[ONOS-3882, 3883, 3892]Implement Yang container, list, leaf, leaf-list parsing

Change-Id: I39039c91385fe0d530d037350ef7833c2459514d
diff --git a/src/main/java/org/onosproject/yangutils/parser/impl/listeners/MandatoryListener.java b/src/main/java/org/onosproject/yangutils/parser/impl/listeners/MandatoryListener.java
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+/*
+ * Copyright 2016 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.yangutils.parser.impl.listeners;
+
+import org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser;
+import org.onosproject.yangutils.parser.impl.TreeWalkListener;
+
+/*
+ * Reference: RFC6020 and YANG ANTLR Grammar
+ *
+ * ABNF grammar as per RFC6020
+ *  mandatory-stmt      = mandatory-keyword sep
+ *                        mandatory-arg-str stmtend
+ *
+ *  mandatory-arg-str   = < a string that matches the rule
+ *                          mandatory-arg >
+ *
+ *  mandatory-arg       = true-keyword / false-keyword
+ *
+ * ANTLR grammar rule
+ *  mandatoryStatement : MANDATORY_KEYWORD (TRUE_KEYWORD | FALSE_KEYWORD) STMTEND;
+ */
+
+/**
+ * Implements listener based call back function corresponding to the "mandatory"
+ * rule defined in ANTLR grammar file for corresponding ABNF rule in RFC 6020.
+ */
+public final class MandatoryListener {
+
+    /**
+     * Creates a new mandatory listener.
+     */
+    private MandatoryListener() {
+    }
+
+    /**
+     * It is called when parser receives an input matching the grammar
+     * rule (mandatory), performs validation and updates the data model
+     * tree.
+     *
+     * @param listener listener's object.
+     * @param ctx context object of the grammar rule.
+     */
+    public static void processMandatoryEntry(TreeWalkListener listener,
+                                          GeneratedYangParser.MandatoryStatementContext ctx) {
+        // TODO method implementation
+    }
+
+    /**
+     * It is called when parser exits from grammar rule (mandatory), it performs
+     * validation and updates the data model tree.
+     *
+     * @param listener listener's object.
+     * @param ctx context object of the grammar rule.
+     */
+    public static void processMandatoryExit(TreeWalkListener listener,
+                                         GeneratedYangParser.MandatoryStatementContext ctx) {
+        // TODO method implementation
+    }
+}
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