XmppPubSubConstants.PubSubApplicationCondition;
+
+/**
+ * Abstracts Publish/Subscribe error message of XMPP PubSub protocol.
+ */
+public class XmppPubSubError {
+
+    private PacketError.Condition baseCondition;
+    private PubSubApplicationCondition applicationCondition;
+
+    public XmppPubSubError(PubSubApplicationCondition applicationCondition) {
+        this.applicationCondition = applicationCondition;
+        this.baseCondition = setBasedOnAppCondition();
+    }
+
+    private PacketError.Condition setBasedOnAppCondition() {
+        return APP_BASE_CONDITION_MAP.getOrDefault(this.applicationCondition,
+                                                   PacketError.Condition.undefined_condition);
+    }
+
+    public PacketError asPacketError() {
+        PacketError packetError = new PacketError(this.baseCondition);
+        if (applicationCondition != null) {
+            packetError.setApplicationCondition(applicationCondition.toString().toLowerCase(),
+                                                PUBSUB_ERROR_NS);
+        }
+        return packetError;
+    }
+
+}