diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index 5d486d1..adb468c 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -1,3 +1,12 @@
+------------------------------------------------------------------------------
+  qJava 2.2.0 [2015.08.x]
+------------------------------------------------------------------------------
+
+  - API convenience extensions
+  - Improved memory reuse while iterating over QTable, QDictionary 
+    and QKeyedTable
+  - Introduce common interface for QTable and QKeyedTable: Table
+
 ------------------------------------------------------------------------------
   qJava 2.1.2 [2015.03.23]
 ------------------------------------------------------------------------------
diff --git a/pom.xml b/pom.xml
index 75c5d12..e0f7711 100644
--- a/pom.xml
+++ b/pom.xml
@@ -11,7 +11,7 @@
     <inceptionYear>2014</inceptionYear>
 
     <packaging>bundle</packaging>
-    <version>2.1.3-SNAPSHOT</version>
+    <version>2.2.0-SNAPSHOT</version>
 
     <organization>
         <name>exxeleron</name>
@@ -25,7 +25,7 @@
             <name>Apache License Version 2.0</name>
             <url>http://www.apache.org/licenses/</url>
             <comments>
-            Copyright (c) 2011-2014 Exxeleron GmbH
+            Copyright (c) 2011-2015 Exxeleron GmbH
         </comments>
         </license>
     </licenses>
diff --git a/src/main/java/com/exxeleron/qjava/QDictionary.java b/src/main/java/com/exxeleron/qjava/QDictionary.java
index 31c8628..97353be 100644
--- a/src/main/java/com/exxeleron/qjava/QDictionary.java
+++ b/src/main/java/com/exxeleron/qjava/QDictionary.java
@@ -73,6 +73,15 @@ public Object getKeys() {
     public Object getValues() {
         return values;
     }
+    
+    /**
+     * Gets a number of entries in {@link QDictionary}.
+     * 
+     * @return a number of entries
+     */
+    public int size() {
+        return length;
+    }
 
     /**
      * <p>
diff --git a/src/main/java/com/exxeleron/qjava/QKeyedTable.java b/src/main/java/com/exxeleron/qjava/QKeyedTable.java
index ab3ec39..b09ae2f 100644
--- a/src/main/java/com/exxeleron/qjava/QKeyedTable.java
+++ b/src/main/java/com/exxeleron/qjava/QKeyedTable.java
@@ -20,7 +20,7 @@
 /**
  * Represents a q keyed table type.
  */
-public class QKeyedTable implements Iterable<QKeyedTable.KeyValuePair> {
+public class QKeyedTable implements Iterable<QKeyedTable.KeyValuePair>, Table {
 
     private final QTable keys;
     private final QTable values;
@@ -64,6 +64,24 @@ public QTable getValues() {
         return values;
     }
 
+    /*
+     * (non-Javadoc)
+     * 
+     * @see com.exxeleron.qjava.Table#size()
+     */
+    public int getRowsCount() {
+        return keys.getRowsCount();
+    }
+
+    /*
+     * (non-Javadoc)
+     * 
+     * @see com.exxeleron.qjava.Table#getColumnsCount()
+     */
+    public int getColumnsCount() {
+        return keys.getColumnsCount() + values.getColumnsCount();
+    }
+
     /**
      * <p>
      * Returns an iterator over a key/value pairs stored in the keyed table.
diff --git a/src/main/java/com/exxeleron/qjava/QTable.java b/src/main/java/com/exxeleron/qjava/QTable.java
index d07f7eb..a4595a1 100644
--- a/src/main/java/com/exxeleron/qjava/QTable.java
+++ b/src/main/java/com/exxeleron/qjava/QTable.java
@@ -24,7 +24,7 @@
 /**
  * Represents a q table type.
  */
-public final class QTable implements Iterable<QTable.Row> {
+public final class QTable implements Iterable<QTable.Row>, Table {
 
     private final String[] columns;
     private final Object[] data;
@@ -68,29 +68,25 @@ public QTable(final String[] columns, final Object[] data) {
     }
 
     /**
-     * Gets a column index for specified name.
+     * Gets a column index for specified name or <code>null</code> if column doesn't exist in the table.
      * 
      * @param column
      *            Name of the column
-     * @return 0 based column index
+     * @return 0 based column index, <code>null</code> if column with given name is not defined
      */
-    public int getColumnIndex( final String column ) {
+    public Integer getColumnIndex( final String column ) {
         return columnsMap.get(column);
     }
 
-    /**
-     * Gets a number of rows in current {@link QTable}.
-     * 
-     * @return a number of rows
+    /* (non-Javadoc)
+     * @see com.exxeleron.qjava.Table#size()
      */
     public int getRowsCount() {
         return rowsCount;
     }
 
-    /**
-     * Gets a number of columns in current {@link QTable}.
-     * 
-     * @return a number of columns
+    /* (non-Javadoc)
+     * @see com.exxeleron.qjava.Table#getColumnsCount()
      */
     public int getColumnsCount() {
         return columns.length;
diff --git a/src/main/java/com/exxeleron/qjava/Table.java b/src/main/java/com/exxeleron/qjava/Table.java
new file mode 100644
index 0000000..853fa00
--- /dev/null
+++ b/src/main/java/com/exxeleron/qjava/Table.java
@@ -0,0 +1,35 @@
+/**
+ *  Copyright (c) 2011-2015 Exxeleron GmbH
+ * 
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ * 
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package com.exxeleron.qjava;
+
+/**
+ * Common interface for tables.
+ */
+public interface Table {
+
+    /**
+     * Gets a number of rows in current {@link QKeyedTable}.
+     * 
+     * @return a number of rows
+     */
+    public abstract int getRowsCount();
+
+    /**
+     * @return a number of columns
+     */
+    public abstract int getColumnsCount();
+
+}
\ No newline at end of file
diff --git a/src/test/java/com/exxeleron/qjava/TestCollections.java b/src/test/java/com/exxeleron/qjava/TestCollections.java
new file mode 100644
index 0000000..8821af6
--- /dev/null
+++ b/src/test/java/com/exxeleron/qjava/TestCollections.java
@@ -0,0 +1,149 @@
+/**
+ *  Copyright (c) 2011-2015 Exxeleron GmbH
+ *
+ *  Licensed under the Apache License, Version 2.0 (the "License");
+ *  you may not use this file except in compliance with the License.
+ *  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing, software
+ *  distributed under the License is distributed on an "AS IS" BASIS,
+ *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *  See the License for the specific language governing permissions and
+ *  limitations under the License.
+ */
+package com.exxeleron.qjava;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertArrayEquals;
+
+import java.lang.reflect.Array;
+import java.util.Iterator;
+
+import org.junit.Test;
+
+import com.exxeleron.qjava.QKeyedTable.KeyValuePair;
+import com.exxeleron.qjava.QTable.Row;
+
+public class TestCollections {
+
+    private static QTable getTestTable() {
+        final String[] columns = new String[] { "f", "i", "s" };
+        final Object[] data = new Object[] { new double[] { -1.1, 0.0, 10.32 }, new int[] { 10, 0, -2 }, new String[] { "foo", "bar", "" } };
+        final QTable table = new QTable(columns, data);
+        return table;
+    }
+
+    @Test
+    public void testQTable() {
+        final QTable t = getTestTable();
+
+        assertEquals(null, t.getColumnIndex("unknown"));
+        assertEquals(Integer.valueOf(0), t.getColumnIndex("f"));
+
+        assertEquals(t, t);
+        assertEquals(t, getTestTable());
+
+        int i = 0;
+        final Iterator<Row> it = t.iterator();
+
+        while ( it.hasNext() ) {
+            final Row row = it.next();
+            final Iterator<Object> cit = row.iterator();
+            int j = 0;
+
+            while ( cit.hasNext() ) {
+                final Object v = cit.next();
+                assertEquals(t.get(i).get(j), v);
+                j++;
+            }
+
+            assertEquals(t.getColumnsCount(), j);
+            i++;
+        }
+
+        assertEquals(t.getRowsCount(), i);
+    }
+
+    @Test
+    public void testQKeyedTable() {
+        final QKeyedTable t = new QKeyedTable(getTestTable(), getTestTable());
+
+        assertEquals(t, t);
+        assertEquals(t.getKeys(), getTestTable());
+        assertEquals(t.getValues(), getTestTable());
+
+        int i = 0;
+        final Iterator<KeyValuePair> it = t.iterator();
+
+        while ( it.hasNext() ) {
+            final KeyValuePair kv = it.next();
+
+            Iterator<Object> cit = kv.getKey().iterator();
+            int j = 0;
+
+            while ( cit.hasNext() ) {
+                final Object v = cit.next();
+                assertEquals(t.getKeys().get(i).get(j), v);
+                j++;
+            }
+
+            cit = kv.getValue().iterator();
+            j = 0;
+
+            while ( cit.hasNext() ) {
+                final Object v = cit.next();
+                assertEquals(t.getValues().get(i).get(j), v);
+                j++;
+            }
+
+            i++;
+        }
+
+        assertEquals(t.getRowsCount(), i);
+    }
+
+    @Test
+    public void testQDictionary() {
+        final String[] keys = new String[] { "foo", "bar", "z" };
+        final Object[] values = new Object[] { 1, "val", null };
+
+        QDictionary d = new QDictionary(keys, values);
+        assertEquals(d, d);
+        assertEquals(d, new QDictionary(keys, values));
+
+        int i = 0;
+        Iterator<com.exxeleron.qjava.QDictionary.KeyValuePair> it = d.iterator();
+
+        while ( it.hasNext() ) {
+            com.exxeleron.qjava.QDictionary.KeyValuePair kv = it.next();
+
+            assertEquals(Array.get(d.getKeys(), i), kv.getKey());
+            assertEquals(Array.get(d.getValues(), i), kv.getValue());
+
+            i++;
+        }
+
+        assertEquals(d.size(), i);
+
+        d = new QDictionary(keys, getTestTable());
+        assertEquals(d, d);
+        assertEquals(d, new QDictionary(keys, getTestTable()));
+
+        i = 0;
+        it = d.iterator();
+
+        while ( it.hasNext() ) {
+            com.exxeleron.qjava.QDictionary.KeyValuePair kv = it.next();
+
+            assertEquals(Array.get(d.getKeys(), i), kv.getKey());
+            assertArrayEquals(((QTable) d.getValues()).get(i).toArray(), ((Row) kv.getValue()).toArray());
+
+            i++;
+        }
+
+        assertEquals(d.size(), i);
+    }
+
+}