What is Stream API?

1. What is the Stream API?

The Stream API is a key component of Java starting from version 8, enabling easy and flexible data collection processing in a functional style. A stream is a sequence of elements that supports sequential and parallel data operations.

2. Benefits of Using Stream API

• Reduces boilerplate code.
• Improves readability and conciseness, especially when working with collections.
• Built-in support for parallel processing (parallel streams).
• Rich set of methods for filtering, sorting, mapping, and collecting data.

3. Key Characteristics of Streams

1. Seamless integration with lambda expressions. Stream API is designed with functional operations in mind.
2. Streams do not store data. They only move, filter, or transform the provided elements.
3. Streams are immutable. They do not modify the source collection.
4. Streams cannot be reused. Once consumed, a stream is considered exhausted.
5. No random access by index.
6. Lazy operations. Execution is deferred until a terminal operation is invoked.

4. Interfaces for Working with Streams

The Stream API consists of the following key interfaces:

• java.util.stream.Stream<T> – for objects.
• DoubleStream, IntStream, LongStream – for primitive types.

5. Creating Streams

Streams can be created in several ways:

1. From a collection:

List<String> words = Arrays.asList("hello", "hola", "hallo", "ciao");
Stream<String> stream = words.stream();

2. Using Stream.of:

Stream<String> stream = Stream.of("hello","hola", "hallo", "ciao");

3. From an array:

String[] words = {"hello", "hola", "hallo", "ciao"};
Stream<String> stream = Stream.of(words);

4. For primitives:

int[] nums = {1, 2, 3, 4, 5};
System.out.println(Arrays.stream(nums).count());

int[] nums = {1, 2, 3, 4, 5};
System.out.println(IntStream.of(nums).count());

5. Using generators and iterators:

Stream<Double> s = Stream.generate(Math::random).limit(5);

Stream<Integer> s = Stream.iterate(1, n -> n * 2).limit(5);

6. Using builder:

Stream<String> s = Stream.<String>builder().add("h").add("e").add("l").add("l").add("o").build();
s.forEach(System.out::print); // "hello"

7. Ranges:

// stream of 1, 2, 3
IntStream s = IntStream.range(1, 4);
// stream of 1, 2, 3, 4
IntStream s = IntStream.rangeClosed(1, 4);

6. Stream API Collection Processing Examples

Example before using Stream API:

List<Student> studentsScore = new ArrayList<>();
for(Student s : students) {
    if(s.getScore() > 90.0) {
        studentsScore.add(s);
    }
}
Collections.sort(studentsScore, new Comparator<>() {
    public int compare(Student s1, Student s2) {
        return Double.compare(
           s1.getScore(), s2.getScore());
    }
});

Rewritten using Stream API:

List<Student> studentsScore = students
        .stream()
        .filter(s -> s.getScore() >= 90.0)
        .sorted(Comparator.comparing(Student::getScore))
        .collect(Collectors.toList());

A data stream is a data transmission channel. It represents a sequence of objects and operates on a data source like an array or collection. The Stream API defines several stream-related interfaces within the java.util.stream package.

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