Index: trunk/src/org/openstreetmap/josm/gui/mappaint/StyleCache.java
===================================================================
--- trunk/src/org/openstreetmap/josm/gui/mappaint/StyleCache.java	(revision 9769)
+++ trunk/src/org/openstreetmap/josm/gui/mappaint/StyleCache.java	(revision 9770)
@@ -82,3 +82,11 @@
         return internPool.putUnique(this);
     }
+
+    /**
+     * Get the size of the intern pool. Only for tests!
+     * @return size of the intern pool
+     */
+    public static int getInternPoolSize() {
+        return internPool.size();
+    }
 }
Index: trunk/test/functional/org/openstreetmap/josm/gui/mappaint/StyleCacheTest.java
===================================================================
--- trunk/test/functional/org/openstreetmap/josm/gui/mappaint/StyleCacheTest.java	(revision 9770)
+++ trunk/test/functional/org/openstreetmap/josm/gui/mappaint/StyleCacheTest.java	(revision 9770)
@@ -0,0 +1,85 @@
+// License: GPL. For details, see LICENSE file.
+package org.openstreetmap.josm.gui.mappaint;
+
+import static org.junit.Assert.assertEquals;
+
+import java.awt.Color;
+import java.awt.Graphics2D;
+import java.awt.image.BufferedImage;
+import java.io.File;
+import java.io.InputStream;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+import org.openstreetmap.josm.JOSMFixture;
+import org.openstreetmap.josm.Main;
+import org.openstreetmap.josm.data.Bounds;
+import org.openstreetmap.josm.data.osm.DataSet;
+import org.openstreetmap.josm.data.osm.visitor.paint.Rendering;
+import org.openstreetmap.josm.data.osm.visitor.paint.StyledMapRenderer;
+import org.openstreetmap.josm.gui.NavigatableComponent;
+import org.openstreetmap.josm.gui.progress.NullProgressMonitor;
+import org.openstreetmap.josm.io.Compression;
+import org.openstreetmap.josm.io.OsmReader;
+
+/**
+ * Test {@link StyleCache}.
+ *
+ * Verifies, that the intern pool is not growing when repeatedly rendering the
+ * same set of primitives (and clearing the calculated styles each time).
+ *
+ * If it grows, this is an indication that the {@code equals} and {@code hashCode}
+ * implementation is broken and two identical objects are not recognized as equal
+ * or produce different hash codes.
+ *
+ * The opposite problem (different objects are mistaken as equal) has more visible
+ * consequences for the user (wrong rendering on the map) and is not recognized by
+ * this test.
+ */
+public class StyleCacheTest {
+
+    private static final int IMG_WIDTH = 1400;
+    private static final int IMG_HEIGHT = 1050;
+
+    private static Graphics2D g;
+    private static BufferedImage img;
+    private static NavigatableComponent nc;
+    private static DataSet dsCity;
+
+    @BeforeClass
+    public static void load() throws Exception {
+        JOSMFixture.createPerformanceTestFixture().init(true);
+        img = new BufferedImage(IMG_WIDTH, IMG_HEIGHT, BufferedImage.TYPE_INT_ARGB);
+        g = (Graphics2D) img.getGraphics();
+        g.setClip(0, 0, IMG_WIDTH, IMG_WIDTH);
+        g.setColor(Color.BLACK);
+        g.fillRect(0, 0, IMG_WIDTH, IMG_WIDTH);
+        nc = Main.map.mapView;
+        nc.setBounds(0, 0, IMG_WIDTH, IMG_HEIGHT);
+
+        MapPaintStyles.readFromPreferences();
+        try (
+            InputStream fisC = Compression.getUncompressedFileInputStream(new File("data_nodist/neubrandenburg.osm.bz2"));
+        ) {
+            dsCity = OsmReader.parseDataSet(fisC, NullProgressMonitor.INSTANCE);
+        }
+    }
+
+    @Test
+    public void testStyleCacheInternPool() throws Exception {
+        Bounds bounds = new Bounds(53.56, 13.25, 53.57, 13.26);
+        Rendering visitor = new StyledMapRenderer(g, nc, false);
+        nc.zoomTo(bounds);
+        Integer internPoolSize = null;
+        for (int i = 0; i < 10; i++) {
+            visitor.render(dsCity, true, bounds);
+            MapPaintStyles.getStyles().clearCached();
+            int newInternPoolSize = StyleCache.getInternPoolSize();
+            if (internPoolSize == null) {
+                internPoolSize = newInternPoolSize;
+            } else {
+                assertEquals("intern pool size", internPoolSize.intValue(), newInternPoolSize);
+            }
+        }
+    }
+}