Allow null as parameter

Change-Id: I9601bca4af0dadf706a2e0ca2502595d10e7ee74

@@ -41,7 +41,7 @@ public final class PositionalParameterStringFormatter {
             if (!current.contains("{}")) {
                 return current;
             }
-            current = current.replaceFirst("\\{\\}", parameter.toString());
+            current = current.replaceFirst("\\{\\}", String.valueOf(parameter));
         }
         return current;
     }

+/*
+ * 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.onlab.util;
+
+import static org.junit.Assert.*;
+import static org.onlab.util.PositionalParameterStringFormatter.format;
+
+import org.junit.Test;
+
+public class PositionalParameterStringFormatterTest {
+
+    @Test
+    public void testFormat0() {
+        String fmt = "Some string 1 2 3";
+        assertEquals("Some string 1 2 3", format(fmt));
+    }
+
+    @Test
+    public void testFormat1() {
+        String fmt = "Some string {} 2 3";
+        assertEquals("Some string 1 2 3", format(fmt, 1));
+    }
+
+    @Test
+    public void testFormat2() {
+        String fmt = "Some string {} 2 {}";
+        assertEquals("Some string 1 2 3", format(fmt, 1, "3"));
+    }
+
+    @Test
+    public void testFormatNull() {
+        String fmt = "Some string {} 2 {}";
+        assertEquals("Some string 1 2 null", format(fmt, 1, null));
+    }
+
+    @Test
+    public void testFormatExtraBracket() {
+        String fmt = "Some string {} 2 {}";
+        assertEquals("Some string 1 2 {}", format(fmt, 1));
+    }
+
+    @Test
+    public void testFormatMissingBracket() {
+        String fmt = "Some string 1 2 3";
+        assertEquals("Some string 1 2 3", format(fmt, 7));
+    }
+}