Question

I really should know this, but for some reason I don’t understand the following.

My abstract class contains the following abstract method:

protected abstract RuleDTO createRowToBeCloned(RuleDTO ruleDTO);

I also have another class as follows:

EvaluationRuleDTO extends from RuleDTO

Then in a subclass of my abstract class I have the following implementation which is not allowed due to “must override or implement a supertype method”:

protected EvaluationRuleDTO createRowToBeCloned(EvaluationRuleDTO ruleDTO) {

However, the following is allowed:

protected EvaluationRuleDTO createRowToBeCloned(RuleDTO ruleDTO) {

I realize this is probably a basic question but I am a little bemused. How come I can I can return a subclass of RuleDTO in the overridden method, but I can’t pass in a subclass?

Thanks

Answer 1

You’re breaking the Liskov principle: everything a superclass can do, a subclass must be able to do. The superclass declares a method accepting any kind of RuleDTO. But in your subclass, you only accept instances of EvaluationRuleDTO. What would happen if you did the following?

RuleDTO rule = new EvaluationRuleDTO();
rule.createRowToBeCloned(new RuleDTO());

An EvaluationRuleDTO is a RuleDTO, so it must fulfill the contract defined by RuleDTO.

The method in the subclass may return an instance of EvaluationRuleDTO instead of a RuleDTO, though, because the contract is to return a RuleDTO, and EvaluationRuleDTO is a RuleDTO.