Java 8 Stream API 等效于嵌套 for 循环

Question

是否可以用单个 Java8 Stream API 指令代替嵌套的 for 循环来替换此示例中执行的处理？

 public static void main(String[] args) {
    List<BeanA> aList = Arrays.asList(new BeanA(), new BeanA(), new BeanA());
    List<BeanB> bList = Arrays.asList(new BeanB(), new BeanB(), new BeanB());

    List<Bean> result = new ArrayList<>();
    for (BeanA a : aList) {
        for (BeanB b : bList) {
            result.add(new Bean(a, b));
        }
    }
    System.out.println("size:" + result.size());
    System.out.println("result:" + result);
}

public static class BeanA {
}

public static class BeanB {
}

public static class Bean {
    private BeanA a;
    private BeanB b;

    public Bean(BeanA a, BeanB b) {
        this.a = a;
        this.b = b;
    }
}

Answer 1

您可以使用 Stream 来实现，但即使使用 Stream 也无法避免嵌套迭代，因为您想要生成 aList.size () * bList.size() Bean 的实例。

List<Bean> result =
    aList.stream()
         .flatMap(a->bList.stream().map(b->new Bean(a,b)))
         .collect(Collectors.toList());