3 Examples Of Flow + Collect() Method Of Inwards Coffee 8

Hello guys, you lot may know that Java 8 brought Stream API which supports a lot of functional programming operations like filter, map, flatMap, reduce, as well as collect. In this article, you lot volition acquire close the collect() method. The collect() method of Stream shape tin endure used to accumulate elements of whatever Stream into a Collection. In Java 8, you lot volition ofttimes write code which converts a Collection similar a List or Set to Stream as well as and so applies simply about logic using functional programming methods similar the filter, map, flatMap as well as and so converts the effect dorsum to the Collection similar a List, Set, Map, or ConcurrentMap in Java. In this final part, the collect() method of Stream helps. It allows you lot to accumulate the effect into pick fo container you lot desire similar a list, set, or a map.

Programmers ofttimes confuse that collect() method belongs to Collector shape but that's non true. It is defined inward Stream shape as well as that's why you lot tin telephone band it on Stream afterwards doing whatever filtering or mapping. It accepts a Collector to accumulate elements of Stream into specified Collection.

The Collector shape provides dissimilar methods e.g. toList(), toSet(), toMap(), as well as toConcurrentMap() to collect the effect of Stream into List, Set, Map, as well as ConcurrentMap inward Java.

It every bit good provides a particular toCollection() method which tin endure used to collect Stream elements into a specified Collection similar ArrayList, Vector, LinkedList or HashSet.

It's every bit good a terminal performance which agency afterwards calling this method on Stream, you lot cannot telephone band whatever other method on Stream.

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Java 8 Stream.collect() Examples

In this article, we'll come across a twain of examples of Stream's collect() method to collect the effect of current processing into a List, Set, as well as Map inward Java. In other words, you lot tin every bit good say we'll convert a given Stream into List, Set, as well as Map inward Java

1. Stream to List using collect()

You tin collect the effect of a Stream processing pipeline inward a listing past times using the Collectors.toList() method. Just transcend the Collectors.toList() to collect() method every bit shown below:

List<String> listOfStringStartsWithJ
 = listOfString
     .stream()
     .filter( sec -> s.startsWith("J"))
     .collect(Collectors.toList());


The listing returned past times the collect method volition receive got all the String which starts alongside "J" inward the same guild they seem inward the original listing because both Stream as well as List croak on elements inward order. This is an of import item which you lot should know because you lot ofttimes require to procedure as well as collect elements inward order.

If you lot desire to acquire to a greater extent than close ordered as well as unordered collection I advise you lot bring together Set doesn't provider ordering as well as doesn't allow duplicate, whatever duplicate from Stream volition endure discarded as well as guild of elements volition endure lost.

Here is an illustration to convert Stream to Set using collect() as well as Collectors inward Java 8:

The laid upwardly of String inward this illustration contains all the String which starts alongside missive of the alphabet C like C and C++. The guild volition endure lost as well as whatever duplicate volition endure removed.

3. Stream to Map using toMap()

You tin exercise a Map from elements of Stream using collect() as well as Collectors.toMap() method. Since a Map similar HashMap store 2 objects i.e. key as well as value as well as Stream contains simply ane element, you lot require to supply the logic to extract key as well as value object from Stream element.

For example, if you lot receive got a Stream of String as well as so you lot tin exercise a Map where the key is String itself as well as value is their length, every bit shown inward the next example:

Map<String, Integer> stringToLength 
   = listOfString
        .stream()
        .collect(
            Collectors.toMap(Function.identity(), String::length));

The Function.identity() used hither denotes that same object is used every bit a key. Though you lot require to endure a niggling fighting careful since Map doesn't allow duplicate keys if your Stream contains duplicate elements than this conversion volition fail.

In that case, you lot require to utilization simply about other overloaded toMap() method every bit good accepts an declaration to resolve conflict inward instance of duplicate keys.  Also, toMap() doesn't supply whatever guarantee on what form of Map is returned. This is simply about other of import item you lot should remember.

If you lot desire to acquire to a greater extent than close dealing alongside Collections as well as Stream I advise you lot convey a await at simply about other Pluralsight gem, ArrayList, HashSet, or LinkedList. 

There is every bit good a toCollection() method inward the Collectors shape which allows you lot to convert Stream to whatever collection. In the next example, nosotros volition acquire how to collect Stream elements into an ArrayList.

ArrayList<String> stringWithLengthGreaterThanTwo 
  = listOfString
      .stream()
      .filter( sec -> s.length() > 2)
      .collect(Collectors.toCollection(ArrayList::new));

Since ArrayList is a list, it provides ordering guarantee, thence all the elements inward the ArrayList volition endure inward the same guild they seem inward original List as well as Stream.

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Java Program to Use Stream.collect() method

Here is our consummate Java plan to demonstrate the utilization of collect() method of Stream shape to convert Stream into dissimilar Collection classes inward Java e.g. List, Set, Map, as well as Collection itself.

import java.util.ArrayList; import java.util.Arrays; import java.util.List; import java.util.Map; import java.util.Set; import java.util.function.Function; import java.util.stream.Collectors;  public class Code {    public static void main(String[] args) {      List<String> listOfString = Arrays.asList("Java", "C", "C++", "Go",         "JavaScript", "Python", "Scala");     System.out.println("input listing of String: " + listOfString);      // Example 1 - converting Stream to List using collect() method     List<String> listOfStringStartsWithJ                               = listOfString.stream()                                             .filter(s -> s.startsWith("J"))                                             .collect(Collectors.toList());      System.out.println("list of String starts alongside missive of the alphabet J: "         + listOfStringStartsWithJ);      // Example 2 - converting Stream to Set     Set<String> setOfStringStartsWithC                        = listOfString.stream()                                     .filter(s -> s.startsWith("C"))                                     .collect(Collectors.toSet());      System.out.println("set of String starts alongside missive of the alphabet C: "         + setOfStringStartsWithC);      // Example three - converting Stream to Map     Map<String, Integer> stringToLength                            = listOfString.stream()                                          .collect(Collectors.toMap(Function.identity(),                                                                     String::length));     System.out.println("map of string as well as their length: " + stringToLength);      // Example - Converting Stream to Collection e.g. ArrayList     ArrayList<String> stringWithLengthGreaterThanTwo                         = listOfString.stream()                                       .filter(s -> s.length() > 2)                                       .collect(Collectors.toCollection(ArrayList::new));     System.out.println("collection of String alongside length greather than 2: "         + stringWithLengthGreaterThanTwo);    } }  Output input list of String:  [Java, C, C++, Go, JavaScript, Python, Scala] list of String starts with missive of the alphabet J:  [Java, JavaScript] set of String starts with missive of the alphabet C:  [C++, C] map of string and their length:  {Java=4, C++=3, C=1, Scala=5, JavaScript=10, Go=2, Python=6} collection of String with length greather than 2:  [Java, C++, JavaScript, Python, Scala]

That's all close how to utilization the collect() method of Stream shape inward Java 8. Along alongside collect(), you lot tin utilization the Collectors method to convert Stream to List, Set, Map, or whatever other Collection of your choice. Just explore the Collectors Javadoc to acquire to a greater extent than close those methods.

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Thanks for reading this article so far. If you lot similar this Java 8 Stream tutorial as well as so delight portion alongside your friends as well as colleagues. If you lot receive got whatever questions or feedback as well as so delight drib a note.

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