Question

Going through this tutorial https://developers.google.com/web-toolkit/doc/latest/tutorial/RPC it is mentioned that to set up a call back Object it is necessary to do the following:

// Set up the callback object.
AsyncCallback<StockPrice[]> callback = new AsyncCallback<StockPrice[]>() {
  public void onFailure(Throwable caught) {
    // TODO: Do something with errors.
  }

  public void onSuccess(StockPrice[] result) {
    updateTable(result);
  }
};

However I noticed AsyncCallback is an interface. As far as I knew interfaces could not be instantiated. How is this possible?

Answer 1

This is an example of using an Anonymous Class to implement a callback in Java. This is equivalent to defining a class that implements that interface. To clarify, this:

new AsyncCallback() {
    ...
}

is equivalent to this:

public class MyCallback implements AsyncCallback {
    ...
}

In fact, if you wanted to, you could create your own class in a separate Java file, call it MyCallback, and then do this:

AsyncCallback<StockPrice[]> callback = new MyCallback();

It’s all the same.