如何启用枚举继承 [英] How to enable enum inheritance

问题描述

我正在编写一个库，它有一组预定义的枚举值。
假设，我的枚举如下所示。

I'm writing a library, which has a predefined set of values for an enum. Let say, my enum looks as below.

public enum EnumClass {
    FIRST("first"),
    SECOND("second"),
    THIRD("third");

    private String httpMethodType;

}

现在使用此库的客户可能需要添加更多值。假设客户端需要添加 CUSTOM_FIRST 和 CUSTOM_SECOND 。这不会覆盖任何现有值，但会使枚举有5个值。

Now the client, who is using this library may need to add few more values. Let say, the client needs to add CUSTOM_FIRST and CUSTOM_SECOND. This is not overwriting any existing values, but makes the enum having 5 values.

在此之后，我应该可以使用类似< ？扩展EnumClass> 以获得5种不变的可能性。

After this, I should be able to use something like <? extends EnumClass> to have 5 constant possibilities.

实现这一目标的最佳方法是什么？

What would be the best approach to achieve this?

推荐答案

你不能有 enum 扩展另一个 enum ，并且您不能通过继承将值添加到现有的枚举。

You cannot have an enum extend another enum, and you cannot "add" values to an existing enum through inheritance.

但是， enum s可以实现 interface s。

However, enums can implement interfaces.

我要做的是让原来的 enum 实现一个标记界面（即没有方法声明），那么你的客户可以创建自己的枚举实现相同的接口。

What I would do is have the original enum implement a marker interface (i.e. no method declarations), then your client could create their own enum implementing the same interface.

然后你的 enum 值会被他们共同的界面引用。

Then your enum values would be referred to by their common interface.

为了加强要求，你可以让你的界面声明相关的方法，例如：在你的情况下，行中的某些东西是get String getHTTPMethodType（）; 。

In order to strenghten the requirements, you could have your interface declare relevant methods, e.g. in your case, something in the lines of public String getHTTPMethodType();.

这将迫使实现 enum s为该方法提供实现。

That would force implementing enums to provide an implementation for that method.

此设置加上适当的API文档应有助于以相对受控的方式添加功能。

This setting coupled with adequate API documentation should help adding functionality in a relatively controlled way.

自包含示例（不介意这里的懒名称）

Self-contained example (don't mind the lazy names here)

package test;

import java.util.ArrayList;
import java.util.List;

public class Main {

    public static void main(String[] args) {
        List<HTTPMethodConvertible> blah = new ArrayList<>();
        blah.add(LibraryEnum.FIRST);
        blah.add(ClientEnum.BLABLABLA);
        for (HTTPMethodConvertible element: blah) {
            System.out.println(element.getHTTPMethodType());
        }
    }

    static interface HTTPMethodConvertible {
        public String getHTTPMethodType();
    }
    static enum LibraryEnum implements HTTPMethodConvertible {
        FIRST("first"),
        SECOND("second"),
        THIRD("third");
        String httpMethodType;
        LibraryEnum(String s) {
            httpMethodType = s;
        }
        public String getHTTPMethodType() {
            return httpMethodType;
        }
    }
    static enum ClientEnum implements HTTPMethodConvertible {
        FOO("GET"),BAR("PUT"),BLAH("OPTIONS"),MEH("DELETE"),BLABLABLA("POST");
        String httpMethodType;
        ClientEnum(String s){
            httpMethodType = s;
        }
        public String getHTTPMethodType() {
            return httpMethodType;
        }
    }
}

输出

first
POST

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