Stream in Java

Introduced in Java 8, the Stream API is used to process collections of objects. A stream is a sequence of objects that various methods which can pipelined to produce the desired result.

The features of Java stream are

- A stream is not a data structure instead it takes input from the collections, Arrays, or I/O channels.

- Streams do not change the original data structure, they only provide the result as per the pipelined methods.

- Each intermediate operation is lazily executed and returns a stream as a result, hence various intermediate operations can be pipelined. Terminal operations mark the end of the stream and return the result.

 

Different Operations On Streams-
Intermediate Operations:

1. map: The map method is used to returns a stream consisting of the results of applying the given function to the elements of this stream.
   List number = Arrays.asList(2,3,4,5);
   List square = number.stream().map(x->x*x).collect(Collectors.toList());
2. filter: The filter method is used to select elements as per the Predicate passed as argument.
   List names = Arrays.asList("Reflection","Collection","Stream");
   List result = names.stream().filter(s->s.startsWith("S")).collect(Collectors.toList());
3. sorted: The sorted method is used to sort the stream.
   List names = Arrays.asList("Reflection","Collection","Stream");
   List result = names.stream().sorted().collect(Collectors.toList());

Terminal Operations:

1. collect: The collect method is used to return the result of the intermediate operations performed on the stream.
   List number = Arrays.asList(2,3,4,5,3);
   Set square = number.stream().map(x->x*x).collect(Collectors.toSet());
2. forEach: The forEach method is used to iterate through every element of the stream.
   List number = Arrays.asList(2,3,4,5);
   number.stream().map(x->x*x).forEach(y->System.out.println(y));
3. reduce: The reduce method is used to reduce the elements of a stream to a single value.
   The reduce method takes a BinaryOperator as a parameter.List number = Arrays.asList(2,3,4,5);
   int even = number.stream().filter(x->x%2==0).reduce(0,(ans,i)-> ans+i);

@Transactional(readOnly = true)

public List<PostsListResponseDto> findAllDesc() {

  return postsRepository.findAllDesc().stream()

    .map(PostsListResponseDto::new) == .map(posts -> new PostsListResponseDto(posts))

    .collect(Collectors.toList()));

}

=> postsRepository.findAllDesc() 를 통해서 List<Posts>를 받아 온다음에 Stream API를 이용하여 .map을 통하여 필요한 정보만을 담은 PostListResponseDto를 만든 후에, .collect를 이용하여 List로 만들어서 반환한다.