java枚举类型的原理是什么

public abstract class Enum<E extends Enum<E>>
        implements Comparable<E>, Serializable {
 
    // 枚举的常量名，例如MONDAY, TUESDAY
    private final String name;
 
    public final String name() {
        return name;
    }
 
    // 枚举的序号，按顺序从0开始
    private final int ordinal;
 
    public final int ordinal() {
        return ordinal;
    }
 
    protected Enum(String name, int ordinal) {
        this.name = name;
        this.ordinal = ordinal;
    }
 
    public String toString() {
        return name;
    }
    
    public static <T extends Enum<T>> T valueOf(Class<T> enumType,
                                                String name) {
        T result = enumType.enumConstantDirectory().get(name);
        if (result != null)
            return result;
        if (name == null)
            throw new NullPointerException("Name is null");
        throw new IllegalArgumentException(
            "No enum constant " + enumType.getCanonicalName() + "." + name);
    }
    ...
}