(Currently 52) programming from 51 years old Note FileOutputStream android.content.Context.openFileOutput (java.lang.String, int)'on a null object reference

Suppose you have this code

SampleActivity

Context context;
@Override 
protected void onCreate(Bundle savedInstanceState){
   super.onCreate(savedInstanceState);
   setContantView(R.layout.sample);
   context = getApplicationContext();

   new AsyncTask().execute();
}


public void fileWrite(){
   try{
       openFileOutput("fileName",Context.MODE_APPEND);
   }
}

AsyncTask

@Override 
protected void onPostExecute(String s){
   super.onPostExecute(s);

   SampleActivity sampleA = new SampleActivity();
   sampleA.fileWrite();
}

With this FileOutputStream android.content.Context.openFileOutput(java.lang.String, int)' on a null object reference I get an error.

Since context is not generated when the instance is created in the code, it becomes NullPointerExeption.

If you want to avoid it> https://qiita.com/old_cat/items/ff4f2116192fd536fb59 As described in, implement the callback method and handle the context.

SampleActivity

Context context;
@Override 
protected void onCreate(Bundle savedInstanceState){
   super.onCreate(savedInstanceState);
   setContantView(R.layout.sample);
   context = getApplicationContext();

   AsyncTaskCallBack asyncTaskCallBack = new AsyncTaskCallBack();
   AsyncTask asyncTask = new AsyncTask(asyncTaskCallBack);
}

public class AsyncTaskCallBack(){
   //You can also use fileOutputStream here
   //But try calling another method
   public void one(){
      fileWriter();
   }
}

public void fileWrite(){
   try{
       openFileOutput("fileName",Context.MODE_APPEND);
   }
}

AsyncTask

SampleActivity.AsyncTaskCallBack asyncTaskCallBack;

public AsyncTask(SampleActivity.AsyncTaskCallBack asyncTaskCallBack){
   this.asyncTaskCallBack = asyncTaskCallBack;
}

@Override 
protected void onPostExecute(String s){
   super.onPostExecute(s);
   asyncTaskCallBack.one();
}