Difference Betwixt Map() As Well As Flatmap() Inwards Coffee Eight Stream

The map() in addition to flatmap() are 2 of import operations inward novel functional Java 8. Both represents functional functioning in addition to they are also methods inward java.util.stream.Stream class. The substitution difference betwixt map() in addition to flatmap() function is that when you lot usage map(), it applies a role on each chemical ingredient of current in addition to stores the value returned yesteryear the role into a novel Stream. This way 1 current is transformed into only about other e.g. a Stream of String is transformed into a Stream of Integer where each chemical ingredient is length of corresponding Stream. Key affair to retrieve is that the role used for transformation inward map() returns a unmarried value. If map() uses a function, which, instead of returning a unmarried value returns a Stream of values than you lot stimulate got a Stream of Stream of values, in addition to flatmap() is used to apartment that into a Stream of values.


For example, if nosotros stimulate got a Stream of String containing {"12", "34"} in addition to a method getPermutations() which returns a listing of permuations of given String. When you lot apply that role into each String of Stream using map you lot volition acquire something similar [["12","21"],["34","43"]], but if you lot usage flatmap, you lot acquire a Stream of Strings e.g. ["12", "21", "34", "43"]. In this aticle, we'll run into duad of working examples to sympathise the deviation betwixt map() in addition to flatmap() inward Java better.

I know it's non slow to sympathise the map() in addition to flatMap() function, peculiarly if you lot stimulate got non done whatever functional programming before. I was inward the same situation, It took me only about fourth dimension to actually sympathise purpose of map and flatMap in addition to cheers to Java SE 8 for Really Impatient, which helped me ot understnad these substitution functional concepts better. The explanation given inward this majority is actually groovy in addition to fifty-fifty if you lot don't stimulate got whatever functional programming experience, you lot volition understnad these novel things amongst niggling flake of effort. I highly recommend this majority to all Java developers who wishing to acquire Java 8.

How Stream.map() industrial plant inward Java 8

The Stream.map() role performs map functional functioning i.e. it stimulate got a Stream in addition to transform it to only about other Stream. It applies a role on each chemical ingredient of Stream in addition to shop render value into novel Stream. This way you lot tin give the axe transform a Stream of String into a Stream of Integer where Integer could hold upward length of String if you lot render the length() function. This is a real powerful role which is real helpful spell dealing amongst collection inward Java.

Here is an illustration of Stream.map() inward Java 8:

List listOfIntegers = Stream.of("1", "2", "3", "4")                .map(Integer::valueOf)                .collect(Collectors.toList());

In this example, nosotros stimulate got a Stream of String values which stand upward for numbers, yesteryear using map() role nosotros stimulate got converted this Stream to Stream of Integers. How? yesteryear appling Integer.valueOf() on each chemical ingredient of Stream. That's how "1" converted to intger 1 in addition to thence on. Once transformation is done, nosotros stimulate got collected the consequence into a List yesteryear converting Stream to List using Collectors.

How Stream.flatMap() industrial plant inward Java 8

The Stream.flatMap() function, every bit cite suggests, is the combination of a map in addition to a apartment operation. This way you lot commencement apply map role in addition to than flattens the result. Key deviation is the role used yesteryear map functioning returns a Stream of values or listing of values rather than unmarried value, that's why nosotros ask flattening. When you lot apartment a Stream of Stream, it gets converted into Stream of values.

To sympathise what flattening a current consists in, reckon a construction similar [ [1,2,3],[4,5,6],[7,8,9] ] which has "two levels". It's basicall a big List containing 3 to a greater extent than List.  Flattening this way transforming it inward a "one level" construction e.g. [ 1,2,3,4,5,6,7,8,9 ] i.e. only 1 list.

In short,
Before flattening - Stream of List of Integer
After flattening - Stream of Integer

Here is a code illustration to sympathise the flatMap() role better:

List evens = Arrays.asList(2, 4, 6); List odds = Arrays.asList(3, 5, 7); List primes = Arrays.asList(2, 3, 5, 7, 11);         List numbers = Stream.of(evens, odds, primes)                .flatMap(list -> list.stream())                .collect(Collectors.toList());         System.out.println("flattend list: " + numbers);  Output: flattend list: [2, 4, 6, 3, 5, 7, 2, 3, 5, 7, 11]

You tin give the axe run into that nosotros stimulate got 3 lists which are merged into 1 yesteryear using flatMap() function. For mapping you lot tin give the axe run into nosotros stimulate got used list.stream() role which returns multiple values instead of unmarried value. Finally, nosotros stimulate got collected the flattend current into a list. If  you want, you lot tin give the axe impress the in conclusion listing using forEach() method.

Stream.map() vs Stream.flatMap() inward Java 8

In short, hither are the key difference betwixt map() vs flatMap() inward Java 8:

• The role you lot overstep to map() functioning returns a unmarried value.
• The role you lot overstep to flatMap() opeartion returns a Stream of value.
• flatMap() is combination of map in addition to apartment operation. 
• map() is used for transformation only, but flatMap() is used for both transformation in addition to flattening. 

Now let's run into a sample Java computer programme to sympathise the differnce betwixt flatMap() in addition to map() better.

Java Program to demo deviation betwixt map vs flatMap

Here is our sample Java computer programme to demonstrate the existent deviation betwixt the map() in addition to flatMap() role of Stream cast inward Java 8. As I told before, map() is used to transform 1 Stream into only about other yesteryear applying a role on each chemical ingredient in addition to flatMap() does both transformation every bit good every bit flattening. The flatMap() role tin give the axe stimulate got a Stream of List in addition to render Stream of values combined from all those list. In the illustration below nosotros stimulate got collected the consequence inward a List but you lot tin give the axe also impress them using forEach() method of Java 8.

import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.stream.Collectors;  /**  * Java Program to demonstrate deviation betwixt map()  * vs flatMap() role inward Java 8. Both are defined  * inward Stream class.   *  * @author WINDOWS 8  */ public class Java8Demo {      public static void main(String args[]) {          // foods which helps inward weight loss         List<String> loseWeight = new ArrayList<>();         loseWeight.add("avocados");         loseWeight.add("beans");         loseWeight.add("salad");         loseWeight.add("oats");         loseWeight.add("broccoli");                          System.out.println("list of String : " + loseWeight);                  // let's usage map() method to convert listing of weight         // lose food, which are String to listing of ints         // which are length of each nutrient String                  List listOfInts = loseWeight.stream()                 .map(s -> s.length())                 .collect(Collectors.toList());                  System.out.println("list of ints generate yesteryear map(): " + listOfInts);                   // flatMap() example, let's commencement creat a listing of list         List<List> listOfListOfNumber = new ArrayList<>();         listOfListOfNumber.add(Arrays.asList(2, 4));         listOfListOfNumber.add(Arrays.asList(3, 9));         listOfListOfNumber.add(Arrays.asList(4, 16));                  System.out.println("list of listing : " + listOfListOfNumber);                  // let's usage flatMap() to flatten this listing into         // listing of integers i.e. 2,4,3,9,4,16                  List listOfIntegers = listOfListOfNumber.stream()                 .flatMap( list -> list.stream())                 .collect(Collectors.toList());                  System.out.println("list of numbers generated yesteryear flatMap : " + listOfIntegers);                       }  }  Output list of String : [avocados, beans, salad, oats, broccoli] list of ints generate yesteryear map(): [8, 5, 5, 4, 8] list of list : [[2, 4], [3, 9], [4, 16]] list of numbers generated yesteryear flatMap : [2, 4, 3, 9, 4, 16]

You tin give the axe run into that inward commencement example, the role used yesteryear map() method returns a unmarried value, the length of Stirng passed to it, spell inward instance of flatMap() the method returns a stream, which is basically your multiple values.

That's all virtually difference betwixt map() in addition to flatMap() inward Java 8. You should usage map() if you lot jsut desire to transform 1 Stream into only about other where each chemical ingredient gets converted to 1 unmarried value. Use flatMap() if the role used yesteryear map functioning render multiple values in addition to you lot desire only 1 listing containing all values. If you lot are nevertheless confused betwixt map() vs flatMap() in addition to thence become read Java SE 8 for Really Impatient By Cay S. Horstmann, 1 of the best majority to acquire virtually novel features of Java 8.

Further Learning
The Complete Java MasterClass
tutorial)

How to usage Stream cast inward Java 8 (tutorial)

How to usage filter() method inward Java 8 (tutorial)

How to usage forEach() method inward Java 8 (example)

How to bring together String inward Java 8 (example)

How to convert List to Map inward Java 8 (solution)

How to usage peek() method inward Java 8 (example)

5 Books to Learn Java 8 from Scratch (books)

How to convert current to array inward Java 8 (tutorial)

Java 8 Certification FAQ (guide)

Java 8 Mock Exams in addition to Practice Test (test)

Thanks for reading this article thence far. If you lot similar this article in addition to thence delight portion amongst your friends in addition to colleagues. If you lot stimulate got whatever question, doubt, or feedback in addition to thence delight drib a comment in addition to I'll seek to respond your question.

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