Java 枚举核心操作与实战技巧
Java 从 JDK 5 开始引入 enum 关键字，提供了一种类型安全的常量表示方式。枚举本质上是一种受限制的类，编译器会自动生成继承自 java.lang.Enum 的子类。
枚举基础方法
所有枚举自动继承以下核心方法：
| 方法 | 作用 |
|---|---|
values() | 返回该枚举所有常量的数组 |
name() | 返回常量名称字符串 |
ordinal() | 返回常量的声明顺序（从0开始） |
getDeclaringClass() | 返回所属枚举类型 |
compareTo() | 基于 ordinal 比较大小 |
方法使用示例
public class EnumMethodShow {
enum Season { SPRING, SUMMER, AUTUMN, WINTER }
public static void main(String[] args) {
// 遍历所有常量
for (Season s : Season.values()) {
System.out.printf("%s 的序号为: %d%n", s, s.ordinal());
}
Season summer = Season.SUMMER;
System.out.println("名称: " + summer.name());
System.out.println("所属类: " + summer.getDeclaringClass().getSimpleName());
System.out.println("与 SPRING 比较: " + summer.compareTo(Season.SPRING));
System.out.println("等于 SUMMER: " + summer.equals(Season.SUMMER));
System.out.println("直接比较: " + (summer == Season.SUMMER));
}
}枚举的高级特性
自定义属性与构造方法
枚举可通过构造方法实现常量赋值，这是 Java 区别于 C/C++ 枚举的重要特性：
public enum HttpStatus {
SUCCESS(200, "请求成功"),
NOT_FOUND(404, "资源不存在"),
SERVER_ERROR(500, "服务器内部错误");
private final int statusCode;
private final String message;
HttpStatus(int code, String msg) {
this.statusCode = code;
this.message = msg;
}
public int getStatusCode() { return statusCode; }
public String getMessage() { return message; }
}实现接口与抽象方法
枚举可实现接口，甚至每个常量重写抽象方法，实现多态行为：
interface Operation {
double apply(double x, double y);
}
public enum Calculator implements Operation {
ADD {
public double apply(double a, double b) { return a + b; }
},
SUBTRACT {
public double apply(double a, double b) { return a - b; }
},
MULTIPLY {
public double apply(double a, double b) { return a * b; }
};
}典型应用场景
状态机实现
public enum OrderState {
CREATED {
public OrderState next() { return PAID; }
},
PAID {
public OrderState next() { return SHIPPED; }
},
SHIPPED {
public OrderState next() { return COMPLETED; }
},
COMPLETED {
public OrderState next() { return this; }
};
public abstract OrderState next();
}策略模式封装
利用枚举嵌套实现策略分发，避免大量 if-else 或 switch：
public enum SalaryStrategy {
WORKDAY(PayRule.WEEKDAY_RULE),
WEEKEND(PayRule.WEEKEND_RULE),
HOLIDAY(PayRule.HOLIDAY_RULE);
private final PayRule rule;
SalaryStrategy(PayRule rule) {
this.rule = rule;
}
public double calculate(double hours, double rate) {
return rule.compute(hours, rate);
}
private enum PayRule {
WEEKDAY_RULE {
double compute(double h, double r) {
return h > 8 ? 8 * r + (h - 8) * r * 1.5 : h * r;
}
},
WEEKEND_RULE {
double compute(double h, double r) { return h * r * 2; }
},
HOLIDAY_RULE {
double compute(double h, double r) { return h * r * 3; }
};
abstract double compute(double hours, double rate);
}
}专用工具类
EnumSet 高效集合
// 创建空集合后添加
EnumSet<HttpStatus> clientErrors = EnumSet.noneOf(HttpStatus.class);
clientErrors.add(HttpStatus.NOT_FOUND);
// 基于范围创建
EnumSet<HttpStatus> successRange = EnumSet.range(HttpStatus.SUCCESS, HttpStatus.NOT_FOUND);
// 包含全部常量
EnumSet<HttpStatus> all = EnumSet.allOf(HttpStatus.class);EnumMap 键值映射
EnumMap<Season, String> activities = new EnumMap<>(Season.class);
activities.put(Season.SPRING, "踏青");
activities.put(Season.SUMMER, "游泳");
activities.put(Season.AUTUMN, "登山");
activities.put(Season.WINTER, "滑雪");
for (Map.Entry<Season, String> entry : activities.entrySet()) {
System.out.println(entry.getKey() + " 适合 " + entry.getValue());
}设计注意事项
- 枚举构造方法只能为
private或默认（实际也是private） - 常量定义必须在最前面，后面才能跟字段和方法
- 枚举不可继承其他类（已隐式继承
java.lang.Enum），但可实现多个接口 - 优先使用
==而非equals()比较枚举常量，更高效且避免空指针
