java-stream

Lazy Filtering CSV Files I had the need to filter through millions of log records, stored as numerous CSV files. The size of the records greatly exceeded my available memory so I wanted to go with a lazy approach. Java 8 Streams API With jdk8 we have the Streams API which paired with Apache commons-csv allows us to easily accomplish this. public class LazyFilterer { private static Iterable<CSVRecord> getIterable(String fileName) throws IOException { return CSVFormat .DEFAULT .withFirstRecordAsHeader() .parse(new BufferedReader(new FileReader(fileName))); } public static void main(String[] args

I'm interested in building a Huffman Coding prototype. To that end, I want to begin by producing a histogram of the characters that make up an input Java String . I've seen many solutions on SO and elsewhere (e.g: here that depend on using the collect() methods for Stream s as well as static imports of Function.identity() and Collectors.counting() in a very specific and intuitive way. However, when using a piece of code eerily similar to the one I linked to above: private List<HuffmanTrieNode> getCharsAndFreqs(String s){ Map<Character, Long> freqs = s.chars().collect(Collectors.groupingBy

I am sending result in descending order but I get output with ascending order List<myEntity> myData = new ArrayList<>(); Map<Integer,List<myEntity>> myid = new LinkedHashMap<>(); try { myData = myService.getData(id); myid = myData.stream().collect(Collectors.groupingBy(myEntity::getDataId)); Here mydata is sorted by desc order but after creating collections by group data id my list get sorted with ascending order. I want my collection list to be descending order not ascending order. As @Holger described in Java 8 is not maintaining the order while grouping , Collectors.groupingBy() returns a

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

How can I write the following in Java 8 streams? int total = 0; for (ObjectA obja : rootObj.getListA()) { for (ObjectB objb : obja.getListB()) { total += objb.getCount() * obja.getCount(); } } return total; The canonical solution for converting nested for loops to Stream API usage is via flatMap : return rootObj.getListA().stream() .flatMapToInt(objA->objA.getListB().stream() .mapToInt(objB->objB.getCount() * objA.getCount())) .sum(); This allows you to perform an operation for each inner iteration. However, in the special case of summing you may simplify the operation as it doesn’t matter