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Java 8 - Streams - Transformations - Flat Map


  • when a transformation on a stream results in a stream of stream, we use flatMap to flatten the result back to a stream.
  • There is also flatMapToInt, flatMapToDouble and flatMapToLong which yield IntStream, DoubleStream and LongStream respectively.

integer stream one to three
Stream<Integer> oneToThree = Stream.of(1, 2, 3);

integer stream one to two
Stream<Integer> oneToTwo = Stream.of(1, 2);

integer to stream function
Function<Integer, Stream> integerToStreamFunction = i -> IntStream.range(i, 5).boxed();

 integer to double stream function
Function<Integer, DoubleStream> integerToDoubleStreamFunction = i -> DoubleStream.of(i);



 USAGE

 log.info("verifying that flatMap operation flattens a stream of integer stream to an integer stream: ");

oneToThree.flatMap(number -> integerToStreamFunction.apply(number)).forEach(number -> log.info(number.toString()));


log.info("verifying that flatMapToDouble operation flattens a stream of integer stream to a double stream: ");

oneToTwo.flatMapToDouble(number -> integerToDoubleStreamFunction.apply(number)) .forEach(number -> log.info(""+number));


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