Question

我正在使用方法链编写DerivedClass（扩展SuperClass）。例如，

public class SuperClass {
    protected int responseCode;
    protected String responseMessage;

    public void setResponseCode(int code) {
            this.responseCode = code;
    }

    public int getResponseCode() {
            return responseCode;
    }

    public SuperClass withResponseCode(int code) {
            setResponseCode(code);
            return this;
    }

    public void setResponseMessage(String message) {
            this.responseMessage = message;
    }

    public String getResponseMessage() {
            return responseMessage;
    }

    public SuperClass withResponseMessage(String message) {
            setResponseMessage(message);
            return this;
    }

}

和

public class DerivedClass extends SuperClass {
    protected String credentialType;
    protected String credentialId;

    public void setCredentialType(String type) {
            this.credentialType = type;
    }

    public String getCredentialType() {
            return credentialType;
    }

    public DerivedClass withCredentialType(String type) {
            setCredentialType(type);
            return this;
    }


    public void setCredentialId(String id) {
            this.credentialId = id;
    }

    public String getCredentialId() {
            return credentialId;
    }

    public DerivedClass withCredentialId(String id) {
            setCredentialId(id);
            return this;
    }

    public static void main(String[] args) {
            DerivedClass dc = new DerivedClass();
            /*dc.setResponseCode(200);
            dc.setResponseMessage("SUCCESS");
            dc.setCredentialType("MobileIdentifier");
            dc.setCredentialId("678882");*/

            dc.withResponseCode(200)
                    .withResponseMessage("SUCCESS")
                    .withCredentialType("MobileIdentifier")
                    .withCredentialId("678882");

            System.out.println("Derived Class: Code - " + dc.getResponseCode() + " - Response Message - " + dc.getResponseMessage() + " - Credential Type - " + dc.getCredentialType());
    }

}

在编写上述内容时，我得到了：

DerivedClass.java:41: error: cannot find symbol
                    .withCredentialType("MobileIdentifier")
                    ^
symbol:   method withCredentialType(String)
location: class SuperClass
1 error

当CredentialType是DerivedClass中的字段而不是SuperClass中的字段时，为什么会出现这种情况？如何从SuperClass＆amp; amp; DerivedClass使用DerivedClass对象？

Answer 1

正如其他人所说的那样，您收到了编译错误，因为SuperClass的方法会返回SuperClass类型，而DerivedClass类型不包含SuperClass＆＃39;方法。

解决此问题的正确方法是使SuperClass抽象并在public abstract class SuperClass<T extends SuperClass<T>> { // TODO attributes, getters and setters public T withResponseCode(int code) { setResponseCode(code); return (T) this; // this cast is absolutely safe! } // TODO other SuperClass builder methods }上使用带有递归类型的泛型（也称为F-bound type）：

DerivedClass

然后，您可以按如下方式定义public class DerivedClass extends SuperClass<DerivedClass> { // TODO attributes, getters and setters public DerivedClass withCredentialType(String type) { setCredentialType(type); return this; } // TODO other DerivedClass builder methods }：

SuperClass

并且不会再出现编译错误。这种方法的缺点是它强制你使jsonconvert.serializeobject类抽象，以便只使用派生类。

Answer 2

方法withReponseMessage()返回SuperClass类型的对象。

类SuperClass没有名为withCredentialType()的方法，这就是您收到错误消息的原因。

解决此问题的一个策略是使SuperClass抽象，然后在那里定义所有API方法，即使这些方法是抽象的。否则，您将必须转换从第一个方法调用返回的对象（这可能使您的程序在将来容易受到不匹配异常的影响。

Answer 3

您需要将其向下转换为DerivedClass，或者从头开始使用DerivedClass实例。

Java：派生类中的方法链接

3 个答案: