collectors

Differences between Collectors.toMap() and Collectors.groupingBy() to collect into a Map

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TLDR : To collect into a Map that contains a single value by key (Map<MyKey,MyObject>), use Collectors.toMap(). To collect into a Map that contains multiple values by key (Map<MyKey, List<MyObject>>), use Collectors.groupingBy(). Collectors.toMap() By writing : chargePoints.stream().collect(Collectors.toMap(Point::getParentId, c -> c)); The returned object will have the Map<Long,Point> type. Look at the Collectors.toMap() function that you … Read more

Java 8 grouping using custom collector?

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When grouping a Stream with Collectors.groupingBy, you can specify a reduction operation on the values with a custom Collector. Here, we need to use Collectors.mapping, which takes a function (what the mapping is) and a collector (how to collect the mapped values). In this case the mapping is Person::getName, i.e. a method reference that returns … Read more

How to add elements of a Java8 stream into an existing List

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NOTE: nosid’s answer shows how to add to an existing collection using forEachOrdered(). This is a useful and effective technique for mutating existing collections. My answer addresses why you shouldn’t use a Collector to mutate an existing collection. The short answer is no, at least, not in general, you shouldn’t use a Collector to modify … Read more

Java 8 Collectors.toMap SortedMap

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I don’t think you can get much better than this: .collect(Collectors.toMap(keyMapper, valueMapper, (v1,v2) ->{ throw new RuntimeException(String.format(“Duplicate key for values %s and %s”, v1, v2));}, TreeMap::new)); where the throw lambda is the same as throwingMerger() but I can’t directly call that since it’s package private (you can of course always make your own static method … Read more

Stream.skip behavior with unordered terminal operation

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Recall that the goal of stream flags (ORDERED, SORTED, SIZED, DISTINCT) is to enable operations to avoid doing unnecessary work. Examples of optimizations that involve stream flags are: If we know the stream is already sorted, then sorted() is a no-op; If we know the size of the stream, we can pre-allocate a correct-sized array … Read more

Java Streams: Replacing groupingBy and reducing by toMap

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This pattern became evident by experience with using both collectors. You’ll find several Q&As on Stackoverflow, where a problem could be solved with either collector, but one of them seems a better fit for the particular task. This is a variation of the difference between Reduction and Mutable Reduction. In the first case, we use … Read more

NullPointerException in Collectors.toMap with null entry values

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You can work around this known bug in OpenJDK with this: Map<Integer, Boolean> collect = list.stream() .collect(HashMap::new, (m,v)->m.put(v.getId(), v.getAnswer()), HashMap::putAll); It is not that much pretty, but it works. Result: 1: true 2: true 3: null (this tutorial helped me the most.) EDIT: Unlike Collectors.toMap, this will silently replace values if you have the same … Read more

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