public MapWritable() {
    super();
    this.instance = new HashMap<Writable, Writable>();
  }
@SuppressWarnings("unchecked")
  @Test
  public void testMapWritable() {
    Text[] keys = {
        new Text("key1"),
        new Text("key2"),
        new Text("Key3"),
    };
    
    BytesWritable[] values = {
        new BytesWritable("value1".getBytes()),
        new BytesWritable("value2".getBytes()),
        new BytesWritable("value3".getBytes())
    };

    MapWritable inMap = new MapWritable();
    for (int i = 0; i < keys.length; i++) {
      inMap.put(keys[i], values[i]);
    }

    MapWritable outMap = new MapWritable(inMap);
    assertEquals(inMap.size(), outMap.size());
    
    for (Map.Entry<Writable, Writable> e: inMap.entrySet()) {
      assertTrue(outMap.containsKey(e.getKey()));
      assertEquals(0, ((WritableComparable) outMap.get(e.getKey())).compareTo(
          e.getValue()));
    }
    
    // Now for something a little harder...
    
    Text[] maps = {
        new Text("map1"),
        new Text("map2")
    };
    
    MapWritable mapOfMaps = new MapWritable();
    mapOfMaps.put(maps[0], inMap);
    mapOfMaps.put(maps[1], outMap);
    
    MapWritable copyOfMapOfMaps = new MapWritable(mapOfMaps);
    for (int i = 0; i < maps.length; i++) {
      assertTrue(copyOfMapOfMaps.containsKey(maps[i]));
      MapWritable a = (MapWritable) mapOfMaps.get(maps[i]);
      MapWritable b = (MapWritable) copyOfMapOfMaps.get(maps[i]);
      assertEquals(a.size(), b.size());
      for (Writable key: a.keySet()) {
        assertTrue(b.containsKey(key));
        
        // This will work because we know what we put into each set
        
        WritableComparable aValue = (WritableComparable) a.get(key);
        WritableComparable bValue = (WritableComparable) b.get(key);
        assertEquals(0, aValue.compareTo(bValue));
      }
    }
  }