diff --git a/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java b/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java
index 0fc8eaa..49fb268 100644
--- a/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java
+++ b/core/api/src/main/java/org/onlab/onos/event/AbstractEvent.java
@@ -26,7 +26,7 @@
 
     private final long time;
     private final T type;
-    private S subject;
+    private final S subject;
 
     /**
      * Creates an event of a given type and for the specified subject and the
diff --git a/core/net/src/test/java/org/onlab/onos/net/flow/FlowIdTest.java b/core/net/src/test/java/org/onlab/onos/net/flow/FlowIdTest.java
new file mode 100644
index 0000000..92c3bd0
--- /dev/null
+++ b/core/net/src/test/java/org/onlab/onos/net/flow/FlowIdTest.java
@@ -0,0 +1,65 @@
+/*
+ * Copyright 2014 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.onos.net.flow;
+
+import org.junit.Test;
+
+import com.google.common.testing.EqualsTester;
+
+import static org.hamcrest.MatcherAssert.assertThat;
+import static org.hamcrest.Matchers.is;
+import static org.hamcrest.Matchers.notNullValue;
+import static org.onlab.junit.ImmutableClassChecker.assertThatClassIsImmutable;
+
+/**
+ * Unit tests for flow id class.
+ */
+public class FlowIdTest {
+
+    final FlowId flowId1 = FlowId.valueOf(1);
+    final FlowId sameAsFlowId1 = FlowId.valueOf(1);
+    final FlowId flowId2 = FlowId.valueOf(2);
+
+    /**
+     * Checks that the DefaultFlowRule class is immutable.
+     */
+    @Test
+    public void testImmutability() {
+        assertThatClassIsImmutable(FlowId.class);
+    }
+
+    /**
+     * Checks the operation of equals(), hashCode and toString() methods.
+     */
+    @Test
+    public void testEquals() {
+        new EqualsTester()
+                .addEqualityGroup(flowId1, sameAsFlowId1)
+                .addEqualityGroup(flowId2)
+                .testEquals();
+    }
+
+    /**
+     * Checks the construction of a FlowId object.
+     */
+    @Test
+    public void testConstruction() {
+        final long flowIdValue = 7777L;
+        final FlowId flowId = FlowId.valueOf(flowIdValue);
+        assertThat(flowId, is(notNullValue()));
+        assertThat(flowId.value(), is(flowIdValue));
+    }
+}