insert values into root node

block-storage/src/test/java/org/lucares/pdb/map/PersistentMapTest.java

@@ -26,17 +26,21 @@ public class PersistentMapTest {
 		FileUtils.delete(dataDirectory);
 	}
 
-	public void test() throws Exception {
+	public void testSingleValue() throws Exception {
 		final Path file = dataDirectory.resolve("map.db");
 
 		try (final DiskStorage ds = new DiskStorage(file)) {
 			final PersistentMap map = new PersistentMap(ds);
 
-			final String value1 = "value1";
-			map.put("key1", value1);
-			final String actualValue1 = map.getAsString("key1");
+			final String value = "value1";
+			final String key = "key1";
 
-			Assert.assertEquals(actualValue1, value1);
+			Assert.assertNull(map.getAsString(key));
+
+			Assert.assertNull(map.put(key, value));
+			final String actualValue = map.getAsString(key);
+
+			Assert.assertEquals(actualValue, value);
 		}
 	}
 }