Collect Elements of a Stream into a Collection

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Collect with toList() and toSet()

Elements from a Stream can be easily collected into a container by using the Stream.collect operation:

    .asList("apple", "banana", "pear", "kiwi", "orange")
    .filter(s -> s.contains("a"))
// prints: [apple, banana, pear, orange]

Other collection instances, such as a Set, can be made by using other Collectors built-in methods. For example, Collectors.toSet() collects the elements of a Stream into a Set.

Explicit control over the implementation of List or Set

According to documentation of Collectors#toList() and Collectors#toSet(), there are no guarantees on the type, mutability, serializability, or thread-safety of the List or Set returned.

For explicit control over the implementation to be returned, Collectors#toCollection(Supplier) can be used instead, where the given supplier returns a new and empty collection.

// syntax with method reference
        .filter(s -> s != null && s.length() <= 3)

// syntax with lambda
        .filter(s -> s != null && s.length() <= 3)
        .collect(Collectors.toCollection(() -> new LinkedHashSet<>()))

Collecting Elements using toMap

Collector accumulates elements into a Map, Where key is the Student Id and Value is Student Value.

List<Student> students = new ArrayList<Student>(); 
  students.add(new Student(1,"test1"));
  students.add(new Student(2,"test2"));
  students.add(new Student(3,"test3"));
  Map<Integer, String> IdToName =
      .collect(Collectors.toMap(Student::getId, Student::getName));

Output :

{1=test1, 2=test2, 3=test3}

The Collectors.toMap has another implementation `Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,

Function<? super T, ? extends U> valueMapper,
BinaryOperator<U> mergeFunction)`.The mergeFunction is mostly used to select either new value or retain old value if the key is repeated when adding a new member in the Map from a list.

The mergeFunction often looks like: (s1, s2) -> s1 to retain value corresponding to the repeated key, or (s1, s2) -> s2 to put new value for the repeated key.

Collecting Elements to Map of Collections

Example: from ArrayList to Map<String, List<>>

Often it requires to make a map of list out of a primary list. Example: From a student of list, we need to make a map of list of subjects for each student.

List<Student> list = new ArrayList<>();
list.add(new Student("Davis", SUBJECT.MATH, 35.0));
list.add(new Student("Davis", SUBJECT.SCIENCE, 12.9));
list.add(new Student("Davis", SUBJECT.GEOGRAPHY, 37.0));

list.add(new Student("Sascha", SUBJECT.ENGLISH, 85.0));
list.add(new Student("Sascha", SUBJECT.MATH, 80.0));
list.add(new Student("Sascha", SUBJECT.SCIENCE, 12.0));
list.add(new Student("Sascha", SUBJECT.LITERATURE, 50.0));

list.add(new Student("Robert", SUBJECT.LITERATURE, 12.0));

Map<String, List<SUBJECT>> map = new HashMap<>(); -> {
            map.computeIfAbsent(s.getName(), x -> new ArrayList<>()).add(s.getSubject());


{ Robert=[LITERATURE], 

Example: from ArrayList to Map<String, Map<>>

List<Student> list = new ArrayList<>();
list.add(new Student("Davis", SUBJECT.MATH, 1, 35.0));
list.add(new Student("Davis", SUBJECT.SCIENCE, 2, 12.9));
list.add(new Student("Davis", SUBJECT.MATH, 3, 37.0));
list.add(new Student("Davis", SUBJECT.SCIENCE, 4, 37.0));

list.add(new Student("Sascha", SUBJECT.ENGLISH, 5, 85.0));
list.add(new Student("Sascha", SUBJECT.MATH, 1, 80.0));
list.add(new Student("Sascha", SUBJECT.ENGLISH, 6, 12.0));
list.add(new Student("Sascha", SUBJECT.MATH, 3, 50.0));

list.add(new Student("Robert", SUBJECT.ENGLISH, 5, 12.0));

Map<String, Map<SUBJECT, List<Double>>> map = new HashMap<>(); -> {
    map.computeIfAbsent(student.getName(), s -> new HashMap<>())
            .computeIfAbsent(student.getSubject(), s -> new ArrayList<>())



{ Robert={ENGLISH=[12.0]}, 
Sascha={MATH=[80.0, 50.0], ENGLISH=[85.0, 12.0]}, 
Davis={MATH=[35.0, 37.0], SCIENCE=[12.9, 37.0]} }


Goal | Code | — | — | Collect to a List | Collectors.toList() | Collect to an ArrayList with pre-allocated size | Collectors.toCollection(() -> new ArrayList<>(size)) | Collect to a Set | Collectors.toSet() | Collect to a Set with better iteration performance | Collectors.toCollection(() -> new LinkedHashSet<>()) | Collect to a case-insensitive Set<String> | Collectors.toCollection(() -> new TreeSet<>(String.CASE_INSENSITIVE_ORDER)) | Collect to an EnumSet<AnEnum> (best performance for enums)| Collectors.toCollection(() -> EnumSet.noneOf(AnEnum.class)) | Collect to a Map<K,V> with unique keys | Collectors.toMap(keyFunc,valFunc) | Map MyObject.getter() to unique MyObject | Collectors.toMap(MyObject::getter, Function.identity()) | Map MyObject.getter() to multiple MyObjects | Collectors.groupingBy(MyObject::getter)

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