스트림 (Stream)
- 배열, 컬렉션 등의 데이터를 하나씩 참조하여 처리 가능한 기능
- for문의 사용을 줄여 코드를 간결하게 함
- 스트림은 크게 3가지로 구성
-> Stream 생성
-> 중개 연산
-> 최종 연산
데이터소스객체.Stream생성().중개연산().최종연산();
스트림 생성
String[] arr = new String[]{"a", "b", "c"}; // 스트링 배열 생성
Stream stream = Arrats.stream(arr); // 배열을 스트림으로 변경
ArrayList list = new ArrayList(Arrays.asList(1,2,3));
Stream stream = list.stream();
스트림 중개연산
IntStream intStream = InstStream.range(1, 10).filter(n -> n % 2 == 0);
// 1부터 9까지의 데이터에서 짝수를 골라냄
IntStream intStream = IntStream.range(1,10).map(n -> n+1);
// 1부터 9까지의 데이터 모두 1씩 더해줘서 2부터 10까지의 데이터가 됨
스트림 최종연산
IntStream.range(1,5).sum()
IntStream.range(1,5).average().getAsDouble()
IntStream.range(1,5).min().getAsInt();
IntStream.range(1,5).max().getAsInt();
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.OptionalInt;
import java.util.stream.Collectors;
import java.util.stream.IntStream;
import java.util.stream.Stream;
public class Main {
public static void main(String[] args) {
System.out.println("== 배열 스트림 == ");
String[] arr = new String[]{"a", "b", "c"};
System.out.println("== fori ==");
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
System.out.println("== forEach ==");
for (String item: arr) {
System.out.println(item);
}
System.out.println("== to Stream ==");
Stream stream1 = Arrays.stream(arr);
stream1.forEach(System.out::println);
System.out.println("== 컬렉션 스트림 ==");
ArrayList list1 = new ArrayList(Arrays.asList(1, 2, 3));
System.out.println(list1);
Stream stream2 = list1.stream();
stream2.forEach(num -> System.out.println("num = " + num));
System.out.println("== 스트림 builder ==");
Stream streamBuilder = Stream.builder().add(1).add(2).add(3).build();
streamBuilder.forEach(System.out::println);
System.out.println("== 스트림 generate ==");
Stream streamGenerate = Stream.generate( () -> "abc" ).limit(3);
streamGenerate.forEach(System.out::println);
System.out.println("== 스트림 iterate ==");
Stream streamIterate = Stream.iterate(10, n -> n * 2).limit(3);
streamIterate.forEach(System.out::println);
System.out.println("== 기본타입 스트림 ==");
IntStream intStream = IntStream.range(1, 5);
intStream.forEach(System.out::println);
System.out.println("== Filtering ==");
IntStream intStream2 = IntStream.range(1, 10).filter(n -> n % 2 == 0);
intStream2.forEach(System.out::println);
System.out.println("== Mapping ==");
IntStream intStream3 = IntStream.range(1, 10).map(n -> n + 1);
intStream3.forEach(n -> System.out.print(n + " "));
System.out.println();
System.out.println("== Sorting ==");
IntStream intStream4 = IntStream.builder().add(5).add(1).add(3).add(4).add(2).build();
IntStream intStreamSort = intStream4.sorted();
intStreamSort.forEach(System.out::println);
System.out.println("== sum, average ==");
int sum = IntStream.range(1, 5).sum();
System.out.println("sum = " + sum);
double average = IntStream.range(1, 5).average().getAsDouble();
System.out.println("average = " + average);
System.out.println("== min, max ==");
int min = IntStream.range(1, 5).min().getAsInt();
System.out.println("min = " + min);
int max = IntStream.range(1, 5).max().getAsInt();
System.out.println("max = " + max);
System.out.println("== reduce ==");
Stream<Integer> stream3 = new ArrayList<Integer>(Arrays.asList(1, 2, 3, 4, 5)).stream();
System.out.println(stream3.reduce((x, y) -> x + y).get());
System.out.println("== forEach == ");
IntStream.range(1,10).filter(n -> n == 5).forEach(System.out::println);
}
}
import java.util.stream.IntStream;
public class Practice {
public static void main(String[] args) {
int[] arr = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
int sum = 0;
for (int num: arr) {
if (num % 2 == 0) {
sum += num;
}
}
System.out.println("sum = " + sum);
int sum2 = IntStream.range(1, 11).filter(x -> x % 2 == 0).sum();
System.out.println("sum2 = " + sum2);
}
}