数据成员上的 Java ClassCastException

Question

我有以下内容:类 BaseModel 包含 BaseForm:

public class BaseModel {
    BaseForm form;

    public BaseForm getForm() { return form; }
    public void setForm(BaseForm form) { this.form = form; }
}

MyModel 类扩展了 BaseModel； MyForm 类扩展了 BaseForm。也就是说，存在并行继承，

MyModel -> BaseModel

MyForm -> BaseForm

public class MyModel extends BaseModel { 
    // ... 'form' (inner data member) is set to MyForm which extends BaseForm
}

下面给了我一个 ClassCaseException。我需要接收此特定模型中包含的特定表格:

MyForm myForm = (MyForm)specificModel.getForm();

java.lang.ClassCastException: BaseForm cannot be cast to MyForm

我知道我无法转换到特定类别，但我该怎么办？

Answer 1

如果您在MyModel中设置给成员form的对象实际上是一个MyForm(而不是BaseForm )，那么就不会抛出异常。请参阅下面的示例(我已经测试过代码):

public class MyModel extends BaseModel { 

    // ... 'form' (inner data member) is set to MyForm which extends BaseForm
    MyModel(MyForm mf) {
        this.form = mf;
    }

    public static void main(String[] args) {
        MyForm mf = new MyForm();
        MyModel specificModel = new MyModel(mf);
        MyForm myForm = (MyForm) specificModel.getForm();  // No exception now
    }
}

public class BaseModel {
    BaseForm form;

    public BaseForm getForm() { return form; }
    public void setForm(BaseForm form) { this.form = form; }
}

public class MyForm extends BaseForm {
}

public class BaseForm {
}