### Question:
Listener4Assist implements ListenerAssist { public void setCallback(@NonNull Listener4Callback callback) { this.callback = callback; } Listener4Assist(ListenerModelHandler<T> handler);  Listener4Assist(ListenerModelHandler.ModelCreator<T> creator); void setCallback(@NonNull Listener4Callback callback); void setAssistExtend(@NonNull AssistExtend assistExtend); AssistExtend getAssistExtend(); void infoReady(DownloadTask task, BreakpointInfo info, boolean fromBreakpoint); void fetchProgress(DownloadTask task, int blockIndex, long increaseBytes); void fetchEnd(DownloadTask task, int blockIndex); synchronized void taskEnd(DownloadTask task, EndCause cause,
                                     @Nullable Exception realCause); @Override boolean isAlwaysRecoverAssistModel(); @Override void setAlwaysRecoverAssistModel(boolean isAlwaysRecoverAssistModel); @Override void setAlwaysRecoverAssistModelIfNotSet(boolean isAlwaysRecoverAssistModel);  }### Answer:
@Test public void setCallback() { assist.setCallback(callback); assertThat(assist.callback).isEqualTo(callback); }