Change TextView text from another class using interface

Question

I'm trying to make some utils functions to use in a bigger app later(download file from url, upload file to url etc) So in MainActivity I have only 2 buttons that on click call static methods from Utils class. However, I want on MainActivity to have some indicators of how things working on download/upload methods(connecting, connection success/fail, percent of download etc) so I put on MainActivity a TextView that will show that. I made an interface ICallback that contains void setConnectionStatus(String status) and from Utils class I use this to send to MainActivity the status.

Here are some parts of the code :

public class MainActivity extends AppCompatActivity implements ICallback {

Button btnDownloadDB, btnUploadDB, btnUploadPics;
TextView txtStatus;
ProgressBar pb;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    //Initialize stuffs
    initViews();

    //Setting listeners
    btnDownloadDB.setOnClickListener(v -> {
        txtStatus.setText(R.string.connecting);
        pb.setVisibility(View.VISIBLE);
        Utils.downloadFile(DOWNLOAD_DB, DB_FILE_NAME);
    });
}

@Override
public void setConnectionStatus(String status) {
    Log.d("MIHAI", status);
    txtStatus.setText(status);
}

The interface :

public interface ICallback {
void setConnectionStatus(String status); }

And the Utils class :

public class Utils {

static ICallback callback= new MainActivity();

public static void downloadFile(String downloadURL, String fileName) {

    IFileTransferClient client = ServiceGenerator.createService(IFileTransferClient.class);
    Call<ResponseBody> responseBodyCall = client.downloadFile(downloadURL);

    responseBodyCall.enqueue(new Callback<ResponseBody>() {
        @Override
        public void onResponse(Call<ResponseBody> call, Response<ResponseBody> response) {
            Log.d("MIHAI", "connection ok");
            callback.setConnectionStatus("Connection successful");
        }

        @Override
        public void onFailure(Call<ResponseBody> call, Throwable t) {
            Log.d("MIHAI", "err...fail");
            callback.setConnectionStatus("Connection failed. Check internet connection.");
        }
    });
}

The problem appear on MainActivity, when I try to set text of the txtStatus TextView getting a null reference error even if the txtStatus is initialized on initViews() method. The Logs are working fine so I get the right status in MainActivity. I tried to initialize the TextView again in that function before seting the text and im getting : "java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:183)"

Is there any chance to make this work? Thank you for reading.

Kind regards, Mihai

Is the error thrown from the listener or the setConnectionStatus method? — 66Gramms
– 66Gramms, Commented Dec 2, 2021 at 10:09

Jenea Vranceanu · Accepted Answer · 2021-11-27 20:25:47Z

2

There are multiple problems with your solution but the main one is this line:

static ICallback callback= new MainActivity();

First of all, never hold a static reference to Activity, Fragment, Context or any Context related classes. These classes are either bound to a Context or represent the Context itself. You may leak memory this way. But that is the other problem.

What is the actual problem in your code is that new MainActivity() in Utils class creates an absolutely different instance of MainActivity that has nothing to do with MainActivity that is responsible for displaying your UI in the runtime.

What you should do instead is pass an instance of ICallback to the function as an argument:

public static void downloadFile(String downloadURL, String fileName, ICallback callback) {
...
}

And remove static ICallback callback= new MainActivity();.

Note: when you pass a callback object to a function make sure when it is called your Activity is not in a finished state.

answered Nov 27, 2021 at 20:25

Jenea Vranceanu

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1 Comment

Mihai Stanciu Over a year ago

Thank you very much. Did what you suggested and works. Please, tell me what are the other problems with the solution. I really want to learn more. Thank you!

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