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

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 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)

Basic Programs

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