WSL2: Find and Delete Zone.Identifier files

Introduction

Java8 has come up with great features than previous java versions, Stream is a part of util package, map() and flatMap() are two important intermediate methods of Stream to perform Stream transformation and flattening operations.

Stream<t> will be input for map() and then Stream<R> will be output of map() after applying functional logic on each element of Stream<T>. For example Stream<String> which holds Employee names of organisation, after applying map() output will be Stream<Integer> which stores length of employee names, so here the functional logic is getting length of employee name.

map() method is only doing transfermation of one Stream into another type of Stream

map() method

 Method map() will take every element of Stream as an input and returns one output to each and every input, thats why it is called as one-to-one mapping function.

map() with non nested list/Stream

Stream transformation of list with map()

From the above figure you can clearly understand that map() will take stream of list of Three names of a persons, then we do calculation of length of each name and then converting into new stream, you have to notice that each element is taking as an input and each input can get output. Similar we done with upper case conversion example.

Each input has output when we use map() to transform into new Stream

In the above figure Stream of names converted/transformed into Stream of lengths( Integers) and in scond example converted into Steam of uppercase names.

		List<String> names = new ArrayList<String>();
		names.add("Sachin");
		names.add("Dravid");
		names.add("Ganguly");

		Stream<String> namesStream = names.stream();

		Stream<Integer> namesLengthStream = namesStream.map(name -> name.length());

		List<Integer> namesLengthList = namesLengthStream.collect(Collectors.toList());

		System.out.println("names list:- " + names);
		System.out.println("Names length list:- " + names.stream().map(na -> na.length()).collect(Collectors.toList()));
		System.out.println("Names inti uppercase as  list:- " + names.stream().map(na -> na.toUpperCase()).collect(Collectors.toList()));

output:

names list:- [Sachin, Dravid, Ganguly]
Names length list:- [6, 6, 7]
Names inti uppercase as  list:- [SACHIN, DRAVID, GANGULY]

map() with nested list/Stream

Here we can apply map() on nested list/stream, but here we have to use nested map() to perform operations on nested list

nested map() with nested list/stream

		List<String> names = new ArrayList<String>();
		names.add("Sachin");
		names.add("Dravid");
		names.add("Ganguly");

		List<String> namesTwo = new ArrayList<String>();
		namesTwo.add("SachinTwo");
		namesTwo.add("DravidTwo");
		namesTwo.add("GangulyTwo");

		List<String> names3 = new ArrayList<String>();
		names3.add("SachinThree");
		names3.add("DravidThree");
		names3.add("GangulyThree");

		List<List<String>> namesList = new ArrayList<>();
		namesList.add(names);
		namesList.add(namesTwo);
		namesList.add(names3);
		System.out.println("Names Nested list:- " + namesList);
		
		System.out.println("Names into uppercase:- "
				+ namesList.stream().map(namesa -> namesa.stream().map(
						el -> el.toUpperCase()).collect(Collectors.toList()))
						.collect(Collectors.toList()));
		
		System.out.println("Names length list:- "
				+ namesList.stream().map(namesa -> namesa.stream().map(
						el -> el.length()).collect(Collectors.toList()))
						.collect(Collectors.toList()));

Names Nested list:- [[Sachin, Dravid, Ganguly], [SachinTwo, DravidTwo, GangulyTwo], [SachinThree, DravidThree, GangulyThree]]
Names into uppercase:- [[SACHIN, DRAVID, GANGULY], [SACHINTWO, DRAVIDTWO, GANGULYTWO], [SACHINTHREE, DRAVIDTHREE, GANGULYTHREE]]
Names length list:- [[6, 6, 7], [9, 9, 10], [11, 11, 12]]

In the output you can notice that map() produce indivedual output for each indivedual input element. If we want to combine all nested streams into a single stream we need to perform flattening operation which is possible with flatMap() method. For example if we want to get output as single stream [SACHIN, DRAVID, GANGULY, SACHINTWO, DRAVIDTWO, GANGULYTWO, SACHINTHREE, DRAVIDTHREE, GANGULYTHREE] is not possible with map(), here we need to use flatMap()

flatMap()

Method flatMap() if Stream is useful to apply mapping and flattening on a Stream/Collection of Streams/Collections format( Collection<Collection<T>>  -> flatMap()  -> Collection<T>).

flatMap() on Stream of Streams

System.out.println("Names flattening with flatMap:- "+
namesList.stream().flatMap(
				x -> x.stream()
				).collect(Collectors.toList()));
		
System.out.println("Names into uppercase with flatMap:- " + 
        namesList.stream()
		.flatMap(namesl -> namesl.stream()).map(
        nl -> nl.toUpperCase()
        ).collect(Collectors.toList()));

Names flattening with flatMap:- [Sachin, Dravid, Ganguly, SachinTwo, DravidTwo, GangulyTwo, SachinThree, DravidThree, GangulyThree]
Names into uppercase with flatMap:- [SACHIN, DRAVID, GANGULY, SACHINTWO, DRAVIDTWO, GANGULYTWO, SACHINTHREE, DRAVIDTHREE, GANGULYTHREE]

References

Java8 Streams API