# 수업 - [주소 : https://slog.gg/p/13855](https://slog.gg/p/13855) # 1강 - [24 11, 25, p 13855, 1강, 스트림 소개, 반복문과 조건문을 사용하는 현대적인 방법](https://youtu.be/RdhX4s_wRJI) ![img](https://i.imgur.com/Mowwn9o.png) # 2강 - [24 11, 25, p 13855, 2강, 1부터 10까지 출력하기, IntStream, forEach 은 종단함수이다.](https://youtu.be/p6xbRRlygpE) ```java import java.util.stream.IntStream; class Main { public static void main(String[] args) { // 일반 for (int i = 1; i <= 10; i++) { System.out.println(i); } // 스트림 IntStream.rangeClosed(1, 10) .forEach(e -> { System.out.println(e); }); // 스트림 IntStream.rangeClosed(1, 10) .forEach(e -> System.out.println(e)); // 스트림 IntStream.rangeClosed(1, 10) .forEach(System.out::println); } } ``` # 3강 - [24 11, 25, p 13855, 3강, 각 요소는 다음 요소를 기다리지 않고 종단함수까지 진행, Trace Current Stream Chain 는 스트림 디버깅에 최고](https://youtu.be/-VRuwtAp0mE) ```java import java.util.stream.IntStream; class Main { public static void main(String[] args) { // 일반 for (int i = 1; i <= 10; i++) { if (i % 2 == 0) continue; System.out.println(i); } // 스트림, 인텔리제이에서 브레이크 포인트 걸어서 Trace Current Stream Chain 뷰를 확인해보세요. IntStream.rangeClosed(1, 10) .filter(e -> e % 2 != 0) .forEach(e -> { System.out.println(e); }); } } ``` # 4강 - [24 11, 25, p 13855, 4강, 배열의 요소에 2 곱하기, map, 스트림 연산은 재료를 변경하지 않는다, 스트림 연산의 특징 : 불변성](https://youtu.be/OXG9YUQ3_JA) ```java import java.util.Arrays; class Main { public static void main(String[] args) { System.out.println("== No Stream =="); noStreamVersion(); System.out.println("== Stream =="); streamVersion(); } private static void noStreamVersion() { int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; System.out.println("= 원본 numbers(변형 전) ="); System.out.println(Arrays.toString(numbers)); for (int i = 0; i < numbers.length; i++) { numbers[i] *= 2; } System.out.println("= 원본 numbers(변형 후) ="); System.out.println(Arrays.toString(numbers)); } private static void streamVersion() { int[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10}; System.out.println("= 원본 numbers(스트림 전) ="); System.out.println(Arrays.toString(numbers)); int[] calculatedNumbers = Arrays.stream(numbers) .map(e -> e * 2) .toArray(); System.out.println("= 원본 numbers(스트림 후) ="); System.out.println(Arrays.toString(numbers)); System.out.println("= 새 배열 calculatedNumbers ="); System.out.println(Arrays.toString(calculatedNumbers)); } } ``` # 5강 - [24 11, 25, p 13855, 5강, int 배열을 String 배열로 바꾸는 방법](https://youtu.be/DBQN2RWxDXI) - 기본형 타입에서 레퍼런스 타입으로 매핑할 때 : mapToObj - 그 외의 경우 : map ```java import java.util.Arrays; class Main { public static void main(String[] args) { System.out.println("== No Stream =="); noStreamVersion(); System.out.println("== Stream v1 =="); streamVersionV1(); System.out.println("== Stream v2 =="); streamVersionV2(); } private static void noStreamVersion() { int[] numbers = new int[]{1, 2, 3}; String[] messages = new String[numbers.length]; for (int i = 0; i < numbers.length; i++) { if (numbers[i] % 2 == 0) continue; messages[i] = numbers[i] + "번"; } System.out.println("= 원본 ="); System.out.println(Arrays.toString(numbers)); System.out.println("= 결과 ="); System.out.println(Arrays.toString(messages)); } private static void streamVersionV1() { Object[] array = Arrays.stream(new int[]{1, 2, 3}) .filter(e -> e % 2 != 0) .mapToObj(e -> e + "번") .toArray(); System.out.println("= 결과 ="); System.out.println(Arrays.toString(array)); } private static void streamVersionV2() { String[] array = Arrays.stream(new int[]{1, 2, 3}) .filter(e -> e % 2 != 0) .mapToObj(e -> e + "번") .toArray(String[]::new); System.out.println("= 결과 ="); System.out.println(Arrays.toString(array)); } } ``` # 6강 - [24 11, 25, p 13855, 6강, String 배열을 int 배열로 바꾸는 방법](https://youtu.be/a6SAfwmU4dc) - 레퍼런스 타입에서 int 타입으로 매핑할 때 : mapToInt - 그 외의 경우 : map ```java import java.util.Arrays; class Main { public static void main(String[] args) { System.out.println("== No Stream =="); noStreamVersion(); System.out.println("== Stream =="); streamVersion(); } private static void noStreamVersion() { String[] messages = new String[]{"1번", "2번", "3번"}; int[] numbers = new int[messages.length]; for (int i = 0; i < messages.length; i++) { String message = messages[i]; // message 에서 가장 마지막 한자를 제거 message = message.substring(0, message.length() - 1); int number = Integer.parseInt(message); if (number % 2 == 0) continue; numbers[i] = number; } System.out.println("= 결과 ="); System.out.println(Arrays.toString(numbers)); } private static void streamVersion() { int[] array = Arrays.stream(new String[]{"1번", "2번", "3번"}) .map(e -> e.substring(0, e.length() - 1)) .mapToInt(Integer::parseInt) .filter(number -> number % 2 != 0) .toArray(); System.out.println("= 결과 ="); System.out.println(Arrays.toString(array)); } } ``` # 7강 - [24 11, 25, p 13855, 7강, int 배열을 List 형태로 변경](https://youtu.be/iw23P6ZhLsc) - 레퍼런스 타입에서 int 타입으로 매핑할 때 : mapToInt - 그 외의 경우 : map ```java import java.util.Arrays; import java.util.List; class Main { public static void main(String[] args) { // v1 List list1 = Arrays .stream(new int[]{10, 20, 30}) .mapToObj(e -> e) // int => Integer .toList(); // v2 List list2 = Arrays .stream(new int[]{10, 20, 30}) .boxed() // int => Integer .toList(); // v3 : 이 경우는 래핑(boxing)할 필요가 없다. List list3 = Arrays .stream(new Integer[]{10, 20, 30}) .toList(); } } ``` # 8강 - [24 11, 25, p 13855, 8강, 스트림 문제를 위한 샘플 데이터 생성](https://youtu.be/WA5qd3YdCHM) ```java import java.util.ArrayList; import java.util.List; class Main { public static void main(String[] args) { List people = new ArrayList<>(); people.add(new Person(1, "Alice", 20, 'F')); people.add(new Person(2, "Bob", 25, 'M')); people.add(new Person(3, "David", 35, 'M')); } } class Person { private int id; private String name; private int age; private char gender; public Person(int id, String name, int age, char gender) { this.id = id; this.name = name; this.age = age; this.gender = gender; } public int getId() { return id; } public String getName() { return name; } public int getAge() { return age; } public char getGender() { return gender; } } ``` # 9강 - [24 11, 25, p 13855, 9강, 남자의 나이의 합, 출력 : 60](https://youtu.be/-wElC3dm0ck) ```java package com.ll; import java.util.ArrayList; import java.util.List; class Main { public static void main(String[] args) { List people = new ArrayList<>(); people.add(new Person(1, "Alice", 20, 'F')); people.add(new Person(2, "Bob", 25, 'M')); people.add(new Person(3, "David", 35, 'M')); // 문제 : 남성들의 나이의 합 System.out.println("== No Stream =="); noStreamVersion(people); System.out.println("== Stream =="); streamVersion(people); } private static void noStreamVersion(List people) { int sum = 0; for (Person person : people) { if (person.getGender() == 'M') { sum += person.getAge(); } } System.out.println("sum of age : " + sum); } private static void streamVersion(List people) { int sum = people .stream() .filter(e -> e.getGender() == 'M') .mapToInt(e -> e.getAge()) .sum(); System.out.println("sum of age : " + sum); } } class Person { private int id; private String name; private int age; private char gender; public Person(int id, String name, int age, char gender) { this.id = id; this.name = name; this.age = age; this.gender = gender; } public int getId() { return id; } public String getName() { return name; } public int getAge() { return age; } public char getGender() { return gender; } } ``` # 10강 - [24 11, 25, p 13855, 10강, 남자의 나이의 평균, 출력 : 30.0](https://youtu.be/tuLURqwCnU0) ```java import java.util.ArrayList; import java.util.List; class Main { public static void main(String[] args) { List people = new ArrayList<>(); people.add(new Person(1, "Alice", 20, 'F')); people.add(new Person(2, "Bob", 25, 'M')); people.add(new Person(3, "David", 35, 'M')); // 문제 : 남성들의 나이의 평균 System.out.println("== No Stream =="); noStreamVersion(people); System.out.println("== Stream =="); streamVersion(people); } private static void noStreamVersion(List people) { int sum = 0; int itemsOfMan = 0; for (Person person : people) { if (person.getGender() == 'M') { sum += person.getAge(); itemsOfMan++; } } double avg = (double) sum / itemsOfMan; System.out.println("avg of age : " + avg); } private static void streamVersion(List people) { double avg = people .stream() .filter(e -> e.getGender() == 'M') .mapToInt(e -> e.getAge()) .average() .orElse(0); System.out.println("avg of age : " + avg); } } class Person { private int id; private String name; private int age; private char gender; public Person(int id, String name, int age, char gender) { this.id = id; this.name = name; this.age = age; this.gender = gender; } public int getId() { return id; } public String getName() { return name; } public int getAge() { return age; } public char getGender() { return gender; } } ``` # 11강 - [24 11, 25, p 13855, 11강, 남자의 이름들, 출력 : Bob, David](https://youtu.be/N5ILahDI6iU) ```java package com.ll; import java.util.ArrayList; import java.util.List; import java.util.stream.Collectors; class Main { public static void main(String[] args) { List people = new ArrayList<>(); people.add(new Person(1, "Alice", 20, 'F')); people.add(new Person(2, "Bob", 25, 'M')); people.add(new Person(3, "David", 35, 'M')); // 문제 : 남성의 이름들 System.out.println("== No Stream =="); noStreamVersion(people); System.out.println("== Stream =="); streamVersion(people); } private static void noStreamVersion(List people) { List names = new ArrayList<>(); for (Person person : people) { if (person.getGender() == 'M') { names.add(person.getName()); } } String manNames = String.join(", ", names); System.out.println("manNames : " + manNames); } private static void streamVersion(List people) { String manNames = people.stream() .filter(person -> person.getGender() == 'M') .map(Person::getName) .collect(Collectors.joining(", ")); System.out.println("manNames : " + manNames); } } class Person { private int id; private String name; private int age; private char gender; public Person(int id, String name, int age, char gender) { this.id = id; this.name = name; this.age = age; this.gender = gender; } public int getId() { return id; } public String getName() { return name; } public int getAge() { return age; } public char getGender() { return gender; } } ``` # 12강 - [24 11, 25, p 13855, 12강, id가 2번인 사람의 이름을 출력 : Bob](https://youtu.be/sx-pBejxQDY) ```java import java.util.ArrayList; import java.util.List; import java.util.Optional; class Main { public static void main(String[] args) { List people = new ArrayList<>(); people.add(new Person(1, "Alice", 20, 'F')); people.add(new Person(2, "Bob", 25, 'M')); people.add(new Person(3, "David", 35, 'M')); // 문제 : 남성의 이름들 System.out.println("== No Stream =="); noStreamVersion(people); System.out.println("== Stream =="); streamVersion(people); } private static void noStreamVersion(List people) { Person found = null; for (Person person : people) { if (person.getId() == 2) { found = person; break; } } if (found == null) { System.out.println("not found"); return; } System.out.println("found : " + found.getName()); } private static void streamVersion(List people) { Optional opPerson = people.stream() .filter(e -> e.getId() == 2) .findFirst(); Person found = opPerson.orElse(null); if (found == null) { System.out.println("not found"); return; } System.out.println("found : " + found.getName()); } } class Person { private int id; private String name; private int age; private char gender; public Person(int id, String name, int age, char gender) { this.id = id; this.name = name; this.age = age; this.gender = gender; } public int getId() { return id; } public String getName() { return name; } public int getAge() { return age; } public char getGender() { return gender; } } ```

자바, 스트림