In spark MLlib, How do i convert string to integer in spark scala?

StringIndexer is the solution. It will fit into the ML pipeline with an estimator and transformer. Essentially once you set the input column, it computes the frequency of each category and numbers them starting 0. You can add IndexToString at the end of pipeline to replace by original strings if required.

You can look at ML Documentation for "Estimating, transforming and selecting features" for further details.

In your case it will go like:

import org.apache.spark.ml.feature.StringIndexer 

val indexer = new StringIndexer().setInputCol("productID").setOutputCol("productIndex") 
val indexed = indexer.fit(df).transform(df)
indexed.show()