Mapping between Numeric Streams – Streams

Mapping between Numeric Streams

In contrast to the methods in the Stream<T> interface, the map() and the flatMap() methods of the numeric stream interfaces transform a numeric stream to a numeric stream of the same primitive type; that is, they do not change the type of the numeric stream.

The map() operation in the stream pipeline below does not change the type of the initial IntStream.

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IntStream.rangeClosed(1, 3)                           // IntStream
         .map(i -> i * i)                             // IntStream
         .forEach(n -> System.out.printf(“%d “, n));  // 1 4 9

The flatMap() operation in the stream pipeline below also does not change the type of the initial stream. Each IntStream created by the mapper function is flattened, resulting in a single IntStream.

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IntStream.rangeClosed(1, 3)                           // IntStream
         .flatMap(i -> IntStream.rangeClosed(1, 4))   // IntStream
         .forEach(n -> System.out.printf(“%d “, n));  // 1 2 3 4 1 2 3 4 1 2 3 4

Analogous to the methods in the Stream<T> interface, the mapToNumType() methods in the numeric stream interfaces transform a numeric stream to a stream of the designated numeric type, where NumType is either Int, Long, or Double.

The mapToDouble() operation in the stream pipeline below transforms the initial IntStream into a DoubleStream.

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IntStream.rangeClosed(1, 3)                           // IntStream
         .mapToDouble(i -> Math.sqrt(i))              // DoubleStream
         .forEach(d -> System.out.printf(“%.2f “, d));// 1.00 1.41 1.73

The methods asLongStream() and asDoubleStream() in the IntStream interface transform an IntStream to a LongStream and a DoubleStream, respectively. Similarly, the method asDoubleStream() in the LongStream interface transforms a LongStream to a DoubleStream.

The asDoubleStream() operation in the stream pipeline below transforms the initial IntStream into a DoubleStream. Note how the range of int values is thereby transformed to a range of double values by the asDoubleStream() operation.

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IntStream.rangeClosed(1, 3)                           // IntStream
         .asDoubleStream()                            // DoubleStream
         .map(d -> Math.sqrt(d))                      // DoubleStream
         .forEach(d -> System.out.printf(“%.2f “, d));// 1.00 1.41 1.73

In the stream pipeline below, the int values in the IntStream are first boxed into Integers. In other words, the initial IntStream is transformed into an object stream, Stream<Integer>. The map() operation transforms the Stream<Integer> into a Stream<Double>. In contrast to using the asDoubleStream() in the stream pipeline above, note the boxing/unboxing that occurs in the stream pipeline below in the evaluation of the Math.sqrt() method, as this method accepts a double as a parameter and returns a double value.

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IntStream.rangeClosed(1, 3)                           // IntStream
         .boxed()                                     // Stream<Integer>
         .map(n -> Math.sqrt(n))                      // Stream<Double>
         .forEach(d -> System.out.printf(“%.2f “, d));// 1.00 1.41 1.73