目前比較常用的枚舉

public enum Planet {
    MERCURY(3.302e+23, 2.439e6),
    VENUS  (4.869e+24, 6.052e6),
    URANUS (8.683e+25, 2.556e7);

    private final double mercury;
    private final double venus;

    Planet(double mercury, double venus) {
        this.mercury = mercury;
        this.venus = venus;
    }
}

這種方式較爲常見。

        for (Planet planet : Planet.values()) {
            double mercury = planet.mercury;
        }

 還有一種“算數的思路”，用IDEA還會有提示，更容易理解。

package test;

import DataStructures.List.DoublyLinkedList;

public enum Operation {
    PLUS("+") {
        public double apply(double x, double y) {return x + y; }
    },
    MINUS("-") {
        public double apply(double x, double y) {return x - y; }
    };

    private final String symbol;
    Operation(String symbol) {
        this.symbol = symbol;
    }
    public abstract double apply(double x, double y);

    public static void main(String[] args) {
        double x = Double.parseDouble("1.2");
        double y = Double.parseDouble("2.4");
        for (Operation operation : Operation.values()) {
            System.out.println(operation.apply(x, y));
        }
    }
}

　　

一個賞心悅目的例子，這裏主要用了EnumMap

package test;

import java.util.*;

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

public class Plant {
    enum LifeCycle {ONE, TWO, THREE}
    final String name;
    final LifeCycle lifeCycle;

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

    @Override
    public String toString() {
        return "Plant{" +
                "name='" + name + '\'' +
                ", lifeCycle=" + lifeCycle +
                '}';
    }

    public static void main(String[] args) {
        Map<Plant.LifeCycle, Set<Plant>> map = new EnumMap<LifeCycle, Set<Plant>>(Plant.LifeCycle.class);
        Plant p1 = new Plant("no1", LifeCycle.ONE);
        Plant p2 = new Plant("no2", LifeCycle.TWO);
        Plant p3 = new Plant("no3", LifeCycle.ONE);
        Plant p4 = new Plant("no4", LifeCycle.THREE);
        Plant[] garden = {p1, p2, p3, p4};
        for (Plant.LifeCycle lifeCycle : Plant.LifeCycle.values()) {
            map.put(lifeCycle, new HashSet<>());
        }
        for (Plant plant : garden) {
            map.get(plant.lifeCycle).add(plant);
        }
        System.out.println(map);
        System.out.println(Arrays.stream(garden)
                .collect(groupingBy(p -> p.lifeCycle))
        );
    }
}

最後用一段極其簡單的lambda也解決了，真是不可小覷。