Map기본연습

기본연습

맵에 값을 넣기

import java.util.HashMap;
import java.util.Map;

public class Exercise1 {

    public static void main(String[] args) {
        
        Map<Integer, String> employees = new HashMap<Integer, String>();
        
        employees.put(Integer.valueOf(1), "James");
        employees.put(Integer.valueOf(2), "David");
        employees.put(Integer.valueOf(3), "Cavin");
        employees.put(Integer.valueOf(4), "Lucy");
        employees.put(Integer.valueOf(5), "Tom");
        
        System.out.println(employees);
    }
}

{1=James, 2=David, 3=Cavin, 4=Lucy, 5=Tom}

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두개의 맵을 합치기

import java.util.HashMap;
import java.util.Map;

public class Exercise1 {

    public static void main(String[] args) {
        
        Map<Integer, String> fruit1 = new HashMap<Integer, String>();
        
        fruit1.put(Integer.valueOf(1), "Apple");
        fruit1.put(Integer.valueOf(2), "Kiwi");
        fruit1.put(Integer.valueOf(3), "Banana");
        fruit1.put(Integer.valueOf(4), "Melon");
        fruit1.put(Integer.valueOf(5), "Orange");
        
        System.out.println(fruit1);
        
        Map<Integer, String> fruit2 = new HashMap<Integer, String>();
        
        fruit2.put(Integer.valueOf(1), "Strawberry");
        fruit2.put(Integer.valueOf(2), "Raspberry");
        fruit2.put(Integer.valueOf(10), "Watermelon");
        fruit2.put(Integer.valueOf(15), "Peach");
        
        System.out.println(fruit2);
        
        fruit1.putAll(fruit2);
        
        System.out.println(fruit1);
    }
}

{1=Apple, 2=Kiwi, 3=Banana, 4=Melon, 5=Orange}
{1=Strawberry, 2=Raspberry, 10=Watermelon, 15=Peach}
{1=Strawberry, 2=Raspberry, 3=Banana, 4=Melon, 5=Orange, 10=Watermelon, 15=Peach}

두개의 맵을 합칠경우에 중복되는 키값이 있다면 덮어쓰기가 된다.

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키로 벨류 값을 추출

import java.util.HashMap;
import java.util.Map;

public class Exercise1 {

    public static void main(String[] args) {
        
        Map<Integer, String> fruits = new HashMap<Integer, String>();
        
        fruits.put(Integer.valueOf(1), "Strawberry");
        fruits.put(Integer.valueOf(2), "Raspberry");
        fruits.put(Integer.valueOf(10), "Watermelon");
        fruits.put(Integer.valueOf(15), "Peach");
        
        System.out.println(fruits.get(Integer.valueOf(1)));
        System.out.println(fruits.get(Integer.valueOf(2)));
        System.out.println(fruits.get(Integer.valueOf(3)));
        System.out.println(fruits.get(Integer.valueOf(10)));
    }
}

Strawberry
Raspberry
null
Watermelon

비어있을때는 null을 반환한다

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키값으로 맵핑을 지울때

import java.util.HashMap;
import java.util.Map;

public class Exercise1 {

    public static void main(String[] args) {
        
        Map<Integer, String> fruits = new HashMap<Integer, String>();
        
        fruits.put(Integer.valueOf(1), "Strawberry");
        fruits.put(Integer.valueOf(2), "Raspberry");
        fruits.put(Integer.valueOf(10), "Watermelon");
        fruits.put(Integer.valueOf(15), "Peach");
        
        System.out.println(fruits);
        
        // Delete
        fruits.remove(Integer.valueOf(1));
        
        System.out.println(fruits);
    }
}

{1=Strawberry, 2=Raspberry, 10=Watermelon, 15=Peach}
{2=Raspberry, 10=Watermelon, 15=Peach}

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모든 맵핑을 지울 때

import java.util.HashMap;
import java.util.Map;

public class Exercise1 {

    public static void main(String[] args) {
        
        Map<Integer, String> fruits = new HashMap<Integer, String>();
        
        fruits.put(Integer.valueOf(1), "Strawberry");
        fruits.put(Integer.valueOf(2), "Raspberry");
        fruits.put(Integer.valueOf(10), "Watermelon");
        fruits.put(Integer.valueOf(15), "Peach");
        
        System.out.println(fruits);
        
        // Delete all
        fruits.clear();
        
        System.out.println(fruits);
    }
}

{1=Strawberry, 2=Raspberry, 10=Watermelon, 15=Peach}
{}

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맵핑의 갯수를 알아보기

import java.util.HashMap;
import java.util.Map;

public class Exercise1 {

    public static void main(String[] args) {
        
        Map<Integer, String> fruits = new HashMap<Integer, String>();
        
        fruits.put(Integer.valueOf(1), "Strawberry");
        fruits.put(Integer.valueOf(2), "Raspberry");
        fruits.put(Integer.valueOf(10), "Watermelon");
        fruits.put(Integer.valueOf(15), "Peach");
        
        System.out.println(fruits.size());
    }
}

4

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루프를 돌며 맵의 내용을 출력

import java.util.HashMap;
import java.util.Map;

public class Exercise1 {

    public static void main(String[] args) {
        
        Map<Integer, String> fruits = new HashMap<Integer, String>();
        
        fruits.put(Integer.valueOf(1), "Strawberry");
        fruits.put(Integer.valueOf(2), "Raspberry");
        fruits.put(Integer.valueOf(10), "Watermelon");
        fruits.put(Integer.valueOf(15), "Peach");
        
        // 루프를 돌며 맵의 키값을 출력
        for(Integer key : fruits.keySet()) {
            System.out.println("key : " + key);
        }
        
        // 루프를 돌며 맵의 벨류 값을 출력
        for(String value : fruits.values()) {
            System.out.println("value : " + value);
        }
        
        // 루프를 돌며 맵의 키와 벨류 값을 출력
        for(Map.Entry<Integer, String> entry : fruits.entrySet()) {
            System.out.println(
                    "key : " + entry.getKey() 
                    + " , value : " + entry.getValue());
        }
    }
}

key : 1
key : 2
key : 10
key : 15
value : Strawberry
value : Raspberry
value : Watermelon
value : Peach
key : 1 , value : Strawberry
key : 2 , value : Raspberry
key : 10 , value : Watermelon
key : 15 , value : Peach

벨류 값 뽑는것을 제일 많이 본것 같은데, 마지막것도 가끔은 쓰이니 숙지할 것. Entry와 entrySet() !!

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읽기전용 맵을 만들기

public class Exercise1 {

    public static void main(String[] args) {

        Map<Integer, String> fruits = new HashMap<Integer, String>();

        fruits.put(Integer.valueOf(1), "Strawberry");
        fruits.put(Integer.valueOf(2), "Raspberry");
        fruits.put(Integer.valueOf(10), "Watermelon");
        fruits.put(Integer.valueOf(15), "Peach");

        // 읽기전용 맵으로 한다  
        Map<Integer, String> readOnlyFruits = Collections.unmodifiableMap(fruits);
        
        for(String fruitssList : readOnlyFruits.values()) {
            System.out.println(fruitssList);
        }
        
        // 맵에 풋을 하면 예외가 발생함 
        readOnlyFruits.put(Integer.valueOf(5), "Orange");
    }
}

경우에 따라서 맵을 읽기전용으로 만들어야 할 때 사용하면 된다.
import java.util.Collection; 이 아닌
import java.util.stream.Collector; 도 아닌
import java.util.Collections; 이니 햇깔려하지 말것

Strawberry
Raspberry
Watermelon
Peach
Exception in thread "main" java.lang.UnsupportedOperationException
    at java.util.Collections$UnmodifiableMap.put(Collections.java:1459)
    at MapExercise.Exercise1.main(Exercise1.java:28)

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