Collect Elements of a Stream into a Collection

Collect with `toList()` and `toSet()`

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

System.out.println(Arrays
    .asList("apple", "banana", "pear", "kiwi", "orange")
    .stream()
    .filter(s -> s.contains("a"))
    .collect(Collectors.toList())
);
// 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
System.out.println(strings
        .stream()
        .filter(s -> s != null && s.length() <= 3)
        .collect(Collectors.toCollection(ArrayList::new))
);

// syntax with lambda
System.out.println(strings
        .stream()
        .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 = students.stream()
      .collect(Collectors.toMap(Student::getId, Student::getName));
  System.out.println(IdToName);

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<>();
list.stream().forEach(s -> {
            map.computeIfAbsent(s.getName(), x -> new ArrayList<>()).add(s.getSubject());
        });
System.out.println(map);

Output:

{ Robert=[LITERATURE], 
Sascha=[ENGLISH, MATH, SCIENCE, LITERATURE], 
Davis=[MATH, SCIENCE, GEOGRAPHY] }

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<>();

list.stream().forEach(student -> {
    map.computeIfAbsent(student.getName(), s -> new HashMap<>())
            .computeIfAbsent(student.getSubject(), s -> new ArrayList<>())
            .add(student.getMarks());
});

System.out.println(map);

Output:

{ 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]} }

Cheat-Sheet

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Table Of Contents

Collect Elements of a Stream into a Collection

Collect with toList() and toSet()

Explicit control over the implementation of List or Set

Cheat-Sheet

Collect with `toList()` and `toSet()`

Explicit control over the implementation of `List` or `Set`