a.java

PCollection<String> resultCollection = productCollection.apply(ParDo
       .of(new DoFn<String, String>() {
           @ProcessElement
           public void process(ProcessContext processContext) {
               double average = processContext.sideInput(averagePrice);
               String strings = processContext.element();
               assert strings!= null;
               String[] splits = strings.split(",");
               double price = Double.parseDouble(splits[3].trim());
               if (price >= average) {
                   processContext.output(String.join(",", splits[0], splits[1], splits[2], splits[3]));
               }
           }
       }).withSideInputs(averagePrice));