Address comments in gerrit 20328

Create a new class XconnecEndpoint to cover both physical port and load balancer
Also change the CLI load balancer identifier to "LB:"

Change-Id: I0b4cd6e474d8b21468d87fcadd9280fdf7d6aafa
diff --git a/app/src/main/java/org/onosproject/segmentrouting/xconnect/api/XconnectEndpoint.java b/app/src/main/java/org/onosproject/segmentrouting/xconnect/api/XconnectEndpoint.java
new file mode 100644
index 0000000..c7a2242
--- /dev/null
+++ b/app/src/main/java/org/onosproject/segmentrouting/xconnect/api/XconnectEndpoint.java
@@ -0,0 +1,64 @@
+/*
+ * Copyright 2019-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.segmentrouting.xconnect.api;
+
+/**
+ * Represents cross connect endpoint.
+ */
+public abstract class XconnectEndpoint {
+    public static final String LB_KEYWORD = "LB:";
+    static final String PORT_PATTERN = "^\\d+$";
+    static final String LOAD_BALANCER_PATTERN = "^" + LB_KEYWORD + "\\d+$";
+
+    /**
+     * Types of endpoint.
+     */
+    public enum Type {
+        /**
+         * The endpoint is specified by an port number.
+         */
+        PORT,
+
+        /**
+         * The endpoint is specified by a load balancer.
+         */
+        LOAD_BALANCER
+    }
+
+    /**
+     * Type of this endpoint.
+     *
+     * @return type
+     */
+    public abstract XconnectEndpoint.Type type();
+
+    /**
+     * Constructs XconnectEndpoint from string.
+     *
+     * @param s string
+     * @return XconnectEndpoint
+     * @throws IllegalArgumentException if given string is in a wrong format
+     */
+    public static XconnectEndpoint fromString(String s) {
+        if (s.matches(XconnectEndpoint.PORT_PATTERN)) {
+            return XconnectPortEndpoint.fromString(s);
+        } else if (s.matches(XconnectEndpoint.LOAD_BALANCER_PATTERN)) {
+            return XconnectLoadBalancerEndpoint.fromString(s);
+        } else {
+            throw new IllegalArgumentException("Illegal endpoint format: " + s);
+        }
+    }
+}