Java 8 Stream API等效于嵌套的for循环

Question

Is it possible to replace the processing performed in this example with a single Java8 Stream API instruction, instead of nested for loops ?

 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

You can do it with Stream s, but even with Stream s you can't avoid the nested iteration, since you want to produce aList.size() * bList.size() instances of Bean .

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