Transforms can have a nested structure, where a complex transform performs multiple simpler transforms (such as more than one ParDo, Combine, GroupByKey, or even other composite transforms). These transforms are called composite transforms. Nesting multiple transforms inside a single composite transform can make your code more modular and easier to understand.
To create your own composite transform, create a subclass of the PTransform class and override the expand method to specify the actual processing logic. You can then use this transform just as you would a built-in transform from the Beam SDK. Within your PTransform subclass, you’ll need to override the expand method. The expand method is where you add the processing logic for the PTransform. Your override of expand must accept the appropriate type of input PCollection as a parameter, and specify the output PCollection as the return value.
Kata: Please implement a composite transform “ExtractAndMultiplyNumbers” that extracts numbers from comma separated line and then multiplies each number by 10.