活动之间的全局对象

Question

at the moment i'm working on an android app, but i've encountered a problem which i cannot solve. I have 2 activities. In one of them i define an ArrayList as public static. This list is used as content for a listview. in that other activity i want to edit items of that specific list or rather delete them.

Problem: on my samsung galaxy S II it works fine (Android version 4.0.4), but on other devices, like SGSIII (4.1 i think) there is a problem with synchronizing that list. so basically if that second activity finishes and returns to the first, all changes seem to be undone...

here is the code of the update function of the listview in the first activity; the changes are not recognized in this method:

private void updateList() { 

    String[] values = new String[hatCommandList.size()];

    for(int i = 0; i < values.length;i++) {             

        values[i] = ((Command)hatCommandList.get(i)).getRealCommand();

    }           
    ArrayAdapter<String> adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, android.R.id.text1, values);     
    ListViewSentCommands.setAdapter(adapter);       
}

as you can see, nothing special. and here the code of the 2nd activity in which i edit the "hatCommandList":

public void onClick(View v) {               
    SettingsActivity.hatCommandList.remove(position);                       
    finish();

}

"position", which is used to point at a position in the list obviously, contains the right value, i checked it already.

Anyway, i hope you can tell me why it works on some device and why it doesnt on some...

Greetings and thanks in advance, Timo

Answer 1

Do not do this...

SettingsActivity.hatCommandList.remove(position);

Most importantly DO NOT create public objects or methods in an Activity and attempt to use them from any other Android application component - an Activity is a special case class which has a life-cycle and should not be treated like a 'normal' Java class.

If you want to share data in this way then I would recommend you create a POJO helper class and hold the ArrayList there. The second Activity can then make changes and when the first Activity resumes, have it check for data changes and refresh the adapter.

Answer 2

Synchronize your list: how to create Synchronized arraylist

Probably your running into a race condition.

Answer 3

Ok, i found the problem:

when the first activity is started, it reads data from a file into that list. that happens in the onCreate function.

The problem is now, that on my SGSII device with android 4.0.4 the focus resumes to the first activity when i finish the second without problems.

BUT on the SGSIII with 4.1 the onCreate method is used once again, even though it actually only resumes. At the moment i have no idea why, but that's a problem which can be solved i guess.

maybe you can give me some tips why the developer of android sdk did change and how i can stop that.

PS: i'm calling the startActivityForResult method to start the 2. activity and also the onActivityResult function.