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For Doers

The Java Stream API: Primitive Streams

2018-10-21

The sample code for this post can be found on GitHub.

While Streams in Java would typically be used on POJO or POJO-like data structures, Java also lets us deal directly with primitive type streams. Thanks to Java's type erasure, something like Stream<int> would not work, as the type argument must be an Object.

However, the smaller and more focused primitive type streams are still quite useful. We can get a summary statistics of any IntStream like so:

@Test
public void intStream_simpleEx() {
    IntStream intStream = IntStream.of(1,4,5,8,9,10);

    IntSummaryStatistics stats = intStream.summaryStatistics();

    assertEquals(10, stats.getMax());
}

We can do the same with a DoubleStream:

@Test
public void doubleStream_simpleEx() {
    DoubleStream doubleStream = DoubleStream.of(1.0, 2.5, 3.5, 6.5, 8.0);

    DoubleSummaryStatistics stats = doubleStream.summaryStatistics();

    assertEquals(5, stats.getCount());
    assertEquals(1.0, stats.getMin(), .01);
}

And with a LongStream:

@Test
public void longStream_simpleEx() {
    LongStream longStream = LongStream.of(100, 101, 102, 103);

    long[] longs = longStream.filter(l -> l >= 101).toArray();

    assertEquals(longs[0], 101);
}

We can turn a Stream<?> into a primitive stream by calling the corresponding mapping function, e.g. mapToInt(..). Here, we'll map a Stream<SimplePair>, where the SimplePair object just has a name (String) and id (int) into an IntStream:

@Test
public void mapToIntStream() {
    IntStream nameLengths = pairs.stream().mapToInt(sp -> sp.getName().length());

    IntSummaryStatistics stats = nameLengths.summaryStatistics();

    assertEquals(5, stats.getMin());
}

We can go the other way any time we want to by calling boxed(). Here, we will move a primitive IntStream into a Stream<Integer>:

@Test
public void intStream_mapToObjectStream() {
    IntStream intStream = IntStream.of(1, 1, 3, 3, 4);
    Stream<Integer> boxed = intStream.boxed();

    Optional<Integer> mathResult = boxed.reduce((first, second) -> first + 2 * second);

    assertEquals(1 + 2 + 6 + 6 + 8, mathResult.orElseThrow(RuntimeException::new).intValue());
}

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