collectors

I have the following class class Person { public String name; public int age; public List<String> hobbies; Person(String name, int age, List<String> hobbies) {this.name = name; this.age = age; this.hobbies = hobbies;} } How do I create a Map of age to hobbies like Map<Integer, Set<String>> ? The Java 8 way I cooked up is: Map<Integer, Set<String>> collect8 = persons.stream() .collect( toMap( p -> p.age, p -> p.hobbies.stream().collect(toSet()), (hobbies1, hobbies2) -> Stream.concat(hobbies1.stream(), hobbies2.stream()).collect(toSet()) ) ); Is there a more idiomatic way of doing this with

I would like to use stream to group, get the sum by type, then find the result of the different by type. So this is my data set. Sample(SampleId=1, SampleTypeId=1, SampleQuantity=5, SampleType=ADD), Sample(SampleId=2, SampleTypeId=1, SampleQuantity=15, SampleType=ADD), Sample(SampleId=3, SampleTypeId=1, SampleQuantity=25, SampleType=ADD), Sample(SampleId=4, SampleTypeId=1, SampleQuantity=5, SampleType=SUBTRACT), Sample(SampleId=5, SampleTypeId=1, SampleQuantity=25, SampleType=SUBTRACT) Sample(SampleId=6, SampleTypeId=2, SampleQuantity=10, SampleType=ADD), Sample(SampleId=7, SampleTypeId=2,

I've tried to find a good way to set up initial capacity of collector in java stream api. The simplest example is there: data.stream() .collect(Collectors.toList()); I just want to pass an int with size of list into collector in order not to resize internal array. The first intention is to do it in such way: data.stream() .collect(Collectors.toList(data.size())); But unfortunately toList isn't overloaded to work with parameter. I found one solution but it smells: data.stream() .collect(Collectors.toCollection(() -> new ArrayList<>(data.size()))); Is there any way to express it simplier? I'd

I played around with a solution using groupingBy , mapping and reducing to the following question: Elegantly create map with object fields as key/value from object stream in Java 8 . Summarized the goal was to get a map with age as key and the hobbies of a person as a Set . One of the solutions I came up with (not nice, but that's not the point) had a strange behaviour. With the following list as input: List<Person> personList = Arrays.asList( new Person(/* name */ "A", /* age */ 23, /* hobbies */ asList("a")), new Person("BC", 24, asList("b", "c")), new Person("D", 23, asList("d")), new

I would like to learn the time complexity of the given statement below.(In Java8) list.stream().collect(groupingBy(...)); Any idea? There is no general answer to that question, as the time complexity depends on all operations. Since the stream has to be processed entirely, there is a base time complexity of O(n) that has to be multiplied by the costs of all operations done per element. This, assuming that the iteration costs itself are not worse than O(n) , which is the case for most stream sources. So, assuming no intermediate operations that affect the time complexity, the groupingBy has to

How can I limit groupBy by each entry? For example (based on this example: stream groupBy ): studentClasses.add(new StudentClass("Kumar", 101, "Intro to Web")); studentClasses.add(new StudentClass("White", 102, "Advanced Java")); studentClasses.add(new StudentClass("Kumar", 101, "Intro to Cobol")); studentClasses.add(new StudentClass("White", 101, "Intro to Web")); studentClasses.add(new StudentClass("White", 102, "Advanced Web")); studentClasses.add(new StudentClass("Sargent", 106, "Advanced Web")); studentClasses.add(new StudentClass("Sargent", 103, "Advanced Web")); studentClasses.add(new

I have a List of List<Map<String, Object>> like this [ {"A": 50, "B": 100, "C": 200, "D": "Auction" }, { "A": 101322143.24, "B": 50243301.2, "C": 569, "D": "Sold Promissory Buyer" }, { "A": 500, "B": 1000, "C": 1500, "D": "Auction" }] I am using this stream API method to convert this list into Map finalSalesReportForSoldProperty.stream().collect(Collectors.toMap(tags -> ((String) tags.get("D")).replaceAll("[\\- ]", ""), Function.identity())); But it throws me java.lang.IllegalStateException: Duplicate key exception, because my list has duplicate keys I want to add the inner elements of