Referencing and using an array from another class

i am creating an expandable list view in android for an app I am making. What I am trying to dynamically add and remove items from an array.

this is the source code I used and am modifying.http://androidtrainningcenter.blogspot.in/2012/07/android-expandable-listview-simple.html

what i did is made a button in another class that has an "add" function. this is supposed to add to the list whatever is in a referenced textfield. However, I cannot figure out how to reference the button in the sourcecode class, so I thought i would do it the other way and reference the array in the button class.

How can I do this? I know that to reference a method from another class I would do:

Methodclass test = new Methodclass();

then use:

 test.getGroupData();

however, I am not sure how to do this with an array and call the .add()method on it.

EDIT

here is the code I want to use:

    MainActivity.java
package info.androidhive.expandablelistview;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.ExpandableListView;
import android.widget.ExpandableListView.OnChildClickListener;
import android.widget.ExpandableListView.OnGroupClickListener;
import android.widget.ExpandableListView.OnGroupCollapseListener;
import android.widget.ExpandableListView.OnGroupExpandListener;
import android.widget.Toast;

public class MainActivity extends Activity {

    ExpandableListAdapter listAdapter;
    ExpandableListView expListView;
    List<String> listDataHeader;
    HashMap<String, List<String>> listDataChild;

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

        // get the listview
        expListView = (ExpandableListView) findViewById(R.id.lvExp);

        // preparing list data
        prepareListData();

        listAdapter = new ExpandableListAdapter(this, listDataHeader, listDataChild);

        // setting list adapter
        expListView.setAdapter(listAdapter);
    }

    /*
     * Preparing the list data
     */
    private void prepareListData() {
        listDataHeader = new ArrayList<String>();
        listDataChild = new HashMap<String, List<String>>();

        // Adding child data
        listDataHeader.add("Top 250");
        listDataHeader.add("Now Showing");
        listDataHeader.add("Coming Soon..");

        // Adding child data
        List<String> top250 = new ArrayList<String>();
        top250.add("The Shawshank Redemption");
        top250.add("The Godfather");
        top250.add("The Godfather: Part II");
        top250.add("Pulp Fiction");
        top250.add("The Good, the Bad and the Ugly");
        top250.add("The Dark Knight");
        top250.add("12 Angry Men");

        List<String> nowShowing = new ArrayList<String>();
        nowShowing.add("The Conjuring");
        nowShowing.add("Despicable Me 2");
        nowShowing.add("Turbo");
        nowShowing.add("Grown Ups 2");
        nowShowing.add("Red 2");
        nowShowing.add("The Wolverine");

        List<String> comingSoon = new ArrayList<String>();
        comingSoon.add("2 Guns");
        comingSoon.add("The Smurfs 2");
        comingSoon.add("The Spectacular Now");
        comingSoon.add("The Canyons");
        comingSoon.add("Europa Report");

        listDataChild.put(listDataHeader.get(0), top250); // Header, Child data
        listDataChild.put(listDataHeader.get(1), nowShowing);
        listDataChild.put(listDataHeader.get(2), comingSoon);
    }
}

how can I change all that hard-coded material and into a dynamic input? as it, what can I use add to the parent list when the user wants, and if the user wants to add to the child list of a specific parent, how do I do that?

Currently, my code has the button for adding and the edittext code in a separate class.  What I want to do is make is so that when the ADD button is pressed, it will add something to the parent list, and if I click on an element in the parent list, i get that index and add a child element to that specific parent's child-list.

how can I do this?  so basically, i want to do something like this:

remove all of this:

array.add() //hardcoded
and add a more dynamic user dependant input.

and make is something similar to this:

     test = edittext to string
     **pressed add putton**
     test is added to the list.
    **click on test in the list**
     **select add item**
     add item to the test childlist

and this should be able to work f

or any number of inputs.

please let me know how to do this as I have tried everything i can think of. Keep in mind that my add button and edit text are in a separate class. My parent and child lists are in one class, but the button and edit-text are in another. am I going to have to move it?