java：强相关的成对类

我有两个相互关联的类：

Msg1 <-> Data1
Msg2 <-> Data2
...

Msg1数据1
Msg2数据2
...

其中，所有“DataX”扩展了超类“Data”，而“MsgX”扩展了类“Msg”。这些超级类目前（简化）为：

---

abstract class Dataabstract class Data< MSG extends Msg<? extends Data<MSG>>> {
       MSG msg;
       void modify( MSG msg ) {
           this.msg = msg;
       };   
}

abstract class Msg< DATA extends Data<? extends Msg<DATA>>> {
    abstract DATA createData();

    void modifyData( DATA data ) {
        data.modify( this );
    };  
}

Msg.java:5: error: method modify in class Data<MSG> cannot be applied to given types;
            data.modify( this );
                ^
  required: CAP#1
  found: Msg<DATA>
  reason: argument mismatch; Msg<DATA> cannot be converted to CAP#1
  where DATA,MSG are type-variables:
    DATA extends Data<? extends Msg<DATA>> declared in class Msg
    MSG extends Msg<? extends Data<MSG>> declared in class Data
  where CAP#1 is a fresh type-variable:
    CAP#1 extends Msg<DATA> from capture of ? extends Msg<DATA>
1 error