collectors

In Java 8, this works: Stream<Class> stream = Stream.of(ArrayList.class); HashMap<Class, List<Class>> map = (HashMap)stream.collect(Collectors.groupingBy(Class::getSuperclass)); But this doesn't: Stream<Class> stream = Stream.of(List.class); HashMap<Class, List<Class>> map = (HashMap)stream.collect(Collectors.groupingBy(Class::getSuperclass)); Maps allows a null key, and List.class.getSuperclass() returns null. But Collectors.groupingBy emits a NPE, at Collectors.java, line 907: K key = Objects.requireNonNull(classifier.apply(t), "element cannot be mapped to a null key"); It works if I create

This question already has an answer here: Java 8 grouping by from one-to-many 4 answers I have the following objects: public class Item { String value; List<Person> owners; Person creator; } public class Person { String name; int id; Person manager; } now i have the a list containing 3 Item objects: i1 -> {value="1", owners=[p1, p2, p3], creator=p4} i2 -> {value="2", owners=[p2, p3], creator=p5} i3 -> {value="3", owners=[p5], creator=p1} the Person objects are as follows: p1 -> {manager=m1, ...} p2 -> {manager=m2, ...} p3 -> {manager=m3, ...} p4 -> {manager=m2, ...} p5 -> {manager=m1, ...} I

For the Collectors.groupingBy() that returns Map<K,List<T>> is it implied that the List<T> is in order that the stream is evaluated? I see no explicit description of the ordering of the list, whereas the concurrent version explicitly states no ordering. If it weren't ordered somehow, I'd expect it to be a Collection though, and I don't see what other ordering it could possibly be, other than order received. I'm hoping it's guaranteed that the last value in each list is the last value received for that group. The documentation for groupingBy() says: Implementation Requirements: This produces a

How do you group first and then apply filtering using Java streams? Example : Consider this Employee class: I want to group by Department with a list of an employee having a salary greater than 2000. public class Employee { private String department; private Integer salary; private String name; //getter and setter public Employee(String department, Integer salary, String name) { this.department = department; this.salary = salary; this.name = name; } } This is how I can do this List<Employee> list = new ArrayList<>(); list.add(new Employee("A", 5000, "A1")); list.add(new Employee("B", 1000, "B1

Assume that you have a List of numbers. The values in the List can be of type Integer , Double etc. When you declare such a List it is possible to declare it using a wildcard ( ? ) or without a wildcard. final List<Number> numberList = Arrays.asList(1, 2, 3D); final List<? extends Number> wildcardList = Arrays.asList(1, 2, 3D); So, now I want to stream over the List and collect it all to a Map using the Collectors.toMap (obviously the code below is just an example to illustrate the problem). Lets start off by streaming the numberList : final List<Number> numberList = Arrays.asList(1, 2, 3D, 4D

I had a document that looked like the following: data.txt 100, "some text" 101, "more text" 102, "even more text" I processed it using regex and returned a new processed documents as the follow: Stream<String> lines = Files.lines(Paths.get(data.txt); Pattern regex = Pattern.compile("([\\d{1,3}]),(.*)"); List<MyClass> result = lines.map(regex::matcher) .filter(Matcher::find) .map(m -> new MyClass(m.group(1), m.group(2)) //MyClass(int id, String text) .collect(Collectors.toList()); This returns a list of MyClass processed. Can run in parallel and everything is ok. The problem is that I now have

I want to init a Map<String, BigDecimal> and want to always put the same BigDecimal value from outside of the stream. BigDecimal samePrice; Set<String> set; set.stream().collect(Collectors.toMap(Function.identity(), samePrice)); However Java complains as follows: The method toMap(Function, Function) in the type Collectors is not applicable for the arguments (Function, BigDecimal) Why can't I use the BigDecimal from outside? If I write: set.stream().collect(Collectors.toMap(Function.identity(), new BigDecimal())); it would work, but that's of course not what I want. The second argument (like