### Question:
MongoStore extends StoreBase { @Override public String[] keys() throws IOException { List<String> keys = new ArrayList<String>(); BasicDBObject sessionKeyQuery = new BasicDBObject(); sessionKeyQuery.put(appContextProperty, this.getName()); DBCursor mongoSessionKeys = this.collection.find(sessionKeyQuery, new BasicDBObject("_id", 1)); while(mongoSessionKeys.hasNext()) { String id = mongoSessionKeys.next().get("_id").toString(); keys.add(id); } return keys.toArray(new String[keys.size()]); }  @Override int getSize(); @Override String[] keys(); @Override Session load(String id); @Override void remove(String id); @Override void clear(); @Override void save(Session session); @Override String getStoreName(); String getConnectionUri(); void setConnectionUri(String connectionUri); String getHosts(); void setHosts(String hosts); String getDbName(); void setDbName(String dbName); String getCollectionName(); void setCollectionName(String collectionName); String getUsername(); void setUsername(String username); String getPassword(); void setPassword(String password); int getConnectionTimeoutMs(); void setConnectionTimeoutMs(int connectionTimeoutMs); int getConnectionWaitTimeoutMs(); void setConnectionWaitTimeoutMs(int connectionWaitTimeoutMs); int getMinPoolSize(); void setMinPoolSize(int minPoolSize); int getMaxPoolSize(); void setMaxPoolSize(int maxPoolSize); String getReplicaSet(); void setReplicaSet(String replicaSet); boolean isUseSecureConnection(); void setUseSecureConnection(boolean useSecureConnection); boolean isUseSlaves(); void setUseSlaves(boolean useSlaves); WriteConcern getWriteConcern(); void setWriteConcern(WriteConcern writeConcern); int getTimeToLive(); void setTimeToLive(int timeToLive);  }### Answer:
@Test public void testKeys() throws Exception { String[] keys = this.mongoStore.keys(); assertNotNull(keys); }