#17 Stream - Map(데이터의 변형)

● 도입

- Stream 안에 있는 데이터를 변형하는데 사용하는 method
- 데이터에 해당 함수가 적용된 결과물을 제공하는 stream을 return

[Stream interface에 있는 map method]

public interface Stream<T> extends BaseStream<T, Stream<T>> {
	<R> Stream<R> map(Function<? super T, ? extends R> mapper);
}

- T type 데이터를 받아서 해당 데이터를 변형한 후 R type 데이터로 return한다.

● 실습
1. 숫자를 2배로 만들자

package com.fastcampus.functionalprogramming.chapter6;

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

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

public class Chapter6Section3 {
	public static void main(String[] args) {
		Function<Integer, Integer> doubleFunc = (Integer x) -> {
			return x * 2;
		};
		
		List<Integer> numberList = Arrays.asList(-4, 3, 6);
		
		// 1.
		// x2가 되게 해보자
		Stream<Integer> numberStream = numberList.stream();
		Stream<Integer> numberStream2 = numberStream.map(doubleFunc);
		// 간단 : Stream<Integer> numberStream2 = numberStream.map(x -> x * 2);
		List<Integer> numberListX2 = numberStream2.collect(Collectors.toList());
		System.out.println(numberListX2);
		
		// 1-1.
		// x3이 되게 해보자
		List<Integer> numberListX3 = numberList.stream().map(x -> x * 3).collect(Collectors.toList());
		System.out.println(numberListX3);
		
		// 2.
		// output 타입을 다르게(input : Integer / output : String)
		Stream<Integer> numberListStream = numberList.stream();
		Stream<String> strStream = numberListStream.map(x -> "Number is " + x);
		List<String> strList = strStream.collect(Collectors.toList());
		System.out.println(strList);
	}
}

2.
2-1. user object가 담긴 List를 이용해 user의 이메일만 담겨있는 List를 만들자

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;

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

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

public class Chapter6Section3 {
	public static void main(String[] args) {
		// User object가 담긴 List를 이용해 user의 이메일만 담겨있는 List를 만들자
		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<String> emailAddresses = users.stream().map(User::getEmailAddress).collect(Collectors.toList());
		// User::getEmailAddress --> Method Reference : 각 object 마다 object 안에 있는 instance method 적용
		// 다른 방법 : List<String> emailAddresses = users.stream().map(user -> user.getEmailAddress()).collect(Collectors.toList());
		System.out.println(emailAddresses);
	}
}

2-2. Order object가 담긴 List를 이용해 createdByUserId 만 모여있는 List를 만들자

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.function.Function;
import java.util.stream.Collectors;
import java.util.stream.Stream;

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

public class Chapter6Section3 {
	public static void main(String[] args) {
		// Order object가 담긴 List를 이용해 createdByUserId 만 모여있는 List를 만들자
		Order order1 = new Order();
		order1.setId(1001);
		order1.setStatus(OrderStatus.CREATED);	   // enum 사용
		order1.setCreatedByUserId(101);
		
		Order order2 = new Order();
		order2.setId(1002);
		order2.setStatus(OrderStatus.ERROR);	   // enum 사용
		order2.setCreatedByUserId(103);
		
		Order order3 = new Order();
		order3.setId(1003);
		order3.setStatus(OrderStatus.PROCESSED);   // enum 사용
		order3.setCreatedByUserId(102);
		
		Order order4 = new Order();
		order4.setId(1004);
		order4.setStatus(OrderStatus.ERROR);	   // enum 사용
		order4.setCreatedByUserId(104);
		
		Order order5 = new Order();
		order5.setId(1005);
		order5.setStatus(OrderStatus.IN_PROGRESS); // enum 사용
		order5.setCreatedByUserId(101);
		
		List<Order> orders = Arrays.asList(order1, order2, order3, order4, order5);
		List<Long> createdByUserIds = orders.stream().map(Order::getCreatedByUserId).collect(Collectors.toList());
		System.out.println(createdByUserIds);
	}
}