EnumMap

EnumMap คืออะไร

import java.util.EnumMap;
import java.util.Map;

public class Main {
    enum Day { MON, TUE, WED }
    public static void main(String[] args) {
        Map<Day, String> plan = new EnumMap<>(Day.class);
        plan.put(Day.MON, "Gym");
        plan.put(Day.WED, "Swim");
        System.out.println(plan);
    }
}

การสร้างต้องใช้คลาส

import java.util.EnumMap;

public class Main {
    enum Priority { LOW, MEDIUM, HIGH }
    public static void main(String[] args) {
        EnumMap<Priority, Integer> counts = new EnumMap<>(Priority.class);
        counts.put(Priority.HIGH, 5);
        System.out.println(counts.get(Priority.HIGH));
    }
}

ความเร็วจากการรองรับด้วยอาร์เรย์

import java.util.EnumMap;

public class Main {
    enum Slot { A, B, C, D }
    public static void main(String[] args) {
        EnumMap<Slot, Integer> m = new EnumMap<>(Slot.class);
        for (Slot s : Slot.values()) m.put(s, s.ordinal() * 10);
        System.out.println(m.get(Slot.C));
    }
}

การวนซ้ำตามลำดับคีย์

import java.util.EnumMap;
import java.util.Map;

public class Main {
    enum Phase { PLAN, BUILD, SHIP }
    public static void main(String[] args) {
        Map<Phase, Integer> hours = new EnumMap<>(Phase.class);
        hours.put(Phase.SHIP, 3);
        hours.put(Phase.PLAN, 8);
        hours.put(Phase.BUILD, 20);
        for (Map.Entry<Phase, Integer> e : hours.entrySet()) {
            System.out.println(e.getKey() + " = " + e.getValue());
        }
    }
}

ไม่อนุญาตให้ใช้คีย์ null

import java.util.EnumMap;

public class Main {
    enum K { A, B }
    public static void main(String[] args) {
        EnumMap<K, String> m = new EnumMap<>(K.class);
        m.put(K.A, null); // null value is fine
        System.out.println("A -> " + m.get(K.A));
        System.out.println("contains A: " + m.containsKey(K.A));
    }
}

การจัดกลุ่มด้วย EnumMap

import java.util.EnumMap;
import java.util.List;
import java.util.Map;

public class Main {
    enum Type { FRUIT, VEG, MEAT }
    record Item(String name, Type type) {}
    public static void main(String[] args) {
        List<Item> items = List.of(new Item("apple", Type.FRUIT), new Item("pear", Type.FRUIT), new Item("beef", Type.MEAT));
        Map<Type, Integer> counts = new EnumMap<>(Type.class);
        for (Item i : items) counts.merge(i.type(), 1, Integer::sum);
        System.out.println(counts);
    }
}

getOrDefault และ computeIfAbsent

import java.util.ArrayList;
import java.util.EnumMap;
import java.util.List;
import java.util.Map;

public class Main {
    enum Group { A, B }
    public static void main(String[] args) {
        Map<Group, List<String>> m = new EnumMap<>(Group.class);
        m.computeIfAbsent(Group.A, k -> new ArrayList<>()).add("first");
        System.out.println(m);
        System.out.println(m.getOrDefault(Group.B, List.of()));
    }
}

ตัวสร้างสำเนา

import java.util.EnumMap;
import java.util.Map;

public class Main {
    enum K { X, Y }
    public static void main(String[] args) {
        EnumMap<K, Integer> a = new EnumMap<>(K.class);
        a.put(K.X, 1);
        EnumMap<K, Integer> b = new EnumMap<>(a);
        b.put(K.Y, 2);
        System.out.println("copy: " + b);
    }
}

EnumMap เทียบกับอาร์เรย์ที่ใช้ ordinal

import java.util.EnumMap;
import java.util.Map;

public class Main {
    enum Season { SPRING, SUMMER, FALL, WINTER }
    public static void main(String[] args) {
        Map<Season, String> mood = new EnumMap<>(Season.class);
        mood.put(Season.SUMMER, "sunny");
        // Safer and clearer than String[] indexed by ordinal
        System.out.println(mood.get(Season.SUMMER));
    }
}

EnumMaps ซ้อนกัน

import java.util.EnumMap;
import java.util.Map;

public class Main {
    enum State { SOLID, LIQUID, GAS }
    enum Transition { MELT, FREEZE }
    public static void main(String[] args) {
        Map<State, Map<Transition, State>> table = new EnumMap<>(State.class);
        Map<Transition, State> solid = new EnumMap<>(Transition.class);
        solid.put(Transition.MELT, State.LIQUID);
        table.put(State.SOLID, solid);
        System.out.println(table.get(State.SOLID).get(Transition.MELT));
    }
}

การเลือกชนิดที่เหมาะสม

import java.util.EnumMap;
import java.util.Map;

public class Main {
    enum Level { DEBUG, INFO, WARN, ERROR }
    public static void main(String[] args) {
        Map<Level, Integer> thresholds = new EnumMap<>(Level.class);
        for (Level l : Level.values()) thresholds.put(l, l.ordinal() * 100);
        System.out.println(thresholds);
    }
}

ตรวจสอบความเข้าใจ

ทบทวน

import java.util.EnumMap;

public class Main {
    enum K { A }
    public static void main(String[] args) {
        EnumMap<K, String> m = new EnumMap<>(K.class);
        m.put(K.A, "done");
        System.out.println("EnumMap course complete: " + m.get(K.A));
    }
}