Index: trunk/test/unit/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelperTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelperTest.java	(revision 12619)
+++ trunk/test/unit/org/openstreetmap/josm/gui/dialogs/properties/TagEditHelperTest.java	(revision 12619)
@@ -0,0 +1,68 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.gui.dialogs.properties;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertFalse;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.stream.Collectors;
+
+import javax.swing.JTable;
+import javax.swing.table.DefaultTableModel;
+
+import org.junit.Rule;
+import org.junit.Test;
+import org.openstreetmap.josm.gui.tagging.ac.AutoCompletionListItem;
+import org.openstreetmap.josm.testutils.JOSMTestRules;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+/**
+ * Unit tests of {@link TagEditHelper} class.
+ */
+public class TagEditHelperTest {
+
+    /**
+     * Setup tests
+     */
+    @Rule
+    @SuppressFBWarnings(value = "URF_UNREAD_PUBLIC_OR_PROTECTED_FIELD")
+    public JOSMTestRules test = new JOSMTestRules();
+
+    private static TagEditHelper newTagEditHelper() {
+        DefaultTableModel propertyData = new DefaultTableModel();
+        JTable tagTable = new JTable(propertyData);
+        Map<String, Map<String, Integer>> valueCount = new HashMap<>();
+        return new TagEditHelper(tagTable, propertyData, valueCount);
+    }
+
+    /**
+     * Checks that autocompleting list items are sorted correctly.
+     */
+    @Test
+    public void testAcItemComparator() {
+        List<AutoCompletionListItem> list = new ArrayList<>();
+        list.add(new AutoCompletionListItem("Bing Sat"));
+        list.add(new AutoCompletionListItem("survey"));
+        list.add(new AutoCompletionListItem("Bing"));
+        list.add(new AutoCompletionListItem("digitalglobe"));
+        list.add(new AutoCompletionListItem("bing"));
+        list.add(new AutoCompletionListItem("DigitalGlobe"));
+        list.sort(TagEditHelper.DEFAULT_AC_ITEM_COMPARATOR);
+        assertEquals(Arrays.asList("Bing", "bing", "Bing Sat", "digitalglobe", "DigitalGlobe", "survey"),
+                list.stream().map(AutoCompletionListItem::getValue).collect(Collectors.toList()));
+    }
+
+    /**
+     * Unit test of {@link TagEditHelper#containsDataKey}.
+     */
+    @Test
+    public void testContainsDataKey() {
+        assertFalse(newTagEditHelper().containsDataKey("foo"));
+        // TODO: complete test
+    }
+}