java - 父类返回子类

Question

很难用语言来解释我在追求什么，但希望下面的代码示例和注释就足够了。基本上，我希望 SubClass sc = new Subclass().method1() 行返回 Subclass 实例。

public class SuperClass {

    public SuperClass method1()
    {
       //do whatever
       return this
    }
}

public class SubClass extends SuperClass {

    //we inherit method 1

    //method2
    public SubClass method2()
    {
       //do whatever
       return this
    }
}

//succesfully returns instance of Sublass, but...
SubClass sc = new Subclass().method2() 

//...the following line returns an instance of SuperClass and not Sublass
//I want Sublass's instance, without having to using overides
//Is this possible?

SubClass sc = new Subclass().method1()

编辑:----------------------------用例场景------------ ------------------

Message myMessage =  new ReverseTransactionMessageBuilder()
                    .policyNo(POLICY_NO) //on ReverseTransactionMessageBuilder
                    .audUserId(AUD_USER_ID) //on inherited MessageBuilder
                    .audDate(new Date()) //on inherited MessageBuilder
                    .processNo(EProcessConstants.FINANCE_MANUAL_ADJUSTMENT.getProcessCd()) //on inherited MessageBuilder
                    .serviceName("finance.ProcessReversalCmd") //on inherited MessageBuilder
                    .create(); //create is overridden so this is ReverseTransactionMessageBuilder

首先您会注意到，sbrattla 方式允许我以任何顺序调用这些 .audDate () .xxx() 方法。使用上面的类构造，您被迫最后调用 sublcass 上的方法(或使用非常丑陋的强制转换)

Answer 1

您需要执行以下操作:

public class SuperClass<T> {

  public T method1() {
    return (T) this;
  }

}

public class SubClass extends SuperClass<SubClass> {

  public SubClass method2() {
    return (SubClass) this;
  }

}

您可以在“Generics Introduction”中阅读有关 Java 泛型的更多信息，但简要说明了您要告诉 SuperClass 将返回的实例转换为代表您定义的类型的 T。在这种情况下，它是子类。