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STUDY/Java

[JAVA] Stream 스트림

홍시31 2021. 11. 5. 11:43
 
 

01 Stream

- 객체 저장소(ArrayList, List)에 저장된 객체들을 처리하는 다른 방식

 

총 인원수 구하기
StreamTest.java
package com.stream;

import java.util.ArrayList;

class Streaminfo {
	String name;
	int kor;
	public Streaminfo() {}
	public Streaminfo(String name, int kor) {
		super();
		this.name = name;
		this.kor = kor;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public int getKor() {
		return kor;
	}
	public void setKor(int kor) {
		this.kor = kor;
	}
	
	
}
public class StreamTest {
	public static void main(String[] args) {
		ArrayList<Streaminfo> alist = new ArrayList<Streaminfo>();
		Streaminfo si = new Streaminfo();
		alist.add(new Streaminfo("백두산", 90));
		alist.add(new Streaminfo("한라산", 85));
		alist.add(new Streaminfo("소백산", 100));
		alist.add(new Streaminfo("광교산", 70));
		alist.add(new Streaminfo("태백산", 99));
		
		System.out.println("이름\t국어점수");
		System.out.println("=================");
		alist.forEach(k->System.out.println(k.getName() + "\t" + k.getKor()));
		int cnt = (int)alist.stream().count();
		System.out.println();
		System.out.println("총 인원은 " + cnt + "명 입니다.");
	}
}
 

① alist.stream().count(); // long형이므로 int로 형변환

 

[결과화면]

 

 

총합 구하기
StreamTest.java
package com.stream;
import java.util.ArrayList;

public class StreamTest {
// 생략
	public static void main(String[] args) {
		ArrayList<Streaminfo> alist = new ArrayList<Streaminfo>();
		Streaminfo si = new Streaminfo();
		alist.add(new Streaminfo("백두산", 90));
		alist.add(new Streaminfo("한라산", 85));
		alist.add(new Streaminfo("소백산", 100));
		alist.add(new Streaminfo("광교산", 70));
		alist.add(new Streaminfo("태백산", 99));
		
		System.out.println("이름\t국어점수");
		System.out.println("=================");
		alist.forEach(k->System.out.println(k.getName() + "\t" + k.getKor()));
		int sum = alist.stream().mapToInt(Streaminfo::getKor).sum();
		System.out.println();
		System.out.println("국어 점수의 총 합은 " + sum + "점 입니다.");
	}
}
 

① alist.stream().mapToInt(클래스명::get).sum();

 

[결과화면]

 

 

평균 구하기
StreamTest.java
package com.stream
import java.util.ArrayList;

public class StreamTest {
// 생략
	public static void main(String[] args) {
		ArrayList<Streaminfo> alist = new ArrayList<Streaminfo>();
		Streaminfo si = new Streaminfo();
		alist.add(new Streaminfo("백두산", 90));
		alist.add(new Streaminfo("한라산", 85));
		alist.add(new Streaminfo("소백산", 100));
		alist.add(new Streaminfo("광교산", 70));
		alist.add(new Streaminfo("태백산", 99));
		
		System.out.println("이름\t국어점수");
		System.out.println("=================");
		alist.forEach(k->System.out.println(k.getName() + "\t" + k.getKor()));
		double avg = alist.stream().mapToInt(Streaminfo::getKor).average().getAsDouble();
		System.out.println();
		System.out.println("국어 점수의 총 평균은 " + avg + "점 입니다.");
	}
}
 

① alist.stream().mapToInt(클래스명::get).average().getAsDouble();

 

[결과화면]

 

조건문 추가하기
StreamTest.java
package com.stream;
import java.util.ArrayList;

public class StreamTest {
// 생략
	public static void main(String[] args) {
		ArrayList<Streaminfo> alist = new ArrayList<Streaminfo>();
		Streaminfo si = new Streaminfo();
		alist.add(new Streaminfo("백두산", 90));
		alist.add(new Streaminfo("한라산", 85));
		alist.add(new Streaminfo("소백산", 100));
		alist.add(new Streaminfo("광교산", 70));
		alist.add(new Streaminfo("태백산", 99));
		
		System.out.println("이름\t국어점수");
		System.out.println("=================");
		alist.forEach(k->System.out.println(k.getName() + "\t" + k.getKor()));
		int cnt = (int)(alist.stream().filter(k->k.getKor()>=90).count());
		System.out.println();
		System.out.println("90점 이상인 인원 수는 " + cnt + "명 입니다.");
	}
}
 

① alist.sream().filter(조건).count(); // (k: 임의변수)

 

[결과화면]

 

 

조건문 추가하기 (목록 출력)
StreamTest.java
package com.stream;
import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

public class StreamTest {
// 생략
	public static void main(String[] args) {
		ArrayList<Streaminfo> alist = new ArrayList<Streaminfo>();
		Streaminfo si = new Streaminfo();
		alist.add(new Streaminfo("백두산", 90));
		alist.add(new Streaminfo("한라산", 85));
		alist.add(new Streaminfo("소백산", 100));
		alist.add(new Streaminfo("광교산", 70));
		alist.add(new Streaminfo("태백산", 99));
		
		System.out.println("이름\t국어점수");
		System.out.println("=================");
		alist.forEach(k->System.out.println(k.getName() + "\t" + k.getKor()));
		List<Streaminfo> k_list = alist.stream().filter(k->k.getKor()>=90).collect(Collectors.toList());
		System.out.println();
		System.out.println("90점 이상인 인원 출력");
		System.out.println("=================");
		k_list.forEach(k->System.out.println(k.getName() + "\t" + k.getKor()));
	}
}
 

① filter: 조건

② collect: List 목록

 

[결과화면]

 

 

 

 

 

 

 

 

 

02 Stream으로 정렬하기

이름별 오름차순
SortTest.java
package com.stream;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class SortTest {
	String name;
	int kor;
	int eng;

	public SortTest() {}

	public SortTest(String name, int kor, int eng) {
	super();
	this.name = name;
	this.kor = kor;
	this.eng = eng;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getKor() {
		return kor;
	}

	public void setKor(int kor) {
		this.kor = kor;
	}

	public int getEng() {
		return eng;
	}

	public void setEng(int eng) {
		this.eng = eng;
	}

	public static void main(String[] args) {
		ArrayList<SortTest> alist = new ArrayList<SortTest>();
		alist.add(new SortTest("백두산", 56, 75));
		alist.add(new SortTest("한라산", 90, 98));
		alist.add(new SortTest("소백산", 66, 56));
		alist.add(new SortTest("태백산", 74, 48));
		alist.add(new SortTest("광교산", 89, 100));
		alist.add(new SortTest("설악산", 100, 81));
		alist.add(new SortTest("도봉산", 99, 99));
		
		System.out.println("이름\t국어점수\t영어점수");
		System.out.println("========================");
		List<SortTest> name_asc = alist.stream().sorted(Comparator.comparing(SortTest::getName)).collect(Collectors.toList());
		name_asc.forEach(k->System.out.println(k.getName() + "\t" + k.getKor() + "\t" + k.getEng()));
	}
}
 

① alist.stream().sorted(Comparator.comparing(클래스명::get)).collect(Collectors.toList());

② getName 오름차순으로 정렬

 

[결과화면]

 

 

이름별 내림차순
SortTest.java
package com.stream;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class SortTest {
// 생략
	public static void main(String[] args) {
		ArrayList<SortTest> alist = new ArrayList<SortTest>();
		alist.add(new SortTest("백두산", 56, 75));
		alist.add(new SortTest("한라산", 90, 98));
		alist.add(new SortTest("소백산", 66, 56));
		alist.add(new SortTest("태백산", 74, 48));
		alist.add(new SortTest("광교산", 89, 100));
		alist.add(new SortTest("설악산", 100, 81));
		alist.add(new SortTest("도봉산", 99, 99));
		
		System.out.println("이름\t국어점수\t영어점수");
		System.out.println("========================");
		List<SortTest> name_asc = alist.stream().sorted(Comparator.comparing(SortTest::getName).reversed()).collect(Collectors.toList());
		name_asc.forEach(k->System.out.println(k.getName() + "\t" + k.getKor() + "\t" + k.getEng()));
	}
}
 

① alist.stream().sorted(Comparator.comparing(클래스명::get).reversed()).collect(Collectors.toList())

 

[결과화면]

 

 

이름별 오름차순 + 국어점수별 오름차순
SortTest.java
package com.stream;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class SortTest {
// 생략
	public static void main(String[] args) {
		ArrayList<SortTest> alist = new ArrayList<SortTest>();
		alist.add(new SortTest("백두산", 56, 75));
		alist.add(new SortTest("한라산", 90, 98));
		alist.add(new SortTest("백두산", 66, 56));
		alist.add(new SortTest("백두산", 74, 48));
		alist.add(new SortTest("한라산", 89, 100));
		alist.add(new SortTest("한라산", 100, 81));
		alist.add(new SortTest("백두산", 99, 99));
		
		System.out.println("이름\t국어점수\t영어점수");
		System.out.println("========================");
		List<SortTest> name_asc = alist.stream().sorted(Comparator.comparing(SortTest::getName).thenComparing(SortTest::getKor)).collect(Collectors.toList());
		name_asc.forEach(k->System.out.println(k.getName() + "\t" + k.getKor() + "\t" + k.getEng()));
	}
}
 

① alist.stream().sorted(Comparator.comparing(클래스명::get).thenComparing(클래스명::get)).collect(Collectors.toList())

 

[결과화면]

 

 

이름별 내림차순 + 국어점수별 내림차순
SortTest.java
package com.stream;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class SortTest {
// 생략
	public static void main(String[] args) {
		ArrayList<SortTest> alist = new ArrayList<SortTest>();
		alist.add(new SortTest("백두산", 56, 75));
		alist.add(new SortTest("한라산", 90, 98));
		alist.add(new SortTest("백두산", 66, 56));
		alist.add(new SortTest("백두산", 74, 48));
		alist.add(new SortTest("한라산", 89, 100));
		alist.add(new SortTest("한라산", 100, 81));
		alist.add(new SortTest("백두산", 99, 99));
		
		System.out.println("이름\t국어점수\t영어점수");
		System.out.println("========================");
		Comparator<SortTest> kor_des = Comparator.comparing(SortTest::getKor).reversed();
		List<SortTest> name_asc = alist.stream().sorted(Comparator.comparing(SortTest::getName).reversed().thenComparing(kor_des)).collect(Collectors.toList());
		name_asc.forEach(k->System.out.println(k.getName() + "\t" + k.getKor() + "\t" + k.getEng()));
	}
}
 

① 국어 점수별 내림차순 객체 생성: Comparator<클래스명> 변수명1 = Comparator.comparing(클래스명::get).reversed();

② 이름별 내림차순 정렬에 ① 추가: alist.stream().sorted(Comparator.comparing(클래스명::get).reversed().thenComparing(변수명1))

 

[결과화면]

 

 

이름별 내림차순 + 국어점수별 오름차순
SortTest.java
package com.stream;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class SortTest {
// 생략
	public static void main(String[] args) {
		ArrayList<SortTest> alist = new ArrayList<SortTest>();
		alist.add(new SortTest("백두산", 56, 75));
		alist.add(new SortTest("한라산", 90, 98));
		alist.add(new SortTest("백두산", 66, 56));
		alist.add(new SortTest("백두산", 74, 48));
		alist.add(new SortTest("한라산", 89, 100));
		alist.add(new SortTest("한라산", 100, 81));
		alist.add(new SortTest("백두산", 99, 99));
		
		System.out.println("이름\t국어점수\t영어점수");
		System.out.println("========================");
		List<SortTest> name_asc = alist.stream().sorted(Comparator.comparing(SortTest::getName).reversed().thenComparing(SortTest::getKor)).collect(Collectors.toList());
		name_asc.forEach(k->System.out.println(k.getName() + "\t" + k.getKor() + "\t" + k.getEng()));
	}
}
 

① alist.stream().sorted(Comparator.comparing(클래스명::get).reversed().thenComparing(클래스명::get))

 

[결과화면]

 

 

이름별 오름차순 + 국어점수별 내림차순
SortTest.java
package com.stream;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class SortTest {
// 생략
	public static void main(String[] args) {
		ArrayList<SortTest> alist = new ArrayList<SortTest>();
		alist.add(new SortTest("백두산", 56, 75));
		alist.add(new SortTest("한라산", 90, 98));
		alist.add(new SortTest("백두산", 66, 56));
		alist.add(new SortTest("백두산", 74, 48));
		alist.add(new SortTest("한라산", 89, 100));
		alist.add(new SortTest("한라산", 100, 81));
		alist.add(new SortTest("백두산", 99, 99));
		
		System.out.println("이름\t국어점수\t영어점수");
		System.out.println("========================");
		Comparator<SortTest> kor_des = Comparator.comparing(SortTest::getKor).reversed();
		List<SortTest> name_asc = alist.stream().sorted(Comparator.comparing(SortTest::getName).thenComparing(kor_des)).collect(Collectors.toList());
		name_asc.forEach(k->System.out.println(k.getName() + "\t" + k.getKor() + "\t" + k.getEng()));
	}
}
 

① 국어 점수별 내림차순 객체 생성: Comparator<클래스명> 변수명1 = Comparator.comparing(클래스명::get).reversed();

② 이름별 오름차순 정렬에 ① 추가: alist.stream().sorted(Comparator.comparing(클래스명::get).thenComparing(변수명1))

 

[결과화면]

 

 

 

 

 

 

 

 

 

03 Map을 이용해서 Stream 사용

 

성별로 그룹지어 국어 총점 구하기
MapTest.java
package com.stream;

import java.util.ArrayList;
import java.util.Map;
import java.util.stream.Collectors;

public class MapTest {
	enum Gender{남자, 여자}
	String name;
	int kor, eng;
	Gender gender;
	
	public MapTest() {	}
	
	public MapTest(String name, int kor, int eng, MapTest.Gender gender) {
		super();
		this.name = name;
		this.kor = kor;
		this.eng = eng;
		this.gender = gender;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getKor() {
		return kor;
	}

	public void setKor(int kor) {
		this.kor = kor;
	}

	public int getEng() {
		return eng;
	}

	public void setEng(int eng) {
		this.eng = eng;
	}

	public Gender getGender() {
		return gender;
	}

	public void setGender(Gender gender) {
		this.gender = gender;
	}

	public static void main(String[] args) {
		ArrayList<MapTest> alist = new ArrayList<MapTest>();
		alist.add(new MapTest("백두산", 80, 65, MapTest.Gender.남자));
		alist.add(new MapTest("한라산", 77, 64, MapTest.Gender.여자));
		alist.add(new MapTest("태백산", 66, 78, MapTest.Gender.남자));
		alist.add(new MapTest("소백산", 78, 71, MapTest.Gender.여자));
		alist.add(new MapTest("백두산", 90, 83, MapTest.Gender.여자));
		alist.add(new MapTest("소백산", 99, 90, MapTest.Gender.여자));
		alist.add(new MapTest("한라산", 100, 89, MapTest.Gender.남자));
		
		System.out.println("이름\t국어점수\t영어점수\t성별");
		System.out.println("=============================");
		alist.forEach(k->System.out.println(k.getName() + "\t" + k.getKor() + "\t" + k.getEng() + "\t" + k.getGender()));
		
		System.out.println();
		System.out.println("=============================");
		Map<MapTest.Gender, Integer> m_sum = alist.stream().collect(Collectors.groupingBy(MapTest::getGender, Collectors.summingInt(MapTest::getKor)));
		System.out.println("남자 국어 총점 = " + m_sum.get(MapTest.Gender.남자));
		
		int w_kor = alist.stream().filter(k->k.getGender()==MapTest.Gender.여자).mapToInt(MapTest::getKor).sum();
		System.out.println("여자 국어 총점 = " + w_kor);
	}
}
 

① enum을 사용해 성별은 남자와 여자만 입력받기

② Map 이용(남자 그룹): Map<MapTest.Gender, Integer> m_sum = alist.stream().collect(Collectors.groupingBy(MapTest::getGender, Collectors.summingInt(MapTest::getKor)));

③ filter 이용(여자 그룹): alist.stream().filter(k->k.getGender()==MapTest.Gender.여자).mapToInt(MapTest::getKor).sum();

 

[결과화면]

 

 

Map 구조로 출력하기
MapTest.java
package com.stream;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

public class MapTest {
	enum Gender{남자, 여자}
	String name;
	int kor, eng;
	Gender gender;
	
	public MapTest() {	}
	
	public MapTest(String name, int kor, int eng, MapTest.Gender gender) {
		super();
		this.name = name;
		this.kor = kor;
		this.eng = eng;
		this.gender = gender;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public int getKor() {
		return kor;
	}

	public void setKor(int kor) {
		this.kor = kor;
	}

	public int getEng() {
		return eng;
	}

	public void setEng(int eng) {
		this.eng = eng;
	}

	public Gender getGender() {
		return gender;
	}

	public void setGender(Gender gender) {
		this.gender = gender;
	}

	public static void main(String[] args) {
		ArrayList<MapTest> alist = new ArrayList<MapTest>();
		alist.add(new MapTest("백두산", 80, 65, MapTest.Gender.남자));
		alist.add(new MapTest("한라산", 77, 64, MapTest.Gender.여자));
		alist.add(new MapTest("태백산", 66, 78, MapTest.Gender.남자));
		alist.add(new MapTest("소백산", 78, 71, MapTest.Gender.여자));
		alist.add(new MapTest("백두산", 90, 83, MapTest.Gender.여자));
		alist.add(new MapTest("소백산", 99, 90, MapTest.Gender.여자));
		alist.add(new MapTest("한라산", 100, 89, MapTest.Gender.남자));
		
		System.out.println("이름\t성별\t국어점수\t영어점수");
		System.out.println("=============================");
		Map<MapTest.Gender, List<MapTest>> m_kor = alist.stream().collect(Collectors.groupingBy(MapTest::getGender));
		m_kor.get(MapTest.Gender.남자).forEach(k->System.out.println(k.getName() + "\t" + k.getGender() + "\t" + k.getKor() + "\t" + k.getEng()));
	}
}
 

① 성별로 그룹지어 Map 구조에 저장

② 성별이 남자일 때 이름, 성별, 국어, 영어 점수 출력

 

[결과화면]

 

 

 

 

 

 

 

 

 

 

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