Java泛型-在这种用例中我可以摆脱原始类型吗

Question

I'm working with the following jaxb class hierarchy:

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(...)
public class RecordModifyType extends RecordBaseType
{
    ...
    public List<FieldModifyType> getField() { ... }
}

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(...)
public class RecordAddType extends RecordBaseType
{
    ...
    public List<FieldAddType> getField() { ... }
}

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(...)
public class FieldModifyType extends FieldBaseType
{
    ...
}

@XmlAccessorType(XmlAccessType.FIELD)
@XmlType(...)
public class FieldAddType extends FieldBaseType
{
    ...
}

I stumbled upon a method in some other class with the following signature ( please note the raw types ):

private void createFields(FTPPartner user, 
                          String addressMasterDataID, 
                          String connectionMasterDataID, 
                          CRTypeEnum crType, 
                          List addrFields, 
                          List connFields)

This method is called from different parts of the class sometimes with List<FieldAddType> as arguments and other times with List<FieldModifyType> .

Now I'm trying to get rid of the raw types in its signature.

Following the get-put principle , I decided to change the signature to

 private void createFields(FTPPartner user, 
                              String addressMasterDataID, 
                              String connectionMasterDataID, 
                              CRTypeEnum crType, 
                              List<? super FieldBaseType> addrFields, 
                              List<? super FieldBaseType> connFields)
 {
     ...
     FieldBaseType someField = new buildField(...);
     addrFields.add(someField);
     ...
 }

... since the createFields implementation is only putting stuff in these collections. The problem is that since RecordModifyType#getFields() returns List<FieldModifyType> and RecordAddType#getFields() returns List<FieldAddType> , I now can not invoke the method, since it only allows for super types of FieldBaseType, not subtypes:

RecordAddType addrRecord = getRecordAddType(...);
RecordAddType connRecord = getRecordAddType(...);
List<FieldAddType> addrFields = addrRecord.getFields();
List<FieldAddType> connFields = connRecord.getFields();
createFields(user, 
             addressMasterDataID,
             connectionMasterDataID,
             crType
             addFields, // This won't compile
             connFields); // This won't compile
marshalRecord(addrRecord);
marshalRecord(connRecord);

Using raw types is working as expected - there are no compile errors and marshalling works.

So my question is - is there a way to preserve the behaviour but get rid of the raw types?

Answer 1

If I understand your problem correctly, the compiler is complaining because it doesn't know that the type ? super FieldBaseType ? super FieldBaseType for the call is the the same type for each parameter. Also, because both concrete types are subclasses of FieldBaseType , that's how the type needs to be bound IMHO.

Try typing the method:

private <T extends FieldBaseType> void createFields(FTPPartner user, 
                          String addressMasterDataID, 
                          String connectionMasterDataID, 
                          CRTypeEnum crType, 
                          List<T> addrFields, 
                          List<T> connFields) {
    // during the call, the compiler knows that addrFields and connFields
    // hold the same type as each other
}

Now the type is inferred at the call point.