将嵌套的For循环转换为流

问题描述

我在理解流时遇到了一些麻烦.我环顾四周，似乎找不到与我的用例匹配的示例.

I'm having some trouble understanding streams. I've looked around and can't seem to find an example that matches my use case.

我有一个嵌套的for循环:

I have an existing nested for loop:

List<ObjectB> objectBs = new ArrayList<ObjectB>();
for (ObjectA objA: objectAList) {
    for (ObjectB objB: objA.getObjectBList()) {
        if (objB.getNumber() != 2) {
            objectBs.add(objB);
        }
    }
}

大量示例展示了如何将objB.getNumber()添加到列表中，而不是objB.

Alot of exampls show how to add objB.getNumber() to a list but not objB.

推荐答案

您可以使用flatMap获取所有ObjectB实例的Stream<ObjectB>并过滤所需编号的ObjectB: /p>

You can use flatMap to obtain a Stream<ObjectB> of all the ObjectB instances and filter the ObjectB's of the required number :

List<ObjectB> objectBs =
    objectAList.stream()
               .flatMap (a -> a.getObjectBList().stream())
               .filter (b -> b.getNumber() != 2)
               .collect (Collectors.toList());

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