DynamoDBMapper for conditional saves

I'm using DynamoDBMapper and would like to conditionally save if and only if the hashkey and range key combination does not exist. I know there are ways to use UUIDs to reduce the possibility of a collision but I would like to protect myself by using conditional saves.

I came across this article that uses DynamoDBSaveExpression however I'm not able to specify that the condition is "hashkey AND rangekey" cannot exist. The API specifies a withConditionalOperator method but I'm not able to see this in my class. I am using the latest aws java sdk also from here.

Any suggestions on how to conditionally save? Or what I may be doing incorrectly?