Java 8如何将流转换为数组大小？

Question

String[] stringArray = streamString.toArray(size -> new String[size]);

如何自动将大小作为流的大小？

Answer 1

该StreamAPI被形成围绕Spliterator其是迭代器的一种高级形式。这些可以报告某些特征，从而可以优化操作Stream。他们还可能报告预期的元素数量，无论是估计的还是精确的。如果A事先知道元素数量，Spliterator它将报告一个SIZED特征。

Stream给定封装操作，您可以使用以下方法测试关于a元素的知识：

public static <T> Stream<T> printProperties(String op, Stream<T> s) {
    System.out.print("characteristics after "+op+": ");
    Spliterator<T> sp=s.spliterator();
    int characteristics=sp.characteristics();
    if(characteristics==0) System.out.println("0");
    else {
        String str;
        for(;;) {
            int flag=Integer.highestOneBit(characteristics);
            switch(flag) {
                case ORDERED: str="ORDERED"; break;
                case DISTINCT: str="DISTINCT"; break;
                case SORTED: str="SORTED"; break;
                case SIZED: str="SIZED"; break;
                case NONNULL: str="NONNULL"; break;
                case IMMUTABLE: str="IMMUTABLE"; break;
                case CONCURRENT: str="CONCURRENT"; break;
                case SUBSIZED: str="SUBSIZED"; break;
                default: str=String.format("0x%X", flag);
            }
            characteristics-=flag;
            if(characteristics==0) break;
            System.out.append(str).append('|');
        }
        System.out.println(str);
    }
    return StreamSupport.stream(sp, s.isParallel());
}

您可以使用它来学习某些操作如何影响有关元素的知识。例如，当您在以下测试程序中使用此方法时：

Stream<Object> stream;
stream=printProperties("received from TreeSet", new TreeSet<>().stream() );
stream=printProperties("applying map", stream.map(x->x) );
stream=printProperties("applying distinct", stream.distinct() );
stream=printProperties("filtering", stream.filter(x->true) );
stream=printProperties("applying sort", stream.sorted() );
stream=printProperties("requesting unordered", stream.unordered() );

System.out.println();

stream=printProperties("received from varargs array", Stream.of("foo", "bar") );
stream=printProperties("applying sort", stream.sorted() );
stream=printProperties("applying map", stream.map(x->x) );
stream=printProperties("applying distinct", stream.distinct() );
stream=printProperties("requesting unordered", stream.unordered() );

System.out.println();

printProperties("ConcurrentHashMap.keySet().stream()",
    new ConcurrentHashMap<>().keySet().stream() );

它将打印：

characteristics after received from TreeSet: SIZED|ORDERED|SORTED|DISTINCT
characteristics after applying map: SIZED|ORDERED
characteristics after applying distinct: ORDERED|DISTINCT
characteristics after filtering: ORDERED|DISTINCT
characteristics after applying sort: ORDERED|SORTED|DISTINCT
characteristics after requesting unordered: SORTED|DISTINCT

characteristics after received from varargs array: SUBSIZED|IMMUTABLE|SIZED|ORDERED
characteristics after applying sort: SUBSIZED|SIZED|ORDERED|SORTED
characteristics after applying map: SUBSIZED|SIZED|ORDERED
characteristics after applying distinct: ORDERED|DISTINCT
characteristics after requesting unordered: DISTINCT

characteristics after ConcurrentHashMap.keySet().stream(): CONCURRENT|NONNULL|DISTINCT

正如JB Nizet解释的那样，如果流事先不知道大小，则必须使用一种策略来收集可能包括重新分配数组的元素。如文档所述：

…使用提供的生成器函数分配返回的数组，以及分区执行或调整大小可能需要的任何其他数组。