[BAEL-2959] A Guide to MapDB code (#7042)

* [BAEL-2959] A Guide to MapDB code

* [BAEL-2959] fixes

* [BAEL-2959] made VOLUME_LOCATION static in SortedTableMapUnitTest

* moved mapdb files from libraries to libraries-2

* [BAEL-2959] formatting fixes

* updated pom.xml

libraries-2/src/test/java/com/baeldung/mapdb/HTreeMapUnitTest.java

@@ -0,0 +1,38 @@
+package com.baeldung.mapdb;
+
+import org.jetbrains.annotations.NotNull;
+import org.junit.Test;
+import org.mapdb.*;
+
+import java.io.IOException;
+
+import static junit.framework.Assert.assertEquals;
+
+public class HTreeMapUnitTest {
+
+    @Test
+    public void givenValidDB_whenHTreeMapAddedToAndRetrieved_CheckedRetrievalCorrect() {
+
+        DB db = DBMaker.memoryDB().make();
+
+        HTreeMap<String, String> hTreeMap = db
+          .hashMap("myTreMap")
+          .keySerializer(Serializer.STRING)
+          .valueSerializer(Serializer.STRING)
+          .create();
+
+        hTreeMap.put("key1", "value1");
+        hTreeMap.put("key2", "value2");
+
+        assertEquals(2, hTreeMap.size());
+
+        //add another value with the same key
+
+        hTreeMap.put("key1", "value3");
+
+        assertEquals(2, hTreeMap.size());
+        assertEquals("value3", hTreeMap.get("key1"));
+
+    }
+
+}