functional-programming,java-8,reduce,java-stream , Java8: Reduce list of elements like sql group by

Java8: Reduce list of elements like sql group by


Tag: functional-programming,java-8,reduce,java-stream

After a stream().filter().map() on a List I have a data structure of type List<List<Object>> that looks like this:


What I want is to group by the value of the first element of the inner list, leave the middle element out and finally sum the last elements for each "group".

[["1", 30],
 ["2", 40]]

Sorry if this is obvious for some of you, but I have yet to find any example of how to achieve this. I assumed it could be done by Stream.reduce(U identity, BiFunction accumulator, BinaryOperator combiner) but so far I haven't succeeded. If someone could provide some example code for this, I believe it would be appreciated by many others too.


The following code may be of help:

List<List<Object>> originalList = Arrays.asList(
        Arrays.asList("1", "a", 20),
        Arrays.asList("1", "b", 10),
        Arrays.asList("2", "a", 10),
        Arrays.asList("2", "b", 30)

final Map<Object, Integer> collectedMap =
                        e -> e.get(0),
                        Collectors.summingInt(e -> (Integer) e.get(2))));


The output is:

{1=30, 2=40}

Basically, what the code does is to group by the first value in the sublist (get(0)). Then it sums the integers by the use of summingInt. However, it groups the entire thing in a Map - if some other collection is required the stream must be slightly changed.

E.g. to collect the whole thing as a List:

final List<List<Object>> collectedList =
             .map(e -> Arrays.asList(e.getKey(), e.getValue()))


Then, the output will be:

[[1, 30], [2, 40]]


Are interfaces a valid substitute for utility classes in Java 8? [duplicate]

This question already has an answer here: Java 8: Interface with static methods instead of static util class 4 answers For the past decade or so, I've been using the pattern below for my Java utility classes. The class contains only static methods and fields, is declared final so...

What is Anamorphism - example in C#

I am trying to wrap my head around the concept of anamorphism. In functional programming, an anamorphism is a generalization of the concept of unfolds on lists. Formally, anamorphisms are generic functions that can corecursively construct a result of a certain type and which is parameterized by functions that determine...

Questions about Vars Clojure

I'm new in Clojure and i read that it is a functional language. It says that Clojure doesn't have variables, still when i find (def n 5), what's the difference between it and a variable? I can change the value of the var after, so is it really that different...

Why is generic of a return type erased when there is an unchecked conversion of a method parameter in Java 8?

Consider the following code sample: import java.util.ArrayList; import java.util.List; public class Main { public static void main(String[] args) { List list = new ArrayList<Integer>(); String response = getProducer(list).get(); } static Producer<String> getProducer(List<Integer> list) { return new Producer<String>(); } } class Producer<T> { T get() { return null; } } When...

How To Calculate The Number Of Days In A Period

For the following Period calculation: Period.between(LocalDate.of(2015,8,1), LocalDate.of(2015,9,2)) the result is: P1M1D This is equivalent to 31 days + 1 day = 32 days. For this Period: Period.between(LocalDate.of(2015,8,1), LocalDate.of(2015,10,2)) the result is : P2M1D This is equivalent to: 31 days (in August) + 30 days (in September) + 1 (in October)...

Java 8: Filtering Map to List by person's status

I'm trying to filter a Map<Long, Person> people and return only the IDs of these people having a status of SUBSCRIBED in a List<Long>. Here is the code in the old-fashioned way: public List<Long> getSubscribedPeople() { final List<Long> subscribedPeople = new ArrayList<>(); for (final Map.Entry<Long, Person> entry : subscribedPeople.entrySet()) {...

Java8 LocalDate Time Adding Hours Not Wroking

I tried like below, but in both the cases it is showing same time? What i am doing wrong. LocalDateTime currentTime ="UTC")); Instant instant = currentTime.toInstant(ZoneOffset.UTC); Date currentDate = Date.from(instant); System.out.println("Current Date = " + currentDate); currentTime.plusHours(12); Instant instant2 = currentTime.toInstant(ZoneOffset.UTC); Date expiryDate = Date.from(instant2); System.out.println("After 12 Hours =...

State in a java.util.function.Function

Suppose that I have a class which implements java.util.function.Function. The Function needs to know about a start date and end date but these will be constant throughout the lifetime of the function. I'm considering implementing the Function with private final fields for the start and end dates, as the apply...

How to parse non-standard month names with DateTimeFormatter

I need to parse (German) dates that come in the following form: 10. Jan. 18:14 8. Feb. 19:02 1. Mär. 19:40 4. Apr. 18:55 2. Mai 21:55 5. Juni 08:25 5. Juli 20:09 1. Aug. 13:42 [...] As you can see, the month names are cut if the month has...

Java 8 : Lambda Function and Generic Wildcards

I have the following class class Book implement Borrowable { @Override public String toString(Function<? extends Borrowable , String> format) { return format.apply(this); } } This gives me an error that i cannot use "apply" on this(Book object). My current formatter is Function<Book, String> REGULAR_FORMAT = book -> "name='" +

How to create a two dimensional array from a stream in Java 8?

I have a text file like this: ids.txt 1000 999 745 123 ... I want to read this file and load it in a two dimensional array. I expect to have an array similar to the one below: Object[][] data = new Object[][] { // { new Integer(1000) }, //...

Java 8 Optional.ifPresent is my code wrong or is it eclipse?

I am new to Java 8 and trying out Null type annotations and Optional. For my example below, I have used String rather than my class and am calling toUpperCase just to call something, in my case I actually call a function passing in a parameter (so don't think I...

How to use lambda expression to call the comparator?

I am doing the leetcode's largest Number problem. Here is the problem: Given a list of non negative integers, arrange them such that they form the largest number. For example, given [3, 30, 34, 5, 9], the largest formed number is 9534330. and there is a 5 lines C# code:...

Java 8 - Difference between Optional.flatmap and

What's the difference between these to methods: Optional.flatMap() and An example would be appreciated....

Merging two List of objects in java 8

Let's consider a java class Parent with Parent with 20 attributes (attrib1, attrib2 .. attrib20) and its corresponding getters and setters. List<Parent> list1 contains n Parent objects Parent objects. List<Parent> list2 contains n Parent objects Parent objects. Now I want to merge two list and avoid duplicate objects based on...

java 8 stream groupingBy sum of composite variable

I have a class Something which contains a instance variable Anything. class Anything { private final int id; private final int noThings; public Anything(int id, int noThings) { = id; this.noThings = noThings; } } class Something { private final int parentId; private final List<Anything> anythings; private int getParentId()...

Java8 LocalDateTime to XMLGregorianCalender Remove “+05:30” Portion

Did like below, LocalDateTime currentUTCTime ="UTC")); String reqPattern = currentUTCTime.format(DateTimeFormatter.ofPattern("yyyy-MM-dd HH:mm:ss:SSS")); System.out.println("Required pattern: " + reqPattern); GregorianCalendar calendar = GregorianCalendar.from(currentUTCTime.atZone(ZoneId.systemDefault())); XMLGregorianCalendar xcal = DatatypeFactory.newInstance().newXMLGregorianCalendar(calendar); System.out.println("But Showing As :" + xcal); I want the output as 2015-06-18 11:59:15:135, but when i set the...

Generic method to perform a map-reduce operation. (Java-8)

How to overload a Function with generic parameter in Java 8? public class Test<T> { List<T> list = new ArrayList<>(); public int sum(Function<T, Integer> function) { return; } public double sum(Function<T, Double> function) { return; } } Error: java: name clash: sum(java.util.function.Function<T,java.lang.Double>) and sum(java.util.function.Function<T,java.lang.Integer>) have the same erasure...

Java8 Nested Streams write back with setter

I´m trying to loop over two lists, filter the nested list and write the result back into the main object with java8 features. locations.forEach(location -> location.getSubList().stream() .filter(this::correctTestDataValue) .collect(Collectors.toList())); So by now the sublist inside location doesnt change, which is obvious because stream and collect do create a new list, which...

Mapping a list to Map Java 8 stream and groupingBy

I have this simple Bean class: public class Book { public Book(Map<String, String> attribute) { super(); this.attribute = attribute; } //key is isbn, val is author private Map<String, String> attribute; public Map<String, String> getAttribute() { return attribute; } public void setAttribute(Map<String, String> attribute) { this.attribute = attribute; } } In...

How to sort HashMap in java depending on the size of value List

I have a hashmap like the following HashMap<String, ArrayList<String>> map=new HashMap<String, ArrayList<String>>(); map.put("USA", Arrays.asList("CA","IA","IL")); map.put("India", Arrays.asList("MUM","CAL")); map.put("Canada", Arrays.asList("TOR")); I want to sort the map depending on the size of the list in the value in ascending order.How can i do that. Is there any nice method to do so?...

How to convert single element list to java 8 optional

How to nicely convert list containing one or zero elements to Optional? The ugly code: List<Integer> integers = new ArrayList<>(); Optional<Integer> optional = integers.size() == 0 ? Optional.empty() : Optional.of(integers.get(0)); ...

Hook into GHC runtime system

I have been looking at how transactional memory is implemented in Haskell, and I am not sure I understand how the STM operations exposed to the programmer hook into the runtime system functions written in C. In ghc/libraries/base/GHC/Conc/Sync.hs of the git repo, I see the following definitions: -- |A monad...

Is this definition of a tail recursive fibonacci function tail-recursive?

I've seen around the following F# definition of a continuation-passing-style fibonacci function, that I always assumed to be tail recursive: let fib k = let rec fib' k cont = match k with | 0 | 1 -> cont 1 | k -> fib' (k-1) (fun a -> fib' (k-2)...

Scala wrong forward reference

I am working through some of the exercises in: Functional Programming in Scala specifically problem 5.2. The issue is that with the following code which I have pieced together from the answer key. sealed trait Stream[+A] { def take(n: Int): Stream[A] = this match { case Cons(hs, ts) if n...

C# + linq(concat) rewrite to java

I have a code example in C# (with linq) but am trying to translate this to java code (java 8). But am getting very confused about what happens. Also i am not able to reproduce the same result in java as from the c# code. public IEnumerable<Node> GetNeighborNodes(Node v) {...

Idiomatic list construction

I'm very new to Haskell and functional programming in general, so I don't really know how to make this code idiomatic: type Coord = Double data Point = Point Coord Coord Coord deriving Show type Polyline = [Point] -- Add a point to a polyline addPoint :: Polyline -> Point...

How to effectively get indices of 1s for given binary string using Scala?

Suppose we have a binary string such as 10010010. All I want is a function returning indices of 1s for that string: indicesOfOnes("10010010") -> List(0, 3, 6) indicesOfOnes("0") -> List() And what I implemented is: def indicesOfOnes(bs: String): List[Int] = { val lb = ListBuffer[Int]() bs.zipWithIndex.foreach { case (v, i)...

Properly implement F# Unit in C#

This question is not about C#/F# compatibility as in this one. I'd like to know the proper way to implement a type like F# Unit in place of using void. Obviously I'll discard this result and I'll not expose this type to the outside world. Wondering if an empty class...

Java 8 lambdas nested Map

I am trying to use Java-8 lambdas to solve the following problem: Given a List<Transaction>, for each Category.minorCategory I require the sum of Transaction.amount per Category.minorCategory and a Map of Transaction.accountNumber with the sum of Transaction.amount per Transaction.accountNumber. I have this working, as per the code below. I now have...

Java 8, using .parallel in a stream causes OOM error

In the book Java 8 In Action, section 7.1.1, the authors state that a stream can benefit from parallel processing by adding the function .parallel(). They provide a simple method called parallelSum(int) to illustrate this. I was curious to see how well it worked so I executed this code: package...

Eclipse Kepler is unable to set the Java jdk 1.8 when maven project is updated

Installed java version is 1.8, while selecting this version in pom.xml and updating the maven project,it automatically jumps from 1.8 to 1.4, due to which I am unable to have Lambda expression specific code. I am using eclipse kepler. Any idea what is happening here ?...

How to handle initial nil value for reduce functions

I would like to learn and use more functional programming in Swift. So, I've been trying various things in playground. I don't understand Reduce, though. The basic textbook examples work, but I can't get my head around this problem. I have an array of strings called "toDoItems". I would like...

Collapse similar case statements in Scala

Is there an elegant way to do something like the following example using just one case statement? foobar match { case Node(Leaf(key, value), parent, qux) => { // Do something with parent & qux } case Node(parent, Leaf(key, value), qux) => { // Do something with parent & qux (code...

Java 8 stream group by min and max

Suppose you run an SQL query against an employees table: SELECT department, team, MIN(salary), MAX(salary) FROM employees GROUP BY department, team And in the java client you map the result set to a list of Aggregate instances by making a DAO call like below: List<Aggregate> deptTeamAggregates = employeeDao.getMinMaxSalariesByDeptAndTeam() And 'Aggregate'...

Play! Framework support for Java 8 Optional

I've got a Play! 2 routes file and controller that looks something like this: GET /books BooksController.getBooks(author: play.libs.F.Option[String]) public class BooksController extends Controller { public static Result getBooks(play.libs.F.Option<String> author) { ... } } I would like to refactor it to use Java 8's Optional, but this causes a compilation error:...

Java 8 - return List (keyset) opposed to List>

I am trying to use java 8 to return me a list of key values(Integers) in which the value (Checkbox) is checked. The map I am trying to process is of the following form. Map<Integer, CheckBox> The aim is to return the key set for all values where the check...

In underscore/lodash, how to avoid duplicate calculation in a `map` method?

Here is my code: var transformed = _(original).map(function (c) { return { lat: wgs2gcj(c.latitude, c.longitude).lat lng: wgs2gcj(c.latitude, c.longitude).lng } }); Let's say wgs2gcj is a function from a third-party library and will take a long time to compute. Is there a way to do the calculation only once?...

How to use Java 8 Collectors groupingBy to get a Map with a Map of the collection?

Imagine these classes class Subject{ private int id; private Type type; private String origin; private String name; Subject(int id, Type type, String origin, String name){ = id; this.type = type; this.origin = origin; = name; } //Getters and Setters } enum Type{ TYPE1, TYPE2 } I have a...

Accessing call stack depth in Scheme

In order to demonstrate the effectiveness of tail recursion, I would like a way to access the depth of the call stack dynamically in Scheme. Is there a way to do this? If not, is there a way to do this in other major functional languages (OCaml, Haskell, etc.)?...

Stuck with java8 lambda expression

I have Map<Integer,Doctor> docLib=new HashMap<>(); to save class of Doctor. Class Doctor has methods:getSpecialization() return a String, getPatients() to return a collection of class Person. In the main method, I type: public Map<String,Set<Person>> getPatientsPerSpecialization(){ Map<String,Set<Person>> res=this.docLib.entrySet().stream(). map(d->d.getValue()). collect(groupingBy(d->d.getSpecialization(), d.getPatients()) //error ); return res; } As you can see, I have...

Passing method as a parameter - Is this possible?

I am trying to migrate to java 8 and have a number of methods in my dao classes which do the following @Override @SuppressWarnings("unchecked") public List<Group> getGroups() { Session session = sessionFactory.openSession(); List<Group> allGroups = (List<Group>)session.createQuery("from Group").list(); session.close(); return allGroups; } Here the same boiler plate and session.close is...

Convert time based on timezone using java.time

How to change the time based on timezone in LocalDateTime, here i have built a date with Time zone as EST, now i need to find the UTC of the corresponding time. please help me how to solve this String str = "16Jun2015_153556"; DateTimeFormatter formatter = DateTimeFormatter.ofPattern("ddMMMyyyy_HHmmss"); formatter.withZone(ZoneId.of("EST5EDT")); LocalDateTime dateTime...

Having trouble stepping through function that reduces an array of functions

When using the reduce method on an array of functions I am having difficulty tracing through how reduce works on the array exactly. comboFunc(num, functionsArr) { return functionsArr.reduce(function (last, current) { return current(last); }, input); } so with functionsArr = [add, multi] and the functions add and multi being function...

Maven error with Java 8

Getting an error with Maven and Java 8 (jdk1.8.0_45). This issue does not occur with Java 7. MCVE Create a sample maven project. For example: mvn archetype:create -DgroupId=testinovke -DartifactId=testinvoke Create the following content in the generated file package testinovke; import java.lang.invoke.MethodHandle; import java.lang.invoke.MethodHandles; import java.lang.invoke.MethodType; public class App {...

Is there a lazy functional (immutable) language where functions have intermediate variables+return?

I apologize if this has an obvious answer. I would like to find a lazy functional programming language where the following pseudo code makes sense: let f = function(x) { let y = x*x // The variables y and z let z = y*2 // are local return z }...

Joining a collection based on members of the type

I have a class A and its members b and c. Now I construct the List<A> with this: add(new A().setb("abcd").setc("123456")); add(new A().setb("efgh").setc("789101")); add(new A().setb("ijkl").setc("112345")); I want to transform this List to string which looks like this abcd,123456 efgh,789101 ijkl,112345 Now the very obvious way would be to have a StringBuilder...

Why I'm getting different java versions

I just upgraded my Java from 1.8 update 31 to update 45. Once done, I checked in the Java Console it shows Java 8 update 45. But, when I checked in the terminal it shows java version "1.8.0_31". I checked using Verify Java Version, and it show You have the...

Java 8 map to only value type collection

I want to convert my Map <K,V> into only a Set <V>. I could not find any example anywhere, including Oracle's documentation here: All I could go further is: myMap.entrySet().parallelStream(). filter((entry) -> entry.getKey().startsWith("a")) .collect(Collectors.toSet()); This returns a Set of Map.Entry. In this example it's Map<String, String> so I would...

operators in predicate as argument in lambda expression

I need to use Predicate as argument in lambda expression. I tried an example code but seeing compiler error. I see that the compiler is treating the same Predicate differently for different arguments. So Predicate arguments n -> true and n -> false works but n -> n%4 == 0...