Fix JENKINS-28161

This just fixes the NPE symptoms, but I'm afraid that
the reason that an event has a null value has some deeper issue somewhere.

src/main/java/com/sonyericsson/hudson/plugins/gerrit/trigger/gerritnotifier/model/BuildMemory.java

@@ -60,11 +60,19 @@ public class BuildMemory {
      *
      * @author James E. Blair <jeblair@hp.com>
      */
-    private class GerritTriggeredEventComparator implements Comparator<GerritTriggeredEvent> {
+    static class GerritTriggeredEventComparator implements Comparator<GerritTriggeredEvent> {
         @Override
         public int compare(GerritTriggeredEvent o1, GerritTriggeredEvent o2) {
-            return new Integer(((java.lang.Object)o1).hashCode()).compareTo(
-                    new Integer(((java.lang.Object)o2).hashCode()));
+            if (o1 == null && o2 == null) {
+                return 0;
+            }
+            if (o1 != null && o2 == null) {
+                return 1;
+            }
+            if (o1 == null && o2 != null) {
+                return -1;
+            }
+            return new Integer(o1.hashCode()).compareTo(o2.hashCode());
         }
     }
 

src/test/java/com/sonyericsson/hudson/plugins/gerrit/trigger/gerritnotifier/model/GerritTriggeredEventComparatorTest.java

@@ -0,0 +1,55 @@
+package com.sonyericsson.hudson.plugins.gerrit.trigger.gerritnotifier.model;
+
+import com.sonyericsson.hudson.plugins.gerrit.trigger.mock.Setup;
+import com.sonymobile.tools.gerrit.gerritevents.dto.events.ChangeMerged;
+import com.sonymobile.tools.gerrit.gerritevents.dto.events.GerritTriggeredEvent;
+import com.sonymobile.tools.gerrit.gerritevents.dto.events.PatchsetCreated;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+
+import java.lang.reflect.Constructor;
+import java.util.Arrays;
+import java.util.Comparator;
+
+import static org.junit.Assert.*;
+
+/**
+ * Tests {@link com.sonyericsson.hudson.plugins.gerrit.trigger.gerritnotifier.model.BuildMemory.GerritTriggeredEventComparator}.
+ *
+ * @author Robert Sandell <rsandell@cloudbees.com>.
+ */
+@RunWith(Parameterized.class)
+public class GerritTriggeredEventComparatorTest {
+
+    private final GerritTriggeredEvent o1;
+    private final GerritTriggeredEvent o2;
+    private final int expected;
+    private final Comparator<GerritTriggeredEvent> comparator;
+
+    @Parameterized.Parameters(name= "{index}: {0}, {1} == {2}")
+    public static Iterable<Object[]> permutations() {
+        PatchsetCreated event = Setup.createPatchsetCreated();
+        ChangeMerged merged = Setup.createChangeMerged();
+        return Arrays.asList(new Object[][]{
+                {null, null, 0},
+                {null, event, -1},
+                {event, null, 1},
+                {event, event, 0},
+                {event, merged, new Integer(event.hashCode()).compareTo(merged.hashCode())},
+                {merged, event, new Integer(merged.hashCode()).compareTo(event.hashCode())}
+        });
+    }
+
+    public GerritTriggeredEventComparatorTest(GerritTriggeredEvent o1, GerritTriggeredEvent o2, int expected) {
+        this.o1 = o1;
+        this.o2 = o2;
+        this.expected = expected;
+        comparator = new BuildMemory.GerritTriggeredEventComparator();
+    }
+
+    @Test
+    public void testCompare() throws Exception {
+        assertEquals(expected, comparator.compare(o1, o2));
+    }
+}