public int count(DynamoDBQueryExpression<T> queryExpression) {
        return mapper.<T>count(model.targetType(), queryExpression);
    }
Evaluates the specified query expression and returns the count of matching
items, without returning any of the actual item data
@param queryExpression The query expression.
@return The count.
@see com.amazonaws.services.dynamodbv2.datamodeling.DynamoDBMapper#count
aws-java-sdk-dynamodb/src/main/java/com/amazonaws/services/dynamodbv2/datamodeling/DynamoDBTableMapper.java