#16 Stream - Filter

● 도입

- 이제부터 Stream interface에 있는 method 들을 하나씩 배워볼 것이다.

- 제일 먼저 Filter method에 대해 배워볼 것이다.

- Filter method는 말 그대로 무언가를 걸러내는 method이다.

[Stream interface에 있는 filter method]

public interface Stream<T> extends BaseStream<T, Stream<T>> {
	Stream<T> filter(Predicate<? super T> predicate);
	// Predicate를 인자(argument)로 받는 것을 알 수 있다.
}

- 만족하는 데이터만 걸러낸다.
- Predicate에 true를 반환하는 데이터만 존재하는 새로운 Stream을 만들어 return 한다.

 

 

● 실습

1.

package com.fastcampus.functionalprogramming.chapter6;

import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Chapter6Section2 {
	public static void main(String[] args) {
		// 양수만 걸러내는 filter를 Stream 형식으로 만들어보자
		Stream<Integer> numberStream = Stream.of(-5, -3, 3, 7, 10);
		Stream<Integer> filterdNumberStream = numberStream.filter(x -> x > 0);
		List<Integer> filterdNumbers = filterdNumberStream.collect(Collectors.toList());
		System.out.println(filterdNumbers);
		
		// 간단
		List<Integer> filterdNumbers2 = Stream.of(-5, -3, 3, 7, 10) // Stream을 만들고
						.filter(x -> x > 0) // filter를 하고
						.collect(Collectors.toList()); // List로 만든다.
		System.out.println(filterdNumbers2);
	}
}

 

2.

2-1. 이메일이 검증(true)된 유저만 걸러내기

package com.fastcampus.functionalprogramming.chapter6.model;

import java.util.List;

public class User {
	private int id;
	private String name;
	private String emailAddress;
	private boolean isVerified;
	private List<Integer> friendUserIds;
	
	public int getId() {
		return id;
	}
	
	public User setId(int id) {
		this.id = id;
		return this;
	}
	
	public String getName() {
		return name;
	}
	
	public User setName(String name) {
		this.name = name;
		return this;
	}
	
	public String getEmailAddress() {
		return emailAddress;
	}
	
	public User setEmailAddress(String emailAddress) {
		this.emailAddress = emailAddress;
		return this;
	}
	
	public boolean isVerified() {
		return isVerified;
	}
	
	public User setVerified(boolean isVerified) {
		this.isVerified = isVerified;
		return this;
	}
	
	public List<Integer> getFriendUserIds() {
		return friendUserIds;
	}
	
	public User setFriendUserIds(List<Integer> friendUserIds) {
		this.friendUserIds = friendUserIds;
		return this;
	}

	@Override
	public String toString() {
		return "User [id=" + id + ", " + (name != null ? "name=" + name + ", " : "")
				+ (emailAddress != null ? "emailAddress=" + emailAddress + ", " : "") + "isVerified=" + isVerified
				+ ", " + (friendUserIds != null ? "friendUserIds=" + friendUserIds : "") + "]";
	}
}

package com.fastcampus.functionalprogramming.chapter6.model;

import java.math.BigDecimal;
import java.time.LocalDateTime;
import java.util.List;

public class Order {
	private long id;
	private LocalDateTime createdAt;
	private long createdByUserId;
	private OrderStatus status;
	private BigDecimal amount;
	private List<OrderLine> orderLines;

	public enum OrderStatus {
		CREATED,
		IN_PROGRESS,
		ERROR,
		PROCESSED
	}

	public long getId() {
		return id;
	}

	public Order setId(long id) {
		this.id = id;
		return this;
	}

	public LocalDateTime getCreatedAt() {
		return createdAt;
	}

	public Order setCreatedAt(LocalDateTime createdAt) {
		this.createdAt = createdAt;
		return this;
	}

	public long getCreatedByUserId() {
		return createdByUserId;
	}

	public Order setCreatedByUserId(long createdByUserId) {
		this.createdByUserId = createdByUserId;
		return this;
	}

	public OrderStatus getStatus() {
		return status;
	}

	public Order setStatus(OrderStatus status) {
		this.status = status;
		return this;
	}

	public BigDecimal getAmount() {
		return amount;
	}

	public Order setAmount(BigDecimal amount) {
		this.amount = amount;
		return this;
	}

	public List<OrderLine> getOrderLines() {
		return orderLines;
	}

	public Order setOrderLines(List<OrderLine> orderLines) {
		this.orderLines = orderLines;
		return this;
	}

	@Override
	public String toString() {
		return "Order [id=" + id + ", " + (createdAt != null ? "createdAt=" + createdAt + ", " : "")
				+ "createdByUserId=" + createdByUserId + ", " + (status != null ? "status=" + status + ", " : "")
				+ (amount != null ? "amount=" + amount + ", " : "")
				+ (orderLines != null ? "orderLines=" + orderLines : "") + "]";
	}
}

package com.fastcampus.functionalprogramming.chapter6;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;

import com.fastcampus.functionalprogramming.chapter6.model.User;

public class Chapter6Section2 {
	public static void main(String[] args) {
		// 2.
		// 이메일이 검증(true)된 유저만 뽑기, 검증 여부는 User class의 isVerified 필드로 구분
		User user1 = new User();
		user1.setId(101);
		user1.setName("Alice");
		user1.setVerified(true);
		user1.setEmailAddress("Alice@garden.com");
		
		User user2 = new User();
		user2.setId(102);
		user2.setName("Bob");
		user2.setVerified(false);
		user2.setEmailAddress("Bob@garden.com");
		
		User user3 = new User();
		user3.setId(103);
		user3.setName("Charlie");
		user3.setVerified(true);
		user3.setEmailAddress("Charlie@garden.com");
		
		List<User> users = Arrays.asList(user1, user2, user3);
		List<User> verifiedUsers = users.stream().filter(user -> user.isVerified()).collect(Collectors.toList());
		System.out.println(verifiedUsers);
		// cf) stream() method : Collection을 Stream으로 바꿔주는 method(SET과 LIST 모두 사용 가능)
		
		// Method Reference를 이용해 더 간단하게
		// filter(user -> user.isVerified()) --> filter(User::isVerified)
		List<User> users2 = Arrays.asList(user1, user2, user3);
		List<User> verifiedUsers2 = users.stream().filter(User::isVerified).collect(Collectors.toList());
		System.out.println(verifiedUsers2);
		
		// 검증 되지 않은 user만 뽑기
		List<User> users3 = Arrays.asList(user1, user2, user3);
		List<User> unVerifiedUsers = users.stream().filter(user -> !user.isVerified()).collect(Collectors.toList());
		System.out.println(unVerifiedUsers);	
	}
}

 

2-2. Stream을 이용해 ERROR 상태인 order만 걸러내기

package com.fastcampus.functionalprogramming.chapter6;

import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.Stream;

import com.fastcampus.functionalprogramming.chapter6.model.Order;
import com.fastcampus.functionalprogramming.chapter6.model.User;
import com.fastcampus.functionalprogramming.chapter6.model.Order.OrderStatus;

public class Chapter6Section2 {
	public static void main(String[] args) {
		// 3.
		// Stream을 이용해 ERROR 상태인 order만 걸러내기
		Order order1 = new Order();
		order1.setId(1001);
		order1.setStatus(OrderStatus.CREATED);	   // enum 사용
		
		Order order2 = new Order();
		order2.setId(1002);
		order2.setStatus(OrderStatus.ERROR);	   // enum 사용
		
		Order order3 = new Order();
		order3.setId(1003);
		order3.setStatus(OrderStatus.PROCESSED);   // enum 사용
		
		Order order4 = new Order();
		order4.setId(1004);
		order4.setStatus(OrderStatus.ERROR);	   // enum 사용
		
		Order order5 = new Order();
		order5.setId(1005);
		order5.setStatus(OrderStatus.IN_PROGRESS); // enum 사용
		
		List<Order> orders = Arrays.asList(order1, order2, order3, order4, order5);
		List<Order> filteredOrders = orders.stream().filter(order -> order.getStatus() == OrderStatus.ERROR).collect(Collectors.toList());
		System.out.println(filteredOrders);
	}
}