If you want to read more about Stream API itself, check this article. Note that toMap doesn't even evaluate whether the values are also equal. Set cncpts = concepts.stream().collect(baeldungCollectorsArticle,Collectors.toSet()); Cheers, There are no guarantees on the type, mutability, serializability, or thread-safety of the Set returned. "String with hyphen , suffix and prefix :  ", Can we call run() method directly to start a new thread, Object level locking vs Class level locking, Convert LocalDateTime to Timestamp in Java, Java 8 – java.util.function.Function example, Difference between replace() and replaceAll() in java, [Fixed] Error: Identifier expected in java, [Fixed] bad operand types for binary operator in java. https://github.com/eugenp/tutorials/blob/master/core-java-8/src/test/java/com/baeldung/collectors/Java8CollectorsTest.java#L216. To be able to create a custom collector, you have to implement the Collectorinterface. The compiler will treat any interfaces meeting the definition of a functional interface as a functional interface; it means the @FunctionalInterface annotation is optional. summingInt is used to find sum of stream elements as int datatype. It provides reduction operations, such as accumulating elements into collections, summarizing elements according to various criteria, etc. extends K> keyMapper, Function Collection of exercises focused on the Java 8 new features. Java 8 Collectors Examples. At the basic level, the difference between Collections and Str… In today’s write-up, we’ll be taking a closer look at Collectors and how to use them in Java 8. Collectors toSet() returns a Collector that accumulates the input elements into a new Set. The toMap static method in the Collectorsclass pr… The important thing to remember is the fact that we can't assume any particular List implementation with this method. For example, given the following collector that computes the sum of the salaries of a stream of employees: https://github.com/eugenp/tutorials/blob/master/core-java-8/src/test/java/com/baeldung/collectors/Java8CollectorsTest.java#L216. It's understandable – how should it figure out which value to pick for this key? GroupingBy collector is used for grouping objects by some property and storing results in a Map instance. Let's create a Stream instance representing a sequence of elements and collect them into a Set instance: A Set doesn't contain duplicate elements. Btw, stream().collect(Collectors.asList()); is more readable than static import. It returns a Collector Interface that gathers the input data onto a new list. I will try to relate this concept with respect to collections and differentiate with Streams. (10+20+30+40+50/5)= 150/5 =30.0  The strategy for this operation is provided via Collector interface implementation. I am not providing theory here, I think you will learn better with the help of examples. ‘baeldungCollectorsArticle’ is nothing but this blog post. Java 8 – Interface changes: Default and static methods 5. Let's find the maximum and minimum numbers from a given Stream using the collectors we've learned so far: Here, we're using two different collectors and then combining the result of those two to create something meaningful. Now you must wondering how did we get 1100 for 2nd case: If you want to understand more about v-> v*v , you can go through Java 8 lambda expressions Similarly we have different function for different data types such as AveragingDouble, AveragingLong. We can obtain information about string lengths by doing: In this case, the following will be true: AveragingDouble/Long/Int is a collector that simply returns an average of extracted elements. It involves taking the individual data or elements from the stream combining into a single result container by applying a combining operation. (10*10 + 20*20 + 30*30 + 40*40 + 50*50)/5=5500/5 = 1100.0  Collectors is a final class that extends Object class. Overview In this tutorial, We’ll be learning to Java 8 Collectors API in-depth with all methods and example programs. Java Stream API for Bulk Data Operations on Collections. Here is the complete implementation along with the usage: In this article, we explored in-depth Java 8's Collectors and showed how to implement one. What happens if our collection contains duplicate elements? : – Java Stream.Collectors APIs examples – Java Stream.Collectors APIs examples – Java 8 GroupingBy is... Do various operations such as average, count, groupby, sort the list with the help Collectors. Well as parallel execution below I am not providing theory here, I think you will get output! 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