具有多个属性的Java流

问题描述：

流中有以下对象:

class Foo{
    String a;
    String b;
    int c;
}

我想根据以下条件过滤视频流:

I would like to filter a stream based on following criteria:

例如流中有条目:foo1和foo2:

foo1和foo2的a和b值相同，但是c属性不同.

foo1 and foo2 have same values for a and b, but they differ in c property.

在这种情况下，我希望删除那些c更高的条目.

I would like to get rid of entries that have c higher in such case.

答

在语义上等同于 Eugene的答案，但有点更简单:

Semantically equivalent to Eugene’s answer, but a bit simpler:

List<Foo> foos = Stream.of(new Foo("a", "b", 1), new Foo("a", "b", 2),
                 new Foo("a", "b", 3), new Foo("a", "bb", 3), new Foo("aa", "b", 3))
    .collect(Collectors.collectingAndThen(
        Collectors.toMap(x -> Arrays.asList(x.getA(), x.getB()), x -> x,
                         BinaryOperator.minBy(Comparator.comparing(Foo::getC))),
            map -> new ArrayList<>(map.values())));

您需要按同时具有这两个属性的键进行分组，并且由于缺少标准的Pair类型，因此可以使用具有两个元素的List或Map.Entry，两者都可以工作.但是使用List更简单(在Java 9中，您会使用List.of(…, …)甚至更简单)并且如果两个属性中可能出现相同的值，则哈希码更好.

You need to group by a key holding both properties and due to the absence of a standard Pair type, you may use a List with two elements or a Map.Entry, both work. But using List is simpler (in Java 9, you would use List.of(…, …) which is even simpler) and has a better hash code if the same values may occur in both properties.

当dowstream操作纯粹是简化操作时(例如选择C属性的最小值)，toMap收集器更适合，因为它不需要处理Optional.

When the dowstream operation is a pure reduction, like selecting the minimum of the C property, the toMap collector fits better as it doesn’t require dealing with Optional.

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