《Effective Java 第三版》——第六章枚举和注解

enum 说的是一类问题，明白原理会运用代码就行

package effectivejava.chapter6.item37;
import java.util.*;
import static java.util.stream.Collectors.groupingBy;
import static java.util.stream.Collectors.toSet;

// Using an EnumMap to associate data with an enum (Pages 171-3)

// Simplistic class representing a plant (Page 171)
class Plant {
    enum LifeCycle { ANNUAL, PERENNIAL, BIENNIAL }

    final String name;
    final LifeCycle lifeCycle;

    Plant(String name, LifeCycle lifeCycle) {
        this.name = name;
        this.lifeCycle = lifeCycle;
    }

    @Override public String toString() {
        return name;
    }

    public static void main(String[] args) {
        Plant[] garden = {
            new Plant("Basil",    LifeCycle.ANNUAL),
            new Plant("Carroway", LifeCycle.BIENNIAL),
            new Plant("Dill",     LifeCycle.ANNUAL),
            new Plant("Lavendar", LifeCycle.PERENNIAL),
            new Plant("Parsley",  LifeCycle.BIENNIAL),
            new Plant("Rosemary", LifeCycle.PERENNIAL)
        };

        // Using ordinal() to index into an array - DON'T DO THIS!  (Page 171)
        Set<Plant>[] plantsByLifeCycleArr =
                (Set<Plant>[]) new Set[Plant.LifeCycle.values().length];
        for (int i = 0; i < plantsByLifeCycleArr.length; i++)
            plantsByLifeCycleArr[i] = new HashSet<>();
        for (Plant p : garden)
            plantsByLifeCycleArr[p.lifeCycle.ordinal()].add(p);
        // Print the results
        for (int i = 0; i < plantsByLifeCycleArr.length; i++) {
            System.out.printf("%s: %s%n",
                    Plant.LifeCycle.values()[i], plantsByLifeCycleArr[i]);
        }

        // Using an EnumMap to associate data with an enum (Page 172)
        Map<Plant.LifeCycle, Set<Plant>> plantsByLifeCycle =
                new EnumMap<>(Plant.LifeCycle.class);
        for (Plant.LifeCycle lc : Plant.LifeCycle.values())
            plantsByLifeCycle.put(lc, new HashSet<>());
        for (Plant p : garden)
            plantsByLifeCycle.get(p.lifeCycle).add(p);
        System.out.println(plantsByLifeCycle);

        // Naive stream-based approach - unlikely to produce an EnumMap!  (Page 172)
        System.out.println(Arrays.stream(garden)
                .collect(groupingBy(p -> p.lifeCycle)));

        // Using a stream and an EnumMap to associate data with an enum (Page 173)
        System.out.println(Arrays.stream(garden)
                .collect(groupingBy(p -> p.lifeCycle,
                        () -> new EnumMap<>(LifeCycle.class), toSet())));
    }
}

/Library/Java/JavaVirtualMachines/jdk-13.0.2.jdk/Contents/Home/bin/java -Dfile.encoding=UTF-8 -classpath /Users/didi/git/effective-java-3e-source-code/bin effectivejava.chapter6.item37.Plant
ANNUAL: [Basil, Dill]
PERENNIAL: [Rosemary, Lavendar]
BIENNIAL: [Carroway, Parsley]
{ANNUAL=[Basil, Dill], PERENNIAL=[Rosemary, Lavendar], BIENNIAL=[Carroway, Parsley]}
{BIENNIAL=[Carroway, Parsley], ANNUAL=[Basil, Dill], PERENNIAL=[Lavendar, Rosemary]}
{ANNUAL=[Basil, Dill], PERENNIAL=[Rosemary, Lavendar], BIENNIAL=[Carroway, Parsley]}

Process finished with exit code 0

package effectivejava.chapter6.item37;
import java.util.*;
import java.util.stream.Stream;

import static java.util.stream.Collectors.*;

// Using a nested EnumMap to associate data with enum pairs - (Pages 174-5)
public enum Phase {
    SOLID, LIQUID, GAS;
    public enum Transition {
        MELT(SOLID, LIQUID), FREEZE(LIQUID, SOLID),
        BOIL(LIQUID, GAS), CONDENSE(GAS, LIQUID),
        SUBLIME(SOLID, GAS), DEPOSIT(GAS, SOLID);

//        // Adding a new phase (Page 175)
//        SOLID, LIQUID, GAS, PLASMA;
//        public enum Transition {
//            MELT(SOLID, LIQUID), FREEZE(LIQUID, SOLID),
//            BOIL(LIQUID, GAS), CONDENSE(GAS, LIQUID),
//            SUBLIME(SOLID, GAS), DEPOSIT(GAS, SOLID),
//            IONIZE(GAS, PLASMA), DEIONIZE(PLASMA, GAS);

        private final Phase from;
        private final Phase to;
        Transition(Phase from, Phase to) {
            this.from = from;
            this.to = to;
        }

        // Initialize the phase transition map
        private static final Map<Phase, Map<Phase, Transition>>
                m = Stream.of(values()).collect(groupingBy(t -> t.from,
                () -> new EnumMap<>(Phase.class),
                toMap(t -> t.to, t -> t,
                        (x, y) -> y, () -> new EnumMap<>(Phase.class))));
        
        public static Transition from(Phase from, Phase to) {
            return m.get(from).get(to);
        }
    }

    // Simple demo program - prints a sloppy table
    public static void main(String[] args) {
        for (Phase src : Phase.values()) {
            for (Phase dst : Phase.values()) {
                Transition transition = Transition.from(src, dst);
                if (transition != null)
                    System.out.printf("%s to %s : %s %n", src, dst, transition);
            }
        }
    }
}

        // Initialize the phase transition map
        private static final Map<Phase, Map<Phase, Transition>>
                m = Stream.of(values()).collect(groupingBy(t -> t.from,
                () -> new EnumMap<>(Phase.class),
                toMap(t -> t.to, t -> t,
                        (x, y) -> y, () -> new EnumMap<>(Phase.class))));

Stream 这段代码有点绕

挺精巧的：用接口模拟可扩展的枚举

第39条把代码跑一遍就清楚了

《Effective Java 第三版》——第六章 枚举和注解

《Effective Java 第三版》——第六章枚举和注解