Generating a short[] using streams

问题

Building upon Populating a List with a contiguous range of shorts I tried generating an array of primitive shorts. This turned out to be surprisingly harder than expected.

Short[] range = IntStream.range(0, 500).mapToObj(value -> (short) value).toArray(Short[]::new) worked but:

short[] range = IntStream.range(0, 500).mapToObj(value -> (short) value).toArray(short[]::new) generated a compiler error:

method toArray in interface Stream<T> cannot be applied to given types;
  required: IntFunction<A[]>
  found: short[]::new
  reason: inference variable A has incompatible bounds
    equality constraints: short
    upper bounds: Object
  where A,T are type-variables:
    A extends Object declared in method <A>toArray(IntFunction<A[]>)
    T extends Object declared in interface Stream

This seems to be an intersection of two problems:

1. The primitive Stream APIs do not provide an implementation for shorts.
2. The non-primitive Stream APIs do not seem to provide a mechanism to return a primitive array.

Any ideas?

回答1:

You may consider using my StreamEx library. It extends standand streams with additional methods. One of the goals of my library is better interoperation with old code. In particular it has IntStreamEx.toShortArray() and IntStreamEx.of(short...):

short[] numbers = IntStreamEx.range(500).toShortArray();
short[] evenNumbers = IntStreamEx.of(numbers).map(x -> x*2).toShortArray();

Note that it's still the stream of int numbers. When calling toShortArray(), they are automatically converted to short type using (short) cast operation, thus overflow is possible. So use with care.

There are also IntStreamEx.toByteArray(), IntStreamEx.toCharArray(), and DoubleStreamEx.toFloatArray().

回答2:

The canonical way would be implementing a custom Collector.

class ShortCollector {
    public static Collector<Integer,ShortCollector,short[]> TO_ARRAY
        =Collector.of(ShortCollector::new, ShortCollector::add,
                      ShortCollector::merge, c->c.get());

    short[] array=new short[100];
    int pos;

    public void add(int value) {
        int ix=pos;
        if(ix==array.length) array=Arrays.copyOf(array, ix*2);
        array[ix]=(short)value;
        pos=ix+1;
    }
    public ShortCollector merge(ShortCollector c) {
        int ix=pos, cIx=c.pos, newSize=ix+cIx;
        if(array.length<newSize) array=Arrays.copyOf(array, newSize);
        System.arraycopy(c.array, 0, array, ix, cIx);
        return this;
    }
    public short[] get() {
        return pos==array.length? array: Arrays.copyOf(array, pos);
    }
}

Then you could use it like

short[] array=IntStream.range(0, 500).boxed().collect(ShortCollector.TO_ARRAY);

The drawback is that Collectors only work for reference types (as Generics doesn’t support primitive types), thus you have to resort to boxed() and collectors can’t utilize information about the number of elements (if ever available). Thus, the performance is likely to be far worse than toArray() on a primitive data stream.

So, a solution striving for higher performance (I limit this to the single threaded case) will look like this:

public static short[] toShortArray(IntStream is) {
    Spliterator.OfInt sp = is.spliterator();
    long l=sp.getExactSizeIfKnown();
    if(l>=0) {
        if(l>Integer.MAX_VALUE) throw new OutOfMemoryError();
        short[] array=new short[(int)l];
        sp.forEachRemaining(new IntConsumer() {
            int ix;
            public void accept(int value) {
                array[ix++]=(short)value;
            }
        });
        return array;
    }
    final class ShortCollector implements IntConsumer {
        int bufIx, currIx, total;
        short[][] buffer=new short[25][];
        short[] current=buffer[0]=new short[64];

        public void accept(int value) {
            int ix = currIx;
            if(ix==current.length) {
                current=buffer[++bufIx]=new short[ix*2];
                total+=ix;
                ix=0;
            }
            current[ix]=(short)value;
            currIx=ix+1;
        }
        short[] toArray() {
            if(bufIx==0)
                return currIx==current.length? current: Arrays.copyOf(current, currIx);
            int p=0;
            short[][] buf=buffer;
            short[] result=new short[total+currIx];
            for(int bIx=0, e=bufIx, l=buf[0].length; bIx<e; bIx++, p+=l, l+=l)
                System.arraycopy(buf[bIx], 0, result, p, l);
            System.arraycopy(current, 0, result, p, currIx);
            return result;
        }
    }
    ShortCollector c=new ShortCollector();
    sp.forEachRemaining(c);
    return c.toArray();
}

You may use it like

short[] array=toShortArray(IntStream.range(0, 500));

来源：https://stackoverflow.com/questions/30790665/generating-a-short-using-streams