How To Convert Flow To List, Set, Map, As Well As Concurrenthashmap Inwards Coffee Eight - Larn Amongst Examples

In Java 8, Stream is ane of the most of import shape every bit it allows a lot of useful functional operations e.g. filter, map, flatmap, etc on a collection of object. Hence, going forrad converting a Collection to Stream, performing operations in addition to and so converting the resultant dorsum to dissimilar Collection classes similar List, Set, in addition to Map volition hold upwards a mutual task. The outset occupation of converting Stream to Collection is solved past times Java designer past times straight adding a stream() method on Collection interface, which agency all other interfaces in addition to classes which derived from Stream besides receive got this method. Though the mo occupation of converting a Stream to List, Set, in addition to Map requires about agreement of Stream API of JDK 8 in addition to that's what yous volition acquire inwards this article.

I'll demonstrate yous measuring past times measuring illustration but the most of import matter to acquire is that Collector interface tin hold upwards used to collect the resultant of Stream to whatever collection classes similar List, Set, Map, in addition to fifty-fifty ConcurrentMap.

It industrial plant amongst a companion collect() method from Stream class. This method accepts a Collector in addition to returns the Collection yous want.  Btw, if yous are simply starting amongst Java programming in addition to so I propose yous to outset acquire through a comprehensive Java course of pedagogy like The Complete Java Masterclass course on Udemy to start on the correct note. It's besides ane of the most up-to-date in addition to comprehensive course.

Ok, So without wasting whatever to a greater extent than time, let's start amongst the work.

1. Stream to List

You tin utilization the toList() method of Collectors shape to convert a Stream to List inwards Java 8. Here is an illustration for converting a current of integers to a listing of integers. The current is from about other listing of Integers.

List<Integer> listOfIntegers            = input.stream().collect(Collectors.toList());

The listing volition incorporate the items or elements inwards the same fellowship they appear inwards Stream in addition to duplicates are besides allowed. Though the listing implementation is non guaranteed e.g. yous can't await an ArrayList or LinkedList, though if yous desire that tin hold upwards done in addition to we'll encounter it shortly.

Btw, if yous are wondering how does the collect method know which type of List to render in addition to so don't worry, they figured it out using fantabulous type inference available to lambda expressions. In short, they know it from the Stream chemical constituent every bit good type of the resultant expected.

1.1 Stream to ArrayList

In the previous example, nosotros encounter that toList() method render a List implementation but at that topographic point is no guarantee over implementation. This may piece of work for most of the cases but inwards many other cases, yous demand concrete implementations e.g. ArrayList. In that case, yous demand to utilization the toCollection() method of Collectors class.

This method accepts a Supplier which returns a new, empty Collection of the appropriate type.

ArrayList<Integer> aList          = input.stream()                .collect(Collectors.toCollection(ArrayList::new));

You tin encounter that nosotros receive got passed ArrayList::new to the collection() method because nosotros wanted to accumulate the resultant of Stream into an ArrayList. If yous desire whatever other Collection e.g. Vector, yous tin besides overstep Vector::new. If yous are wondering what is this code called in addition to so don't worry, it's known every bit constructor reference, similar to a method reference.

1.2 Stream to LinkedList

This is quite similar to the previous example. Instead of ArrayList nosotros receive got converted Stream into a LinkedList in addition to at that topographic point is entirely ane modify required inwards the previous code, instead of ArrayList::new nosotros receive got passed LinkedList::new to the toCollection() method.

LinkedList<Integer> linkedList             = input.stream()                   .collect(Collectors.toCollection(LinkedList::new));

The returned LinkedList volition incorporate all the elements of Stream inwards the same fellowship they appear inwards Stream.  You tin farther see The Complete Java Masterclass course to acquire to a greater extent than virtually Stream inwards Java 8.

2. Stream to Set

You tin convert a Stream to Set past times using the Collectors.toSet() method. Since Set doesn't allow duplicates in addition to doesn't furnish whatever ordering guarantee, whatever duplicate elements from Stream are lost in addition to ordering is besides gone.

Set<Integer> aSet = input.stream().collect(Collectors.toSet());

There are no implementation guarantees provided for the Set returned past times collect method here. You can't assume it a HashSet, but it volition something which implements Set interface. Also, the size of Set volition hold upwards less than or equal to a expose of elements inwards the in conclusion Stream because duplicate elements are lost when yous covert Stream to Set.

2.1 Stream to HashSet

If yous demand a HashSet rather than a Set in addition to so yous demand to utilization the toCollection() method rather than the toSet() method. As nosotros receive got seen inwards the instance of ArrayList, the toCollection() method allows yous to specify which type of Collection shape yous desire past times providing a supplier. Here is an illustration for converting Stream to HashSet inwards Java.

HashSet<Integer> anHashSet            = input.stream()                   .collect(Collectors.toCollection(HashSet::new));

As amongst whatever other Set, when yous convert Stream to HashSet, all duplicate elements volition hold upwards lost in addition to fellowship of elements volition hold upwards gone.

2.2 Stream to LinkedHashSet

Similar to HashSet, yous tin besides utilization toCollection() method of Collectors shape to convert a Stream to LinkedHashSet inwards Java. The primary departure betwixt HashSet in addition to LinkedHashSet is that fellowship is preserved. Elements inwards the LinkedHashSet exists inwards the fellowship they are inserted. Btw, If yous are interested inwards learning to a greater extent than differences yous tin encounter my before article HashSet vs LinkedHashSet inwards Java.

LinkedHashSet<Integer> aLinkedHashSet               = input.stream()                      .collect(Collectors.toCollection(LinkedHashSet::new));

Again, similar whatever other Set, duplicate elements from Stream volition hold upwards lost, thence the size of the LinkedHashSet volition hold upwards less than or equal to the expose of elements inwards the in conclusion Stream.

2.3 Stream to TreeSet

Similar to before examples, yous tin besides utilization the toCollection() method of Collectors shape to convert a Stream to TreeSet inwards Java. Though, yous should shout back that TreeSet is a sorted Set in addition to keeps the elements inwards their natural fellowship or a custom fellowship specified past times Comparator.

TreeSet<Integer> aTreeSet          = input.stream()                 .collect(Collectors.toCollection(TreeSet::new));

In this example, elements inwards the TreeSet volition hold upwards inwards their sorted fellowship which may hold upwards dissimilar from their fellowship inwards Stream. Again, whatever duplicate chemical constituent volition hold upwards lost because TreeSet doesn't allow duplicate in addition to thence the size of TreeSet will hold upwards less than or equal to the size of the in conclusion Stream.

If yous are interested to acquire to a greater extent than virtually dissimilar collection classes in addition to how to piece of work amongst them using Stream, see books)

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Thanks a lot for reading this article so far. If yous similar these Stream to Collection similar List, Set in addition to Map examples in addition to so delight portion amongst your friends. If yous receive got whatever feedback or dubiousness in addition to so delight drib a note.

P. S. - If yous are looking for about gratis courses to acquire novel concepts in addition to features introduced inwards Java 8 in addition to so yous tin besides banking concern jibe out this listing of Free Java 8 courses on FreeCodeCamp. 

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