Adding to an array from a separate class

Question

I have made a listView that is populated with an array. I can add data to this listview by entering in data in a textbox called txtinputs and then click a button called btnAdds this all works fine when I have it all in a single class. However what I want to do is have this on 2 separate screens using 2 separate classes, the txtinputs and the btnAdds in a separate class than the listView. When the data is entered into the txtinputs and the btnAdds is selected it will add that data into the listView on a seperate class. I want the listView in a class called ListDeadlines and my textBox and Button in a class called AddDeadline. I currently add the data into the listView like this.

public ArrayList<String> arrayList;
public ArrayAdapter<String> adapter;
public EditText txtInput;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_list_deadlines);
    final ListView listView = (ListView)findViewById(R.id.listv);
    String[] items= {"HCI","ITM","Presentation"};
    arrayList=new ArrayList<>(Arrays.asList(items));
    adapter=new ArrayAdapter<String>(this,R.layout.list_item,R.id.txtitem,arrayList);
    listView.setAdapter(adapter);

    txtInput=(EditText)findViewById(R.id.txtinputs);
    Button btAdd=(Button)findViewById(R.id.btnAdds);
    btAdd.setOnClickListener(new View.OnClickListener(){
        @Override
    public void onClick(View v) {
            String newItem=txtInput.getText().toString();
            arrayList.add(newItem);
            adapter.notifyDataSetChanged();;


        }
    });

}

I have tried to do it like this in my AddDeadline class like this however I get a runtime on the button click

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_add_deadline);
    getSupportActionBar().setDisplayHomeAsUpEnabled(true);
   final ListDeadlines lst= new ListDeadlines();
    lst.txtInput=(EditText)findViewById(R.id.txtinput);
    Button btAdd=(Button)findViewById(R.id.btnAdd);
    btAdd.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String newItem = lst.txtInput.getText().toString();
            lst.arrayList.add(newItem);
            lst.adapter.notifyDataSetChanged();

        }
    });
}

I want to achieve something like this

Answer 1

In your ListDeadLinesActivity when a user hit the "+" button you want to start AddDeadlineActivity for a result.

You can put this code inside the onClick of that "+" button.

Intent intent = new Intent(this, AddDeadlineActivity.class);
startActivityForResult(intent, ADD_DEADLINE_REQUEST);

ADD_DEADLINE_REQUEST is the request code, you can declare a static int.

@Override
protected void onCreate(Bundle savedInstanceState) {
    // ...

    btAdd.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent returnIntent = getIntent();
            returnIntent.putExtra("result", yourEditText.getText().toString());

            setResult(RESULT_OK,returnIntent);
            finish();
        }
    });
}

In your ListDeadLinesActivity override onActivityResult method,

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    if (requestCode == ADD_DEADLINE_REQUEST){
        if (resultCode == RESULT_OK) {
            String item = data.getStringExtra("result");
            arrayList.add(item);
            adapter.notifyDataSetChanged();
        }
    }
}

Getting a Result from an Activity