170420(목) - Streams <Intermediate Operations>

Streams 

- Mapping

ㆍelements의 형태를 변경하는 process

ㆍsql 에서의 select 방식이라고 생각하면 된다.

- map

ㆍstreams element 변경이나 형식 변경에 사용

ㆍ다른 함수를 인수로 취한다.

students.stream()
        .map(Student::getName)
        .forEach(System.out::println);

- FlatMap

ㆍmap과 별도로 새로 형성된 element의 하위 Stream을 생성

ㆍ마지막에 모든 하위 steam을 단일 steam으로 변경

List<Integer> numbers = Arrays.asList(1, 2, 3, 4);
List<List<Integer>> mapped = 
                     numbers.stream()
                     .map(number -> Arrays.asList(number -1, number, number +1))
                     .collect(Collectors.toList());
System.out.println(mapped); //:> [[0, 1, 2], [1, 2, 3], [2, 3, 4], [3, 4, 5]]
        
List<Integer> flattened = 
                numbers.stream()
                .flatMap(number -> Arrays.asList(number -1, number, number +1).stream())
                .collect(Collectors.toList());
System.out.println(flattened);  //:> [0, 1, 2, 1, 2, 3, 2, 3, 4, 3, 4, 5]

- Filtering

- filter

ㆍpredicate를 인수로 사용함

//Only the students with score >= 60
students.stream()
            .filter(student -> student.getScore() >= 60)
            .collect(Collectors.toList());

- Unique Elements

ㆍ중복 요소 제거

//Get distinct list of names of the students
students.stream()
            .map(Student::getName)
            .distinct()
            .collect(Collectors.toList());

- Limiting

ㆍ조건이 만족되면 나머지는 short circuit에 의해서 skip 된다.

//List of first 3 students who have age > 20
students.stream()
            .filter(s -> s.getAge() > 20)
            .map(Student::getName)
            .limit(3)
            .collect(Collectors.toList());

- Skipping

ㆍ스킵되는 수가 element 보다 많으면 빈값이 리턴된다.

//List of all the students who have age > 20 except the first 3
students.stream()
            .filter(s -> s.getAge() > 20)
            .map(Student::getName)
            .skip(3)
            .collect(Collectors.toList());

- Sorting

students.stream()
           .map(Student::getName)
           .sorted()
           .collect(Collectors.toList());

students.stream()
           .sorted(Comparator.comparing(Student::getName))
           .map(Student::getName)
           .collect(Collectors.toList());

//Sorting names if the Students in descending order
students.stream()
           .map(Student::getName)
           .sorted(Comparator.reverseOrder())
           .collect(Collectors.toList());
//Sorting names if the Students in descending order
students.stream()
           .map(Student::getName)
           .sorted(Comparator.reverseOrder())
           .collect(Collectors.toList());
//Sorting students by First Name and Last Name both
students.stream()
           .sorted(Comparator.comparing(Student::getFirstName).
                              thenComparing(Student::getLastName))
           .map(Student::getName)
           .collect(Collectors.toList());
//Sorting students by First Name Descending and Last Name Ascending
students.stream()
           .sorted(Comparator.comparing(Student::getFirstName)
                              .reversed()
                              .thenComparing(Student::getLastName))
           .map(Student::getName)
           .collect(Collectors.toList());