Java generics - inferring nested type
generics, java, language-design
Solution
Now I would like to have a class like this
OperationExecutor<T extends Operation<S>> extends AsyncTask<T,Void,S>{
@Override
protected S doInBackground(T... params) {
return null;
}
}
The reason this doesn't work is because `S` hasn't been declared anywhere, only referenced as a type argument in `T`'s bound. Java needs `S` to be declared for other references to it to make sense, for example `protected S doInBackground` and `AsyncTask<T,Void,S>`.
One thing you might consider is whether `OperationExecutor` needs to be generic to a specific type of `Operation<S>`. You could do this instead for example:
OperationExecutor<S> extends AsyncTask<Operation<S>, Void, S> {
@Override
protected S doInBackground(Operation<S>... params) {
return null;
}
}
Problem
I have defined following classes: ``` class Operation<S> class GetReservationOperation extends Operation<Reservation> ``` Now I would like to have a class like this: ``` OperationExecutor<T extends Operation<S>> extends AsyncTask<T,Void,S>{ @Override protected S doInBackground(T... params) { return null; } } ``` But this won't compile: ``` OperationExecutor<GetReservationOperation> executor = new .... ``` Why doesn't Java allow this? After some time I came up with the following solution: ``` OperationExecutor<T extends Operation<S>,S> extends AsyncTask<T,Void,S>{ @Override protected S doInBackground(T... params) { return null; } } ``` But this forces me to write the following: ``` OperationExecutor<GetReservationOperation,Reservation> executor = new .... ``` Which looks odd. Is there any way to make it look nicer? EDIT This worked ``` OperationExecutor<S> extends AsyncTask<Operation<S>,Void,S>{ @Override protected S doInBackground(Operation<S>... params) { return null; } } OperationExecutor<Reservation> executor = new .... executor.execute(getReservationOperation); ```