Spark-Java:How to convert Dataset string column of

问题:

I have a Dataset with one column lastModified of type string with format "yyyy-MM-ddThh:mm:ss.SSS+0000" (sample data: 2018-08-17T19:58:46.000+0000).

I have to add a new column lastModif_mapped of type Timestamp by converting the lastModified's value to format "yyyy-MM-dd hh:mm:ss.SSS".

I tried the code below, but the new column is getting the value null in it:

Dataset<Row> filtered = null;
filtered = ds1.select(ds1.col("id"),ds1.col("lastmodified"))
                .withColumn("lastModif_mapped", functions.unix_timestamp(ds1.col("lastmodified"), "yyyy-MM-dd HH:mm:ss.SSS").cast("timestamp")).alias("lastModif_mapped");

Where am I going wrong?