从父类访问子类变量

Question

I have a declaration like this

TransactionItemDto itemDto = getTransactionItemMessage().getTransactionItemDto();

then I have the following piece of code:

if (itemDto instanceof ChangeTransactionItemDto) {
            inputItem.setSiteCode(itemDto.getSiteCode()));
            inputItem.setBinLocation(itemDto.getBinLocation()));

        }

Now, I have variable ' department ' which belongs to ChangeTransactionItemDto only. But I am not able to do this currently inside the if block above: inputItem.setDepartment(itemDto.getDepartment()));

Thats because itemDto is declared of type TransactionItemDto and not ChangeTransactionItemDto .

I can check the value during the debug process if I do itemDto.getDepartment() since internally its of the subclass type. but the statement gives error. How do I set the department belonging to the subclass - ChangeTransactionItemDto on the inputItem ?

Answer 1

由于已验证itemDto是否属于实例ChangeTransactionItemDto ，因此您始终可以调用inputItem.setDepartment(((ChangeTransactionItemDto)itemDto).getDepartment())

Answer 2

It is a design smell if super class needs access to subclass-specific fields.

One work around is to pull the getter/setter up into the super class with a "do nothing" implementation, then those sub classes that use it override it with a "do something" implementation.

This way there is no class in the class hierarchy that needs special treatment - they can all be treated the same way.