在Java 8中使用Lambda将流收集到HashMap中 [英] Collect stream into a HashMap with Lambda in Java 8

问题描述

我有一个HashMap，需要使用某些函数进行过滤:

I have a HashMap which I need to filter using some function:

HashMap<Set<Integer>, Double> container
Map.Entry<Set<Integer>, Double> map = container.entrySet()
            .stream()
            .filter(k -> k.getKey().size() == size)

对于size = 2，以下内容应有效:

For the size = 2 the following should be valid:

containerBeforeFilter = {1,2,3} -> 1.5, {1,2} -> 1.3
containerAfterFilter = {1,2} -> 1.3

在过滤器中应用了函数之后，我想再次将结果收集到HashMap中.但是，当我尝试应用建议的方法在这里，我收到非法声明.

After I applied the function in the filter, I want to collect results again into a HashMap. However, when I try to apply the method suggested here, I'm getting illegal statements.

因此，在过滤器之后应用的以下语句是非法的:

So the following statement, applied after the filter, is illegal:

.collect(Collectors.toMap((entry) -> entry.getKey(), (entry) -> entry.getValue()));

在唯一的标准是满足某些键的情况下，收集不变的地图值的正确方法是什么?

What would be the proper way of collecting unchanged map values, where the only criteria is satisfying some key?

更新

上面代码中的错误是变量 map 的声明类型.应该是 Map ，而不是 Map.Entry .

The mistake in the above code is the declared type of the variable map. It should have been Map rather than Map.Entry.

所以现在的功能代码是:

So the now functional code is:

Map<Set<Integer>, Double> map = container.entrySet()
            .stream()
            .filter(k -> k.getKey().size() == size)
            .collect(Collectors.toMap(entry -> entry.getKey(), entry -> entry.getValue()));

推荐答案

在您的示例中， Collectors.toMap 似乎没有接受 stream.collect 的类型参数.并且仅返回 Map< Object，Object> .

Seems that Collectors.toMap does not pick up the type arguments of stream.collect in your example and only returns a Map<Object,Object>.

作为一种解决方法，您可以自己创建结果图，然后在最后一步中将过滤后的条目添加到结果图:

As a workaround you can create the result map yourself and in the last stream step add the filtered entries to the result map:

Map<Set<Integer>, Double> result = new HashMap<>();
container.entrySet()
    .stream()
    .filter(entry -> entry.getKey().size() == size)
    .forEach(entry -> result.put(entry.getKey(), entry.getValue()));

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