Java基础知识回顾8-枚举

一、基本概念

枚举是Java1.5引入的新特性，通过关键字enum来定义枚举类。枚举类是一种特殊类，它和普通类一样可以使用构造器、定义成员变量和方法，也能实现一个或多个接口，但是枚举类不能继承其他类。

二、解决ifelse

随着业务的发展，需求只会越来越复杂，为了考虑各种各样的情况，代码中不可避免的会出现很多的if-else。一旦代码中的if-else过多，就会大大影响其可读性和可维护性。

我们可以使用枚举类解决这个问题；

package day03;

/**
 * @author qx
 * @date 2023/10/20
 * @des 季节枚举
 */
public enum SeasonEnum {
    SPRING, SUMMER, AUTUMN, WINTER
}

package day03;

/**
 * @author qx
 * @date 2023/10/20
 * @des 枚举测试
 */
public class EnumTest {
    public static void main(String[] args) {
        testSwitch(SeasonEnum.AUTUMN);
    }

    private static void testSwitch(SeasonEnum season) {
        switch (season) {
            case SPRING:
                System.out.println("春天");
                break;
            case SUMMER:
                System.out.println("夏天");
                break;
            case AUTUMN:
                System.out.println("秋天");
                break;
            case WINTER:
                System.out.println("冬天");
                break;
        }
    }
}

三、枚举常用方法

四、EnumMap

EnumMap作为枚举的专属集合，由于枚举类型实例的数量相对固定并且有限，所以EnumMap使用数组来存放与枚举类型对应的值，毕竟数组是一段连续的内存空间，根据程序局部性原理，效率会很高。

EnumMap需要传递一个类型信息，即Class对象，通过这个参赛EnumMap就可以根据类型信息初始化内部数据结构。

package day03;

/**
 * @author qx
 * @date 2023/10/20
 * @des 季节枚举
 */
public enum SeasonEnum {
    SPRING(0, "春天"), SUMMER(1, "夏天"), AUTUMN(2, "秋天"), WINTER(3, "冬天");

    private int code;
    private String msg;

    SeasonEnum(int code, String msg) {
        this.code = code;
        this.msg = msg;
    }

    public String getMsg() {
        return msg;
    }

    public int getCode() {
        return code;
    }
}

package day03;

/**
 * @author qx
 * @date 2023/10/26
 * @des
 */
public class StudyJava {

    private String name;

    private SeasonEnum seasonEnum;

    public StudyJava(String name, SeasonEnum seasonEnum) {
        this.name = name;
        this.seasonEnum = seasonEnum;
    }

    public StudyJava() {
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public SeasonEnum getSeasonEnum() {
        return seasonEnum;
    }

    public void setSeasonEnum(SeasonEnum seasonEnum) {
        this.seasonEnum = seasonEnum;
    }
}

package day03;

import java.util.*;

/**
 * @author qx
 * @date 2023/10/26
 * @des HashMap和EnumMap
 */
public class EnumMapDemo {
    public static void main(String[] args) {
        List<StudyJava> list = new ArrayList<>();
        list.add(new StudyJava("春天", SeasonEnum.SPRING));
        list.add(new StudyJava("春天1", SeasonEnum.SPRING));
        list.add(new StudyJava("夏天", SeasonEnum.SUMMER));
        list.add(new StudyJava("秋天", SeasonEnum.AUTUMN));
        list.add(new StudyJava("秋天1", SeasonEnum.AUTUMN));
        list.add(new StudyJava("冬天", SeasonEnum.WINTER));
        Map<SeasonEnum, Integer> map = new HashMap<>();
        for (StudyJava studyJava : list) {
            SeasonEnum seasonEnum = studyJava.getSeasonEnum();
            Integer count = map.get(seasonEnum);
            if (count != null) {
                map.put(seasonEnum, count + 1);
            } else {
                map.put(seasonEnum, 1);
            }
        }
        System.out.println(map);
        System.out.println("--------------");

        // 使用EnumMap
        Map<SeasonEnum, Integer> enumMap = new EnumMap<>(SeasonEnum.class);
        for (StudyJava studyJava : list) {
            SeasonEnum seasonEnum = studyJava.getSeasonEnum();
            Integer count = enumMap.get(seasonEnum);
            if (count != null) {
                enumMap.put(seasonEnum, count + 1);
            } else {
                enumMap.put(seasonEnum, 1);
            }
        }
        System.out.println(enumMap);

    }
}

输出:

{AUTUMN=2, SUMMER=1, SPRING=2, WINTER=1}
--------------
{SPRING=2, SUMMER=1, AUTUMN=2, WINTER=1}