Map and flatmap in java

Introduction A Java Map implementation is an collection that maps keys to values. Every Map Entry contains key/value pairs, and every key is associated with exactly one value. The keys are unique, so no duplicates are possible. A common implementation of the Map interface is a HashMap: Map students = new HashMap(); students.put( 132, "James"); students.put( 256, "Amy"); students.put( 115, "Young"); System.out.println( "Print Map: " + students); We've created a simple map of students (Strings) and their respective IDs: Print Map: A Java List implementation is a collection that sequentially stores references to elements. Each element has an index and is uniquely identified by it: List list = new ArrayList(Arrays.asList( "James", "Amy", "Young")); System.out.println(list); System.out.println(String.format( "Third element: %s", list.get( 2)); [James, Amy, Young] Third element: Young The key difference is: Maps have two dimensions, while Lists have one dimension. Though, this doesn't stop us from converting Maps to Lists through several approaches. In this tutorial, we'll take a look at how to convert a Java Map to a Java List: • • • • • Convert Map to List of Map.Entry Java 8 introduced us to the Stream API - which were meant as a step towards integrating Functional Programming into Java to make laborious, bulky tasks more readable and simple. Streams work wonderfully with collections, and can aid us in converting a Map to a List. The easiest way to preserve the key-value map...

Java String Java Regex Exception Handling Java Inner classes Java Multithreading Java I/O Java Networking Java AWT & Events Java Swing JavaFX Java Applet Java Reflection Java Date Java Conversion Java Collection Java JDBC Java Misc Java New Features RMI Internationalization Interview Questions Java MCQ flatMap() Method in Java 8 The Stream API was introduced in Java 8 that is used to process the collections of objects. It can be used by importing the java.util.stream package. In this section, we will discuss the Stream.flatMap() method of the Stream API. Also, we will discuss the key differences between the Stream.flatMap() and Stream.map() method in Java 8. Before moving to the topic, first, we will understand the Stream.map() method. Because the flatMap() method is based on the map() method. Java Stream.map() Method The Stream.map() method performs an intermediate operation by using the mapper function. It produces a new stream for each element. It transforms all the streams into a single stream to provide the result. therefore, each element of the stream gets converted into a new stream. Syntax: import java.util.*; public class MapExample Output: Stream flatMap(Function> mapper) The method takes a function as an argument. It accepts T as a parameter and returns a stream of R. R: It is a type parameter that represents the element type of the new stream. mapper: It is a parameter that is a non-interfering, stateless function to apply to each element. It produces a strea...

I have an Artist class as follows: class Artist What will be the solution for internal iteration using stream() and flatMap() with the help of Lambda-Expression ? I have thought of a solution but may be it is incorrect. int totalMember = artists.stream() .filter(d -> d.getOrigin().equals("Kolkata")) .flatMap(e -> e.getMembers()) .map(f -> 1).reduce(0, (a, b) -> a + b); @SoumyaKantiNaskar a bit unclear.. you want the total number of artists from a certain region where members is null? Taking your example: new Artist("Fossils", "Kolkata", Stream.of(new Artist("Rupam Islam", "Kolkata"), new Artist("Deep", "Kolkata"), new Artist("Allan", "Kolkata"), new Artist("Chandramouli", "Kolkata"), new Artist("Tanmoy", "Kolkata"))). Should this produce zero or 5? You want to count the inner Artists also? Your solution will give the expected output i.e 11. You also can use : int totalMembers = (int) artists.stream() .flatMap(d->d.getMembers()) .filter(d->d.getOrigin().equals("Kolkata")) .count(); Difference between both the solutions is I have flattened the list before filtering it and used long count(); instead of reduce(). What later solution does is, it checks the origin from Stream members and not the origin of artists. Hope this help. I would appreciate if someone can discuss the optimized solution. Check this solution too. It may also happen that a band member form a band of particular locality may not be from the same locality. So the exact solution will be : long totalMember = ar...

Building or modernizing a Java enterprise web app has always been a long process, historically. Not even remotely quick. That's the main goal of Jmix is to make the process quick without losing flexibility - with the open-source RAD platform enabling fast development of business applications. Critically, it has very minimal impact on your server's performance, with most of the profiling work done separately - so it needs no server changes, agents or separate services. Simply put, a single Java or Kotlin developer can now quickly implement an entire modular feature, from DB schema, data model, fine-grained access control, business logic, BPM, all the way to the UI. Jmix supports both developer experiences – visual tools and coding, and a host of super useful plugins as well: >> Try out Jmix Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL. The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries. Critically, it has very minimal impact on your server's performance, with most of the profiling work done separately - so it needs no server changes, agents or separate services. Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you'll hav...

In order to flatten this nested collection into a list of strings, we can use forEach together with a Java 8 method reference: public List flattenListOfListsImperatively( List> nestedList) 4. Flattening the List With flatMap This allows us to flatten the nested Stream structure and eventually collect all elements to a particular collection: public List flattenListOfListsStream(List> list) 5. Conclusion

Table of Contents • • • • • • • • Introduction The map() and flatmap() are two very significant operations in Java. They both are methods of java.util.stream.Stream interface in Java 8 that is primarily used for transformation or mapping operations. The Java flatmap() and map() methods were first just used in functional languages but they were soon introduced in Java 8. The map() offers transformation whereas the flatmap() can be used for both transformations and flattening, thus the name flatmap. In this article, we will further explore the map() and flatmap() methods, the key differences between them and will discover the correct and most suitable use of both of them with the aid of relevant examples. Java Map() The Java map() method takes one input value and produced one output hence, it is also referred to as One-To-One mapping. The Syntax of the map() method is as follows: Stream map(Function mapper) • Here, the R is the element type of the new output stream. • The stream is an interface. • The T represents the type of stream elements. • The mapper is a stateless function that is applied to each element. • The function returns the new stream R. The given mapper function is applied to each element of input Stream and results are then stored in an output Stream. The map() function can be used where it is required to map the elements of a particular collection to a certain function, and the new stream containing the updated results is to be returned. For instance, the m...