Added more unit tests.

+package org.onlab.onos.net;
+
+import com.google.common.testing.EqualsTester;
+import org.junit.Test;
+import org.onlab.onos.net.provider.ProviderId;
+
+import static org.junit.Assert.assertEquals;
+import static org.onlab.onos.net.DeviceId.deviceId;
+import static org.onlab.onos.net.Link.Type.DIRECT;
+import static org.onlab.onos.net.Link.Type.INDIRECT;
+import static org.onlab.onos.net.PortNumber.portNumber;
+
+/**
+ * Test of the default link model entity.
+ */
+public class DefaultLinkTest {
+
+    private static final ProviderId PID = new ProviderId("foo");
+    private static final DeviceId DID1 = deviceId("of:foo");
+    private static final DeviceId DID2 = deviceId("of:bar");
+    private static final PortNumber P1 = portNumber(1);
+    private static final PortNumber P2 = portNumber(2);
+
+    public static ConnectPoint cp(DeviceId id, PortNumber pn) {
+        return new ConnectPoint(id, pn);
+    }
+
+    @Test
+    public void testEquality() {
+        Link l1 = new DefaultLink(PID, cp(DID1, P1), cp(DID2, P2), DIRECT);
+        Link l2 = new DefaultLink(PID, cp(DID1, P1), cp(DID2, P2), DIRECT);
+        Link l3 = new DefaultLink(PID, cp(DID1, P2), cp(DID2, P2), DIRECT);
+        Link l4 = new DefaultLink(PID, cp(DID1, P2), cp(DID2, P2), DIRECT);
+        Link l5 = new DefaultLink(PID, cp(DID1, P2), cp(DID2, P2), INDIRECT);
+
+        new EqualsTester().addEqualityGroup(l1, l2)
+                .addEqualityGroup(l3, l4)
+                .addEqualityGroup(l5)
+                .testEquals();
+    }
+
+    @Test
+    public void basics() {
+        Link link = new DefaultLink(PID, cp(DID1, P1), cp(DID2, P2), DIRECT);
+        assertEquals("incorrect src", cp(DID1, P1), link.src());
+        assertEquals("incorrect dst", cp(DID2, P2), link.dst());
+        assertEquals("incorrect type", DIRECT, link.type());
+    }
+
+}

+package org.onlab.onos.net.link;
+
+import org.junit.Test;
+import org.onlab.onos.net.DeviceId;
+import org.onlab.onos.net.PortNumber;
+
+import static org.junit.Assert.assertEquals;
+import static org.onlab.onos.net.DefaultLinkTest.cp;
+import static org.onlab.onos.net.DeviceId.deviceId;
+import static org.onlab.onos.net.Link.Type.DIRECT;
+import static org.onlab.onos.net.PortNumber.portNumber;
+
+/**
+ * Test of the default link description.
+ */
+public class DefaultLinkDescriptionTest {
+
+    private static final DeviceId DID1 = deviceId("of:foo");
+    private static final DeviceId DID2 = deviceId("of:bar");
+    private static final PortNumber P1 = portNumber(1);
+
+    @Test
+    public void basics() {
+        LinkDescription desc = new DefaultLinkDescription(cp(DID1, P1), cp(DID2, P1), DIRECT);
+        assertEquals("incorrect src", cp(DID1, P1), desc.src());
+        assertEquals("incorrect dst", cp(DID2, P1), desc.dst());
+        assertEquals("incorrect type", DIRECT, desc.type());
+    }
+
+}

+package org.onlab.onos.net.link;
+
+import org.junit.Test;
+import org.onlab.onos.event.AbstractEventTest;
+import org.onlab.onos.net.ConnectPoint;
+import org.onlab.onos.net.DefaultLink;
+import org.onlab.onos.net.Link;
+import org.onlab.onos.net.provider.ProviderId;
+
+import static org.onlab.onos.net.DeviceId.deviceId;
+import static org.onlab.onos.net.PortNumber.portNumber;
+
+/**
+ * Tests of the device event.
+ */
+public class LinkEventTest extends AbstractEventTest {
+
+    private Link createLink() {
+        return new DefaultLink(new ProviderId("foo"),
+                               new ConnectPoint(deviceId("of:foo"), portNumber(1)),
+                               new ConnectPoint(deviceId("of:bar"), portNumber(2)),
+                               Link.Type.INDIRECT);
+    }
+
+    @Test
+    public void withTime() {
+        Link link = createLink();
+        LinkEvent event = new LinkEvent(LinkEvent.Type.LINK_ADDED, link, 123L);
+        validateEvent(event, LinkEvent.Type.LINK_ADDED, link, 123L);
+    }
+
+    @Test
+    public void withoutTime() {
+        Link link = createLink();
+        long before = System.currentTimeMillis();
+        LinkEvent event = new LinkEvent(LinkEvent.Type.LINK_ADDED, link);
+        long after = System.currentTimeMillis();
+        validateEvent(event, LinkEvent.Type.LINK_ADDED, link, before, after);
+    }
+
+}