枚举类

引言：什么是枚举？

枚举（Enum）是Java中一种特殊的类，用于定义固定数量的常量。枚举让代码更清晰、更安全。

枚举就像一周的星期几：
├── 固定数量：7天
├── 每天有特定含义：星期一、星期二...
├── 不能随意创建新的一天
└── 取值只能是这7天之一

用枚举表示：
enum Weekday {
    MONDAY, TUESDAY, WEDNESDAY, THURSDAY, FRIDAY, SATURDAY, SUNDAY
}

为什么使用枚举？

枚举的基本使用

定义枚举

public enum Weekday {
    MONDAY,    // 星期一
    TUESDAY,   // 星期二
    WEDNESDAY, // 星期三
    THURSDAY,  // 星期四
    FRIDAY,    // 星期五
    SATURDAY,  // 星期六
    SUNDAY     // 星期日
}

使用枚举

Weekday today = Weekday.FRIDAY;

if (today == Weekday.FRIDAY) {
    System.out.println("今天是周五，明天就是周末了！");
}

// switch语句中使用枚举
switch (today) {
    case MONDAY:
        System.out.println("周一加油！");
        break;
    case FRIDAY:
        System.out.println("周五快乐！");
        break;
    case SATURDAY:
    case SUNDAY:
        System.out.println("周末愉快！");
        break;
    default:
        System.out.println("工作日努力！");
}

枚举的常用方法

values() - 获取所有枚举值

Weekday[] days = Weekday.values();
for (Weekday day : days) {
    System.out.println(day);
}

valueOf() - 根据名称获取枚举值

Weekday day = Weekday.valueOf("FRIDAY");
System.out.println(day);  // FRIDAY

ordinal() - 获取枚举的序号

int index = Weekday.MONDAY.ordinal();
System.out.println("Monday的序号: " + index);  // 0

int index2 = Weekday.FRIDAY.ordinal();
System.out.println("Friday的序号: " + index2);  // 4

name() - 获取枚举名称

String name = Weekday.MONDAY.name();
System.out.println(name);  // MONDAY

带属性的枚举

定义带属性的枚举

public enum Season {
    SPRING("春天", "春暖花开"),
    SUMMER("夏天", "炎热酷暑"),
    AUTUMN("秋天", "秋高气爽"),
    WINTER("冬天", "寒冷刺骨");

    private final String chineseName;
    private final String description;

    Season(String chineseName, String description) {
        this.chineseName = chineseName;
        this.description = description;
    }

    public String getChineseName() {
        return chineseName;
    }

    public String getDescription() {
        return description;
    }
}

使用带属性的枚举

Season currentSeason = Season.SPRING;

System.out.println("中文名: " + currentSeason.getChineseName());
System.out.println("描述: " + currentSeason.getDescription());

带方法的枚举

public enum Operation {
    PLUS("+") {
        @Override
        public double apply(double x, double y) {
            return x + y;
        }
    },
    MINUS("-") {
        @Override
        public double apply(double x, double y) {
            return x - y;
        }
    },
    MULTIPLY("*") {
        @Override
        public double apply(double x, double y) {
            return x * y;
        }
    },
    DIVIDE("/") {
        @Override
        public double apply(double x, double y) {
            if (y == 0) {
                throw new ArithmeticException("除数不能为0");
            }
            return x / y;
        }
    };

    private final String symbol;

    Operation(String symbol) {
        this.symbol = symbol;
    }

    public String getSymbol() {
        return symbol;
    }

    public abstract double apply(double x, double y);
}

// 使用
double result = Operation.PLUS.apply(10, 5);
System.out.println("10 + 5 = " + result);  // 15

枚举实现接口

interface Describable {
    String getDescription();
}

public enum Color implements Describable {
    RED("红色") {
        @Override
        public String getDescription() {
            return "这是红色";
        }
    },
    GREEN("绿色") {
        @Override
        public String getDescription() {
            return "这是绿色";
        }
    },
    BLUE("蓝色") {
        @Override
        public String getDescription() {
            return "这是蓝色";
        }
    };

    private final String chineseName;

    Color(String chineseName) {
        this.chineseName = chineseName;
    }

    public String getChineseName() {
        return chineseName;
    }
}

// 使用
Color color = Color.RED;
System.out.println(color.getChineseName());
System.out.println(color.getDescription());

枚举在switch中的应用

public enum TrafficLight {
    RED, YELLOW, GREEN;
}

TrafficLight light = TrafficLight.GREEN;

switch (light) {
    case RED:
        System.out.println("红灯停");
        break;
    case YELLOW:
        System.out.println("黄灯等一等");
        break;
    case GREEN:
        System.out.println("绿灯行");
        break;
}

枚举的构造方法

私有构造方法

枚举的构造方法自动私有，不能直接调用构造方法创建枚举实例。

public enum Status {
    SUCCESS("成功", 200),
    ERROR("错误", 500),
    NOT_FOUND("未找到", 404);

    private final String description;
    private final int code;

    Status(String description, int code) {
        this.description = description;
        this.code = code;
    }

    public String getDescription() {
        return description;
    }

    public int getCode() {
        return code;
    }
}

// 使用
System.out.println(Status.SUCCESS.getDescription());  // 成功
System.out.println(Status.SUCCESS.getCode());  // 200

小结