Java泛型编译错误-不了解该错误

Question

I know many ask such problems, but I really do not get this one.

(If you need more code, please tell)

public class QuestionManager<Question extends Component & IQuestion<? extends IAnswerStorage>>

public class AccountingQuestionManager<Question extends Component & IQuestion<? extends IAnswerStorage>> extends QuestionManager<Question>

public interface IQuestion<DataStorage extends IAnswerStorage>

IAnswerStorage is an empty interface

These are my 3 generic classes I need for explaining. In a QuestionManager the questions need to be questions ( IQuestions ) [with storage specified]

I now tried the following in the hope that it allows any questions, which are components as well.

QuestionManager<? extends Component & IQuestion<? extends IAnswerStorage>> manager = new AccountingQuestionManager<>(
            "Test test", this);

I also have a function to add questions to a pool of questions:

manager.addQuestion(question);

But I get the 2 following errors:

Incorrect number of arguments for type QuestionManager<Question>; it cannot be parameterized with arguments <? extends Component, IQuestion<? extends IAnswerStorage>>

Syntax error on token "&", , expected

Thanks so much for your help. Let me know if you need more.

Answer 1

In this expression

public class QuestionManager<Question extends Component & IQuestion<? extends IAnswerStorage>>

you are declaring a type parameter named Question which is a subtype of Component and of IQuestion<? extends IAnswerStorage> IQuestion<? extends IAnswerStorage> . That is, the type parameter Question has multiple bounds.

In this expression

QuestionManager<? extends Component & IQuestion<? extends IAnswerStorage>> manager = new AccountingQuestionManager<>("Test test", this);

what you are attempting to do is declare a type argument for the QuestionManager type. A type argument cannot be declared to have multiple bounds. A type argument, by definition, already is a type with whatever bounds it has. You can't redefine it in the expression.

Here are the syntax rules for type arguments.

Answer 2

You may use the generic & in a class declaration, but not in variable assignment. You can safely do

QuestionManager<?> manager = //...

because it is guaranteed in the specification of the type parameter Question that it will already fit ? extends Component & IQuestion<? extends IAnswerStorage> ? extends Component & IQuestion<? extends IAnswerStorage> ? extends Component & IQuestion<? extends IAnswerStorage> .

Answer 3

So that is the way I was able to solve my specific problem (which I would not be able to, if I would be less generic) QuestionManager:

public class QuestionManager<Question extends IQuestion<? extends AnswerStorage>>

extends ExtBorderLayout

And the "addQuestion" method looks like the following

public <QuestionComponent extends IQuestion<? extends AnswerStorage> & Component> void addQuestion(
        QuestionComponent question)

So, making the method generic did the job for me.

Thanks for your help!

Answer 4

You cannot use '&' in the generic part when declaring a variable. It only works when declaring a generic class. When declaring a variable/field that uses a generic class, always provide concrete classes/interfaces for the generic parameters. eg, QuestionManager<Question>