Index: trunk/test/unit/org/openstreetmap/josm/actions/MergeLayerActionTest.java
===================================================================
--- trunk/test/unit/org/openstreetmap/josm/actions/MergeLayerActionTest.java	(revision 11885)
+++ trunk/test/unit/org/openstreetmap/josm/actions/MergeLayerActionTest.java	(revision 11885)
@@ -0,0 +1,81 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.actions;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertNull;
+
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.openstreetmap.josm.Main;
+import org.openstreetmap.josm.data.osm.DataSet;
+import org.openstreetmap.josm.gui.layer.LayerManagerTest.TestLayer;
+import org.openstreetmap.josm.gui.layer.OsmDataLayer;
+import org.openstreetmap.josm.testutils.JOSMTestRules;
+
+import edu.umd.cs.findbugs.annotations.SuppressFBWarnings;
+
+/**
+ * Unit tests for class {@link MergeLayerAction}.
+ */
+public class MergeLayerActionTest {
+
+    /**
+     * Setup test.
+     */
+    @Rule
+    @SuppressFBWarnings(value = "URF_UNREAD_PUBLIC_OR_PROTECTED_FIELD")
+    public JOSMTestRules test = new JOSMTestRules().platform().commands();
+
+    private MergeLayerAction action;
+
+    /**
+     * Setup test.
+     */
+    @Before
+    public void setUp() {
+        if (action == null) {
+            action = new MergeLayerAction();
+        }
+        for (TestLayer testLayer : Main.getLayerManager().getLayersOfType(TestLayer.class)) {
+            Main.getLayerManager().removeLayer(testLayer);
+        }
+    }
+
+    /**
+     * Tests that no error occurs when no source layer exists.
+     */
+    @Test
+    public void testMergeNoSourceLayer() {
+        assertNull(Main.getLayerManager().getActiveLayer());
+        action.actionPerformed(null);
+        assertEquals(0, Main.getLayerManager().getLayers().size());
+    }
+
+    /**
+     * Tests that no error occurs when no target layer exists.
+     */
+    @Test
+    public void testMergeNoTargetLayer() {
+        OsmDataLayer layer = new OsmDataLayer(new DataSet(), "", null);
+        Main.getLayerManager().addLayer(layer);
+        assertEquals(1, Main.getLayerManager().getLayers().size());
+        assertNull(action.merge(layer));
+        assertEquals(1, Main.getLayerManager().getLayers().size());
+    }
+
+    /**
+     * Tests that the merge is done with two empty layers.
+     * @throws Exception if any error occurs
+     */
+    @Test
+    public void testMergeTwoEmptyLayers() throws Exception {
+        OsmDataLayer layer1 = new OsmDataLayer(new DataSet(), "1", null);
+        OsmDataLayer layer2 = new OsmDataLayer(new DataSet(), "2", null);
+        Main.getLayerManager().addLayer(layer1);
+        Main.getLayerManager().addLayer(layer2);
+        assertEquals(2, Main.getLayerManager().getLayers().size());
+        action.merge(layer2).get();
+        assertEquals(1, Main.getLayerManager().getLayers().size());
+    }
+}