如何以子类类型返回静态实例

Question

我目前是两个班。 带有简单属性“ responseCode”的响应。 响应具有静态实例（为KO时为简单响应）。

我还有一个类DetailResponse，它是我的类Response的子类。

我要做的是能够使用Response类中的静态变量KO_RESPONSE返回类型DetailResponse的对象。 有没有办法做到这一点？

我的超级班级回应：

public class Response {
    public static final Response KO_RESPONSE = new A(ReturnCode.KO);

    public ReturnCode responseCode;
    public String comment;

    public Response () {};
    public Response (ReturnCode responseCode) {
        this.responseCode = responseCode;
    }

    public static enum ReturnCode {
        OK,
        KO;
    }
}

我的子类DetailResponse扩展了Response：

public class DetailResponse extends Response {
    public String otherField;
}

BuisinessService类别：

public BuisinessService {
    public DetailResponse sendRequest() {
        String status = sendRequest() // Do something
        if(status.equals("KO")) {
            return Response.KO_RESPONSE; // What I would do but doesn't work because Response is the super class of DetailResponse
        } else {
            DetailResponse detail = new DetailResponse(ReturnCode.OK);
            detail.comment = "comment";
            detail.otherField = "somethingCool";
            return detail;
        }
    }
}

Answer 1

您正在尝试覆盖static变量，Java不允许这样做。 相反，您可以实现以下模式来获取基类和子类的实例。 这是一个例子：

    public class Response {
     public static Response getInstance(){
      return new Response(<args>);
     }
    }

    public class DetailedResponse extends Response {
     public static DetailedResponse getInstance(){
      return new DetailedResponse(<args>);
     }
    }

最后，可以按以下方式实现商务舱：

public BuisinessService {
    public DetailResponse sendRequest() {
        String status = sendRequest() // Do something
        if(status.equals("KO")) {
            return DetailResponse.getInstance(); // What I would do but doesn't work because Response is the super class of DetailResponse
        } else {
            DetailResponse detail = new DetailResponse(ReturnCode.OK);
            detail.comment = "comment";
            detail.otherField = "somethingCool";
            return detail;
        }
    }
}

希望这可以帮助！

Answer 2

静态变量和方法不会继承，但是您可以使用静态块初始化程序

public class Response {
        public static Response KO_RESPONSE;

        public ReturnCode responseCode;
        public String comment;

        public Response () {};
        public Response (ReturnCode responseCode) {
            this.responseCode = responseCode;
        }

        public static enum ReturnCode {
            OK,
            KO;
        }
    }

    public class DetailResponse extends Response {

        static {
            Response.KO_RESPONSE = new DetailResponse(/* .. */);
        }

        public String otherField;
    }

Answer 3

为什么在这里使用A（），但未定义A。

public static final Response KO_RESPONSE = new A(ReturnCode.KO);

可以在此处使用详细响应或其他类型的响应

class DetailResponse extends Response {

    public DetailResponse(ReturnCode ko) {
        super(ko);
    }

    public String otherField;
}

class Response {
    public static final Response KO_RESPONSE = new Response(ReturnCode.KO);

    public ReturnCode responseCode;
    public String comment;

    public Response () {};
    public Response (ReturnCode responseCode) {
        this.responseCode = responseCode;
    }

    public static enum ReturnCode {
        OK,
        KO;
    }
}

此外，您还必须为详细的响应类声明一个参数化的构造函数，该类可用于定义