Java 8 过滤器和列表收集<Map<String, Object>>
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Java 8 Filter and Collect of List<Map<String, Object>>
提问by BoxMan0617
Ok, so I have a List of Maps that I want to filter then collect to another List of Maps. Every time I put that List through the process, I get a List of Objects... Help?!
好的，所以我有一个要过滤的地图列表，然后收集到另一个地图列表。每次我在整个过程中放入该列表时，我都会得到一个对象列表...帮助？！
List<Map<String, Object>> segments = (List<Map<String, Object>>) (List<?>) query.getResultList();
List<Map<String, Object>> segmentsWithMoreVersions = segments.stream()
.filter((Object s) -> {
Object[] ss = (Object[]) s;
if(((Long) ss[2]) == 1)
{
return false;
}
return true;
})
.collect(Collectors.toList());
As you can see... In the filter method I've had use an Object because I can't get the s variable to be a Map. When I do try, I get a ClassCastException.
如您所见...在过滤器方法中，我使用了一个对象，因为我无法将 s 变量设为 Map。当我尝试时，我得到一个 ClassCastException。
Edit: Ok, maybe more information is needed...
So my segmentsvariable is like so:
编辑：好的，也许需要更多信息......所以我的segments变量是这样的：
[
{'name': 'ABC', 'version': 1, 'ct': 1},
{'name': 'AAA', 'version': 1, 'ct': 1},
{'name': 'BFD', 'version': 1, 'ct': 4},
{'name': 'SDE', 'version': 1, 'ct': 1}
]
What I want to do is to filter out all of the Maps that have the ct as 1. The filter method is looking at that Object (because I can't get it to cast to a Map) and checking if it == to 1, and if so it does NOT return it to the stream.
我想要做的是过滤掉所有 ct 为 1 的地图。过滤器方法正在查看该对象（因为我无法将其转换为地图）并检查它是否 == 为 1 ，如果是这样，它不会将其返回到流中。
Edit2: After the comments, I edited to the following:
Edit2：评论后，我编辑为以下内容：
List<Map<String, Object>> segmentsWithMoreVersions = segments.stream()
.filter(m -> ((Long) m.get("ct")) != 1L)
.collect(Collectors.toList());
And I got the following: Caused by: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to java.util.Map
我得到了以下信息： Caused by: java.lang.ClassCastException: [Ljava.lang.Object; cannot be cast to java.util.Map
采纳答案by BoxMan0617
Ok... So the problem was NOT the filter function. Thanks to @Radiodef pointing out the unchecked conversion I found that it was that that was causing the problems. So what I did was change that to this:
好的...所以问题不在于过滤器功能。感谢@Radiodef 指出未经检查的转换，我发现正是这导致了问题。所以我所做的是将其更改为：
List<?> tmp = query.getResultList();
List<Map<String, Object>> segments = new ArrayList<Map<String, Object>>();
for(Object t : tmp)
{
Object[] tt = (Object[]) t;
Map<String, Object> rMap = new HashMap<String, Object>();
rMap.put("name", tt[0]);
rMap.put("version", tt[1]);
rMap.put("ct", tt[2]);
segments.add(rMap);
}
From there the filterworked! THanks guys!
从那里开始filter工作！多谢你们！
回答by Radiodef
So, based on your description, I think the filter you want is this:
所以，根据你的描述，我认为你想要的过滤器是这样的：
segments.stream()
.filter(m -> ((Long) m.get("ct")) != 1L)
.collect(toList());
Or being explicit about the type of the predicate:
或者明确说明谓词的类型：
.filter((Map<String, Object> m) ->
((Long) m.get("ct")) != 1L)
On your edit: you have erroneous data in the Map, or you've misunderstood the way it's represented. It appears that segmentsis actually a List<Object[]>.
在您的编辑中：您在地图中有错误的数据，或者您误解了它的表示方式。看来这segments实际上是一个List<Object[]>.
I don't really have enough information to help you fix it.
我真的没有足够的信息来帮助您修复它。
To try debugging, you could do something like:
要尝试调试，您可以执行以下操作：
segments.stream()
.map(Arrays::toString)
.forEach(System.out::println);
This will tell you what's actually in it if you don't have documentation to refer to.
如果您没有可参考的文档，这将告诉您其中的实际内容。
回答by Nik
This works for me...
这对我有用...
List<Map<String, Object>> segments = new ArrayList<Map<String, Object>>();
List<Map<String, Object>> segmentsWithMoreVersions = segments.stream()
.filter((Map<String, Object> s) -> {
return (Long) s.get("versions") > 1;
})
.collect(Collectors.toList());
