使用 Java 流将 ArrayList<Integer> 转换为 Integer[][]

Question

我有一个 ArrayList，需要将其转换为二维数组。我需要使用 Java 流来实现这一点。private static ArrayList<Integer> GLOBALLIST;Integer[][] TwoDArray = new Integer[2][8];GLOBALLIST = Lists.newArrayList(36,40,44,48,52,56,60,64,100,104,108, 112,116,120,124,128);AtomicInteger counter = new AtomicInteger(0);        TwoDArray = (Integer[][]) ((GLOBALLIST.stream()                .collect(Collectors.groupingByConcurrent(it -> counter.getAndIncrement() / 8))                .values()).toArray(new Integer[2][8]));这是错误说明 ObjectList 无法转换为 Integer[][]

Answer 1

当您的起点是ArrayList，即List支持随机访问时，例如

ArrayList<Integer> list = new ArrayList<>(Arrays.asList(36,40,44,48,52,56,60,64,100,104,108,112,116,120,124,128,132,136,140,144,149,153,157,161,165,169,173,177));

你可以简单地使用

Integer[][] array = IntStream.range(0, (list.size()+7)/8)

    .mapToObj(ix -> list.subList(ix*=8, Math.min(ix+8,list.size())).toArray(new Integer[0]))

    .toArray(Integer[][]::new);

System.out.println(Arrays.deepToString(array));

[[36, 40, 44, 48, 52, 56, 60, 64], [100, 104, 108, 112, 116, 120, 124, 128], [132, 136, 140, 144, 149, 153, 157, 161], [165, 169, 173, 177]]

Answer 2

如果你使用番石榴，那么这里有一个解决方案：

ArrayList<Integer> GLOBALLIST;

    GLOBALLIST = Lists.newArrayList(36,40,44,48,52,56,60,64,100,104,108, 112,116,120,124,128,132,136,140,144,149,153,157,161,165,169,173,177);

    int[][] twoDList = Lists.partition(GLOBALLIST,8)

            .stream()

            .map(Ints::toArray)

            .map(a -> Arrays.copyOf(a, 8))

            .toArray(int[][]::new);

我在gradle 中使用了以下 guava 依赖项：

compile 'com.google.guava:guava:22.0'

Answer 3

随着轻微的修改到blockCollector在这个答案，你可以做这样的：

public static Integer[][] toArray2D(Collection<Integer> list, int blockSize) {

    return list.stream()

               .collect(blockCollector(blockSize))

               .stream()

               .map(sublist -> sublist.toArray(new Integer[sublist.size()]))

               .toArray(length -> new Integer[length][]);

}

public static <T> Collector<T, List<List<T>>, List<List<T>>> blockCollector(int blockSize) {

    return Collector.of(

            ArrayList<List<T>>::new,

            (list, value) -> {

                List<T> block = (list.isEmpty() ? null : list.get(list.size() - 1));

                if (block == null || block.size() == blockSize)

                    list.add(block = new ArrayList<>(blockSize));

                block.add(value);

            },

            (r1, r2) -> { throw new UnsupportedOperationException("Parallel processing not supported"); }

    );

}

测试

List<Integer> list = Arrays.asList(36,40,44,48,52,56,60,64,100,104,108, 112,116,120,124,128,132,136,140,144,149,153,157,161,165,169,173,177);

Integer[][] r = toArray2D(list, 8);

System.out.println(Arrays.deepToString(r));

输出

[[36, 40, 44, 48, 52, 56, 60, 64], [100, 104, 108, 112, 116, 120, 124, 128], [132, 136, 140, 144, 149, 153, 157, 161], [165, 169, 173, 177]]