android - finish the current activity only after the asynctask finishes

Question

I have implement task by interface and callback function. When the user got account wrong, the error message comes up and there are no errors at that point. However, the app stops unfortunately when the user login successfully.

Java:

public interface AsyncResponse {
    void processFinish();
} 

// start this activity from MainActivity when the button is pressed
public class login extends AppCompatActivity implements AsyncResponse{
    ...
    BackgroundTask backgroundTask = new BackgroundTask(this);
    backgroundTask.delegate = this;
    backgroundTask.execute("login",user_ac_s,user_pw_s);

    public void processFinish(){
        if(!MainActivity.user_ID.equals("0"))
            finish();
    }
}

public class BackgroundTask extends AsyncTask<String, Void, String>{ 
    ...
    public AsyncResponse delegate = null;
    ...
    protected void onPostExecute(String result) {
        super.onPreExecute();
        if(task.equals("get_userID")){
            MainActivity.user_ID = result;
            delegate.processFinish();
        }
        if(task.equals("register") || task.equals("login")){
            Toast.makeText(context,result,Toast.LENGTH_LONG).show();
            if(result.contains("success")){
                BackgroundTask get_userID_task = new BackgroundTask(context);
                get_userID_task.execute("get_userID",user_ac);
            }
        }
    }
}

Error: --------- beginning of crash

08-04 05:53:11.807 2667-2667/com.example.kwei.minigame1 E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.kwei.minigame1, PID: 2667 java.lang.NullPointerException: Attempt to invoke interface method 'void com.example.kwei.minigame1.AsyncResponse.processFinish()' on a null object reference at com.example.kwei.minigame1.BackgroundTask.onPostExecute(BackgroundTask.java:132) at com.example.kwei.minigame1.BackgroundTask.onPostExecute(BackgroundTask.java:26) at android.os.AsyncTask.finish(AsyncTask.java:636) at android.os.AsyncTask.access$500(AsyncTask.java:177) at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:653) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:135) at android.app.ActivityThread.main(ActivityThread.java:5254) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

"stops unfortunately" means there's a logcat. Please include it in the question — OneCricketeer, Aug 04 '16 at 05:51
Possible duplicate of [What is a NullPointerException, and how do I fix it?](http://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it) — Yazan, Aug 04 '16 at 06:14

Piyush · Accepted Answer · 2016-08-04T06:15:09.333

1

Your

AsyncResponse delegate = null;

delegate is null. You haven't initialized that !

Edit:

Make constructor

  public AsyncResponse delegate;

  public BackgroundTask(AsyncResponse delegate){
    this.delegate = delagate;
  }

Now execute your async task with

  BackgroundTask backgroundTask = new BackgroundTask(new AsyncResponse()......);
  backgroundTask.delegate = this;// Comment this 
  backgroundTask.execute("login",user_ac_s,user_pw_s);

this keyword defines the activity's context in your case but actually you need to get for AsyncResponse.

edited Aug 04 '16 at 06:15

answered Aug 04 '16 at 05:59

Piyush

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how about backgroundTask.delegate = this in the class login ; – Keroro Chan Aug 04 '16 at 06:01
_this_ means what ? You need to pass your delegate to in async constructor – Piyush Aug 04 '16 at 06:03
backgroundTask.delegate = this change the variable delegate in object backgroundTask, isn't it? – Keroro Chan Aug 04 '16 at 06:06
new AsyncResponse() cannot be allowed as AsyncResponse is interface or abstract. – Keroro Chan Aug 04 '16 at 06:29
Because you have already implements that. So change `BackgroundTask backgroundTask = new BackgroundTask(this);` – Piyush Aug 04 '16 at 06:31
It works now. What is the difference between `backgroundTask.delegate = this;` in login and `this.delegate = delagate;` in constructor? – Keroro Chan Aug 04 '16 at 06:39
1

This _backgroundTask.delegate = this;_ will indicates that it is the context of activity but you need to use for your _AsyncResponse_ class. – Piyush Aug 04 '16 at 06:42
If I want to implement the way like `backgroundTask.delegate = this;`. How can I change it? – Keroro Chan Aug 04 '16 at 06:46
You should implement your `AsyncResponse` in your `BackgroundTask` class. – Piyush Aug 04 '16 at 06:50
OK. Thank you so much. – Keroro Chan Aug 04 '16 at 07:09

android - finish the current activity only after the asynctask finishes

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