无法更新 listView (android)

Question

i was trying to code something(the world will never know what :D) and when im trying to update listview i just get an error....

code in theory: "when i click on item in NavDrawer the listView should get populated with specific data"

code in reality:

public class MainActivity extends AppCompatActivity {
ArrayAdapter<String> adapter;
String[] values;

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


    ListView listview = (ListView) findViewById(R.id.listview);
    values = new String[]{"Android", "iPhone", "WindowsMobile",
            "Blackberry", "WebOS", "Ubuntu", "Windows7"};
    adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, values);
    listview.setAdapter(adapter);
}
public void update(){
    adapter.clear();
    adapter.notifyDataSetChanged();
}

that method is called from this method that exsists in NavDrawerFragment.java

@Override
public void itemClicked(View view, int position) {
    Toast.makeText(getActivity(), "shit is going down", Toast.LENGTH_LONG).show();
    new MainActivity().update();

}

i guess that it is happening because im creating a new object, but i just (in the middle of typing a post) passed adapter as a method parameter(u can inmagine it as a setter)and in itemClicked method i typed adapter.clear() and same shit happend

---

okay, i changed values varible using setter

 public void setValues(String[] values) {
    this.values = values;
    adapter.notifyDataSetChanged();
} 

i called it like that from recyclerview(that exists in the drawer) adapter String values[] = {"lel", "uja"}; mainActivity.setValues(values); String values[] = {"lel", "uja"}; mainActivity.setValues(values); so same crash, maybile som1 would like to get on skype call? i promise, i wont be annoying

---

Apparently I was creating another object after all:

 MainActivity mainActivity = new MainActivity();

I was wondering why I can access that method like this:

mainActivity.setValues(values);

So probably what I'm asking is: How to access method without creating activity?

Answer 1

For an ArrayAdapter, notifyDataSetChanged only works if you use the add() , insert() , remove() , and clear() on the Adapter.

When an ArrayAdapter is constructed, it holds the reference for the List that was passed in. If you were to pass in a List that was a member of an Activity , and change that Activity member later, the ArrayAdapter is still holding a reference to the original List. The Adapter does not know you changed the List in the Activity.

 @Override
    public void itemClicked(View view, int position) {
        //filter your list i.e `values` based on on item click,clearing it and adding new values
        adapter.notifyDataSetChanged();
    }