I have the following class:

public class RemoteService {

  private static class QueryEntry<T> {
    public final IQueryInput<T> input;
    public final FutureCallback<T> callback;
    // constructor not shown here
  }

  private final List<QueryEntry<?>> mQueries;

  public <T> Result doStuff(IQueryInput<T> input, FutureCallback<T> callback) {
    // Simplified code here
    if(mIsServiceAlive) {
      return actuallyMakeQuery(input, callback);
    } else {
      // will make it happen later
      mQueries.add(new QueryEntry(input, callback));
    }
  }

  private <T> Result actuallyMakeQuery(IQueryInput<T> input, FutureCallback<T> callback) {
    // ... magic
    return result;
  }

  //.. other code
  
  private void onServiceAlive() {
    //This method is called when this service is connected to dependencies 
    //and is now live. Here we want to actually make the queries ...
    for(final QueryEntry<?> entry : mQueries) {
      // Compile error here because I can't reference entry<T>.input
      // as T is now lost .... is there any where I can make this happen?
      actuallyMakeQuery(entry.input, entry.callback);
    }
    mQueries.clear();
  }

}

Question here is quite straightforward -> is there anyway I can call actuallyMakeQuery with mQueries? I don't think so because when I cache all the inputs, I lost T because mQueries is using QueryEntry<?>. I also cannot change doStuff signature as it is defined as an interface else where. What can I do here?

CodePudding user response：

You can capture the wildcard by introducing an additional level of indirection:

private <T> Result actuallyMakeQuery(IQueryInput<T> input, FutureCallback<T> callback) {
    // ... magic
    return new Result();
}

private <T> Result actuallyMakeQuery(QueryEntry<T> entry) {
    // no wildcards anymore
    return actuallyMakeQuery(entry.input, entry.callback);
}

Also, you omitted the <> in

mQueries.add(new QueryEntry<>(input, callback));

unnecessarily turning it into rawtypes.

You can then use that new method:

private void onServiceAlive() {
    // This method is called when this service is connected to dependencies
    // and is now live. Here we want to actually make the queries ...
    for (final QueryEntry<?> entry : mQueries) {
        // no problem here anymore
        actuallyMakeQuery(entry);
    }
    mQueries.clear();
}