若没有任何实例包含Class Body 则enum被隐式声明为final

本文参考

今天在Java Language Specification上偶然看到一条关于枚举的语法特点说明

An enum declaration is implicitly final unless it contains at least one enum constant that has a class body

不是很理解此处Class Body的所指，查阅了一些关于Java enum的语法介绍和stack overflow上的回答，对这句话有了一定的理解

https://stackoverflow.com/questions/9891613/why-cant-a-java-enum-be-final

不含Class Body的enum声明

public enum Color1 {

RED("红色"),

GREEN("绿色"),

BLUE("蓝色");

private String description;

Color(String description) {

this.description = description;
}

public String getDescription() {

return description;
}
}

不含Class Body的反编译结果

public final class Color1 extends Enum<Color1> {
public static final /* enum */ Color1 RED = new Color1("RED", 0, "\u7ea2\u8272");
public static final /* enum */ Color1 GREEN = new Color1("GREEN", 1, "\u7eff\u8272");
public static final /* enum */ Color1 BLUE = new Color1("BLUE", 2, "\u84dd\u8272");
private String description;
private static final /* synthetic */ Color1[] $VALUES;

public static Color1[] values() {
return (Color1[])$VALUES.clone();
}

  public static Color1 valueOf(String name) {
      return Enum.valueOf(Color1.class, name);
  }

  private Color1(String string, int n, String description) {
      super(string, n);
      this.description = description;
  }

  public String getDescription() {
      return this.description;
  }

  static {
      $VALUES = new Color1[]{RED, GREEN, BLUE};
  }
}

含Class Body的enum声明

这个例子中无法体现"至少有一个实例包含Class Body"的语法特点，但是后期验证确实如此，有兴趣可以改下代码自行尝试

public enum Color2 {

RED {

@Override

public String getDescription() {

return "红色";
}
},

GREEN {

@Override

public String getDescription() {

return "绿色";
}
},

BLUE {

@Override

public String getDescription() {

return "蓝色";
}
};

public abstract String getDescription();
}

含Class Body的反编译结果

public abstract class Color2 extends Enum<Color2> {
  public static final /* enum */ Color2 RED = new /* Unavailable Anonymous Inner Class!! */;
  public static final /* enum */ Color2 GREEN = new /* Unavailable Anonymous Inner Class!! */;
  public static final /* enum */ Color2 BLUE = new /* Unavailable Anonymous Inner Class!! */;
  private static final /* synthetic */ Color2[] $VALUES;

  public static Color2[] values() {
      return (Color2[])$VALUES.clone();
  }

  public static Color2 valueOf(String name) {
      return Enum.valueOf(Color2.class, name);
  }

  private Color2(String string, int n) {
      super(string, n);
  }

  public abstract String getDescription();

  /* synthetic */ Color2(String x0, int x1, 1 x2) {
      this(x0, x1);
  }

  static {
      $VALUES = new Color2[]{RED, GREEN, BLUE};
  }
}

结论

enum实际上是一种特殊的class声明，enum实例（如上面代码示例中的RED、GREEN和BLUE）都被隐式地声明为public static final

当enum实例包含Class Body时，这种写法实际上生成了一个继承enum的特殊的内部类，所以enum声明不能够是final（声明为final的类无法被继承）

另外一个注意点，编译器不一定为enum声明隐式地添加public修饰符，若enum声明被包含在某个接口或类内，Java接口在编译时会自动为enum类型加上public static修饰符，而Java类在编译时会自动为 enum 类型加上static修饰符，具体可以参考这篇文章：

https://www.cnblogs.com/jingmoxukong/p/6098351.html