Enum的真正面目

Posted 2021-09-04 一楼的开水瓶

一直想写一下关于java中枚举的东西

首先我们看这样一个类

public enum SeasonEnum {

    SPRING("春天", "春暖花开"),
    SUMMER("夏天", "炎炎盛夏"),
    FALL("秋天", "秋高气爽"),
    WINTER("冬天", "大雪纷飞");

    private final String name;
    private final String desc;

    private SeasonEnum(String name, String desc) {
        this.name = name;
        this.desc = desc;
    }

    public static void main(String[] args) {

        System.out.println(SeasonEnum.SPRING); 
        for(SeasonEnum s : SeasonEnum.values()){
            System.out.println(s);
        }
    }
}

我们对齐用javac编译再用javap反编译看看

public final class com.xxxx.SeasonEnum extends java.lang.Enum<com.xxxx.SeasonEnum> {
  public static final com.xxxx.SeasonEnum SPRING;
  public static final com.xxxx.SeasonEnum SUMMER;
  public static final com.xxxx.SeasonEnum FALL;
  public static final com.xxxx.SeasonEnum WINTER;
  public static com.xxxx.SeasonEnum[] values();
  public static com.xxxx.SeasonEnum valueOf(java.lang.String);
  public static void main(java.lang.String[]);
  static {};
}

可以发现其实Enum只是一个语法糖，java编译器会将其编译成一个继承了Enum<E extends Enum<E>>抽象类的一个类，这个类有所定义的四个静态对象，而对枚举的使用SeasonEnum.SPRING其实就是使用类的静态字段

public final class com.xxxx.SeasonEnum extends java.lang.Enum<com.xxxx.SeasonEnum>{
    public static SeasonEnum[] values(){
        return (SeasonEnum[])$VALUES.clone();
    }
    public static SeasonEnum valueOf(String name){
        return (SeasonEnum)Enum.valueOf(enum01/SeasonEnum, name);
    }
    private SeasonEnum(String s, int i){
        super(s, i);
    }
    public static final SeasonEnum SPRING;
    public static final SeasonEnum SUMMER;
    public static final SeasonEnum FALL;
    public static final SeasonEnum WINTER;
    private static final SeasonEnum $VALUES[];
    static {
        SPRING = new SeasonEnum("春天", "春暖花开");
        SUMMER = new SeasonEnum("夏天", "炎炎盛夏");
        FALL = new SeasonEnum("秋天", "秋高气爽");
        WINTER = new SeasonEnum("冬天", "大雪纷飞");
        $VALUES = (new SeasonEnum[] {
            SPRING, SUMMER, FALL, WINTER
        });
    }
}