Java 8 嵌套循环进行流式处理
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Java 8 nested loops to stream
提问by anton4o
Trying to get my head round the Java 8 streams syntax with a simple example. Had a look at the other similar questions on this topic, but could not find any solutions that would match my example and would work for me. Basically I am trying to refactor the following snippet with two nested loops to use the new stream API:
试图通过一个简单的示例来了解 Java 8 流语法。查看了有关此主题的其他类似问题，但找不到与我的示例相匹配并且对我有用的任何解决方案。基本上，我试图用两个嵌套循环重构以下代码段以使用新的流 API：
List<Car> filteredCars = new ArrayList<>();
for (Car car : cars) {
for (Wheel wheel : wheels) {
if (car.getColor() == wheel.getColor() &&
wheel.isWorking() == true ) {
filteredCars.add(car);
break;
}
}
}
return filteredCars;
Managed to come up with this which returns void:
设法想出这个返回无效：
return cars.stream().forEach(
car -> wheels.stream()
.filter(wheel -> wheel.getColor() == car.getColor() &&
wheel.isWorking() == true)
.collect(Collectors.toList()));
What is wrong with the stream syntax above and what am I missing?
上面的流语法有什么问题，我错过了什么？
采纳答案by Eran
You can't perform two terminal operations - forEachand collecton the same Stream.
您不能执行两个终端操作 -forEach并且collect在同一个Stream.
instead, you need to filter the cars list by checking for each car if it has a matching working wheel :
相反，您需要通过检查每辆车是否有匹配的工作轮来过滤汽车列表：
List<Car> filteredCars =
cars.stream()
.filter (
car -> wheels.stream()
.anyMatch(wheel -> wheel.getColor() == car.getColor() &&
wheel.isWorking()))
.collect(Collectors.toList());
回答by fabian
The problem is, you're creating the List(s) inside the forEachand forEachreturns void. This would be the equivalent of the following for loop:
问题是，您正在和返回中创建List(s) 。这将等效于以下 for 循环：forEachforEachvoid
for (Car car : cars) {
List<Car> filteredCars = new ArrayList<>();
for (Wheel wheel : wheels) {
if (car.getColor() == wheel.getColor() &&
wheel.isWorking() == true ) {
filteredCars.add(car);
break;
}
}
}
return filteredCars; // whoops cannot be accessed (scope) !!!
You could use filteron the carsstream and collect the use collecton the filtered stream to achieve the desired results:
您可以filter在cars流上使用并收集collect对过滤流的使用以达到所需的结果：
Predicate<Car> carCheck = car -> wheels.stream().anyMatch(wheel -> car.getColor() == wheel.getColor() && wheel.isWorking());
List<Car> filteredCars = cars.stream().filter(carCheck).collect(Collectors.toList());
