Android error message: Cannot resolve Add method

I am getting the error:

Cannot resolve method 'add (int, com.example.trainer.androidlistviewelement.ListViewFragmentMainActivity.PlaceholderFragment)

CODE:

   package com.example.trainer.androidlistviewelement;

    import android.app.Fragment;
    import android.support.v7.app.ActionBarActivity;
    import android.os.Bundle;
    import android.view.LayoutInflater;
    import android.view.Menu;
    import android.view.MenuItem;
    import android.view.View;
    import android.view.ViewGroup;
    import android.widget.ArrayAdapter;
    import android.widget.ListView;
    import android.widget.TextView;

    import static com.example.trainer.androidlistviewelement.R.id.container;


    public class ListViewFragmentMainActivity extends ActionBarActivity {

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_list_view_fragment_main);
            TextView tv1 = (TextView) findViewById(R.id.textView1);
            tv1.setText(tv1.getText());
            if (savedInstanceState == null) {
                getSupportFragmentManager().beginTransaction().add(R.id.container, new PlaceholderFragment()).commit();
            }
        }


        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
            // Inflate the menu; this adds items to the action bar if it is present.
            getMenuInflater().inflate(R.menu.menu_list_view_fragment_main, menu);
            return true;
        }

        @Override
        public boolean onOptionsItemSelected(MenuItem item) {

            int id = item.getItemId();

            //noinspection SimplifiableIfStatement
            if (id == R.id.action_settings) {
                return true;
            }

            return super.onOptionsItemSelected(item);
        }

        public static class PlaceholderFragment extends Fragment {
            String[] monthsArray = {"Jan", "Feb", "Mar", "Apr"};
            private ListView monthsListView;
            private ArrayAdapter<String> arrayAdapter;

            public PlaceholderFragment() {}
            @Override
            public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
                View rootView = inflater.inflate(R.layout.fragment_list_view_fragment_main, container, false);

                monthsListView = (ListView) rootView.findViewById(R.id.months_list);

                arrayAdapter = new ArrayAdapter<String>(getActivity(), android.R.layout.simple_list_item_1, monthsArray);

                monthsListView.setAdapter(arrayAdapter);
                return rootView;
            }
        }
    }