170420(목) - Streams <Terminal Operations>

Streams

- Matching

- At least one element matches to the predicate

//Check if at least one student has got distinction
Boolean hasStudentWithDistinction = students.stream()
                            .anyMatch(student -> student.getScore() > 80);

     - All element match to the predicate

//Check if All of the students have distinction
Boolean hasAllStudentsWithDistinction = students.stream()
                            .allMatch(student -> student.getScore() > 80);

- None of the elements matches to the predicate

//Return true if None of the students are over distinction
Boolean hasAllStudentsBelowDistinction = students.stream()
                            .noneMatch(student -> student.getScore() > 80);

- Finding

//Returns any student that matches to the given condition
students.stream().filter(student -> student.getAge() > 20)
                                                   .findAny();
//Returns first student that matches to the given condition
students.stream().filter(student -> student.getAge() > 20)
                                                   .findFirst();

- Reducing

ㆍ하나의 element 형태로 출력을 생성하기 위해서 streams내의 element를 반복하여 처리

ㆍ모든 요소의 합, 최대 최소 element 계산 등에 유리

ㆍ함수형 언어의 fold 개념과 비슷하다.

- Summing and Multiplying

//Summing all elements of a stream
Integer sum = numbers.stream()
                .reduce(0, (x, y) -> x + y); //reduce(identity, accumulator)

//Product of all elements in a stream
Integer product = numbers.stream()
                .reduce(1, (x, y) -> x * y);

//Summing without passing an identity
Optional<integer> sum = numbers.stream()
                .reduce((x, y) -> x + y);
//Product without passing an identity
Optional<integer> product = numbers.stream()
                .reduce((x, y) -> x * y);

- Min and Max

//Min of the stream
Optional<integer> min = numbers.stream()
                          .reduce(0, Integer::min);
//Max of the stream
Optional<integer> max = numbers.stream()
                          .reduce(0, Integer::max);

- Optional

ㆍ객체가 할당되었는지, 할당 해제되었는지 나타냄 : Null 반환에 대한 대비책

ㆍfindAny() 에서 Optional  반환 -> map은 Optional instance에서 호출된다.

students.stream()
           .filter(student -> student.getScore() > 80) // filter
           .findAny()         //Any student matching to the filter
           .map(Student::getName)    // mapping to students name
           .ifPresent(System.out::println);  // print if name of the student is not Null