I have a wrapper that may or may not contain data: I group one user to multiple contacts: It groups val mailsByUser: Map<String, List<EmailAdapter>> I want to group all emails to a unique user I want to unwrap the EmailAdapter so that the relation is EmailAdapter.user -> List<EmailAdapter.ma…
Tag: java-stream
Java Stream why does reduce requires two parameters of the functional Interface?
In other words, why does reduce require BinaryOperator, or BiFunction interface? Why doesn’t it use UnaryOperator, or Function interface instead? What is the point of having two parameters as the input for the functional interface? (I know it’s for accumulation, but why doesn’t one parameter…
Using Java stream forEach() in ScheduledExecutorService freezes
The general idea is to have a Runnable running every 10 seconds in background to check some data and if needed make changes in an object. ScheduledExecutorService is instantiated in method main() and the task is scheduled. Runnable task instantiates Crawler object and starts crawling. Most of the times it run…
Java 11: convert List to TreeMap<String, List> using Collectors
I have a list like that I want to convert that List into a TreeMap<String, List<String>> like that: My code so far: I have two problems. First is that TreeMap::new is probably not working because the order is not the same as the original’s List. Second is that I don’t seem to find a wa…
Group by multiple fields and filter by common value of a field
I want to filter this Employee object based on the EmpPFcode . If collegeName has common value for 3 EmpPFcode, we will collect otherwise we will skip that records. So my result would be like below. Below one will skip because collageName is different. I try to do some logic below but it doesn’t not fil…
parallel() not work with the 3-arg iterate()
Here is my code: I expect this will go parallel, but! When i run it, parallel() seems “dead”, numbers from 4 to 19 comes with a “perfect” sequence which was not i want. So i modified the “iterate” part like this: There comes fixed, the parallel() works again. So, Why??? The…
Getting local variable defined in the enclosing scope must be final or effective final
I am getting “local variable defined in the enclosing scope must be final or effective final” as highlighted below. I am not sure how to fix this out in lambda expression. Answer Try replacing this: by this: The entitlePackage will be effectively final. Also your code will be much easier to read a…
Convert one Optional<List> to another Optional<List> in Java
How can I convert Optional List object from one type to another, for an example ProductMultipleOptionViewModel Type 1 Type 2 I want to convert from Optional<List<ProductMultipleOption>>to other Optional<List<ProductMultipleOptionViewModel>>. I tried the below code With the above code, …
Map List to Map with additional transformation
I need to convert a list of objects to a map of sets of one of the objects’ property and I’d like to use Java streams for that. For example there is a List and I’d like to convert it to a …
Convert nested map of streams.groupingBy() into list of POJOs
I want to convert a nested map structure created by java streams + groupingBy into a list of POJOs, where each POJO represents one of the groups and also holds all the matching objects of that group. I have the following code: I use project lombok for convenience here (@Builder, @Data). Please let me know if …