MongoDB Manual recommends ensureIndex :

http://docs.mongodb.org/manual//tutorial/create-an-index/
http://docs.mongodb.org/manual//reference/method/db.collection.ensureIndex/

and deprecates createIndex since very long ago:

http://docs.mongodb.org/manual//reference/method/db.collection.createIndex/

db.collection.createIndex(keys, options)
Deprecated since version 1.8.

Creates indexes on collections.

Java API is the other way around, recommends createIndex:

http://api.mongodb.org/java/current/com/mongodb/DBCollection.html#createIndex(com.mongodb.DBObject) :

createIndex
public void createIndex(DBObject keys)
Calls createIndex(com.mongodb.DBObject, com.mongodb.DBObject) with default index options

and deprecates ensureIndex :

http://api.mongodb.org/java/current/com/mongodb/DBCollection.html#ensureIndex(com.mongodb.DBObject)

ensureIndex
@Deprecated
public void ensureIndex(DBObject keys)
Deprecated. use createIndex(DBObject) instead
Calls ensureIndex(com.mongodb.DBObject, com.mongodb.DBObject) with default options