I've made an AlertDialog with an EditText, and I want that when the user clicks on 'Buscar' (positiveButton) the map shows a Marker in the address entered on the AlertDialog. So I think I need to pass the data from this EditText to the MapFragment in order to use the Geocoder with that data.
Here's my mainActivity.java with the AlertDialog:
package com.example.alex.testing_map;
import android.content.DialogInterface;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.design.widget.NavigationView;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.text.InputType;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;
public class MapsActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {
private String location = "";
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_maps);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    DrawerLayout drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
   ActionBarDrawerToggle drawerToggle = new ActionBarDrawerToggle(
            this, drawerLayout, toolbar, R.string.openDrawer, R.string.closeDrawer);
    drawerLayout.setDrawerListener(drawerToggle);
    drawerToggle.syncState();
    Fragment fragment = new MapFragment();
    FragmentTransaction ft = getSupportFragmentManager().beginTransaction();
    ft.add(R.id.frame_content, fragment);
    ft.commit();
    NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
    navigationView.setNavigationItemSelectedListener(this);
}
@Override
public void onBackPressed() {
    DrawerLayout drawerLayout = (DrawerLayout) findViewById(R.id.drawer_layout);
    if (drawerLayout.isDrawerOpen(GravityCompat.START)) {
        drawerLayout.closeDrawer(GravityCompat.START);
    } else {
        super.onBackPressed();
    }
}
@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_main, menu);
    return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item){
    switch (item.getItemId()){
        case R.id.add_item:
            Intent intent = new Intent(this, AddMail.class);
            this.startActivity(intent);
            break;
        case R.id.search:
            AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setTitle("Dirección");
            final EditText input = new EditText(this);
            input.setInputType(InputType.TYPE_CLASS_TEXT);
            builder.setView(input);
            builder.setPositiveButton("Buscar", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                  location = input.getText().toString();
                }
            });
            builder.setNegativeButton("Cancelar", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    dialog.cancel();
                }
            });
            builder.show();
            break;
        default:
            return super.onOptionsItemSelected(item);
    }
    return true;
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
    Fragment fragment = null;
    switch (item.getItemId()){
        case R.id.mapa:
            fragment = new MapFragment();
            FragmentTransaction ft1 = getSupportFragmentManager().beginTransaction();
            ft1.replace(R.id.frame_content, fragment);
            ft1.commit();
            getSupportActionBar().setTitle(R.string.app_name);
            break;
        case R.id.help:
            fragment = new Help();
            FragmentTransaction ft2 = getSupportFragmentManager().beginTransaction();
            ft2.replace(R.id.frame_content, fragment);
            ft2.commit();
            getSupportActionBar().setTitle(item.getTitle());
            break;
        case R.id.about:
            fragment = new About();
            FragmentTransaction ft3 = getSupportFragmentManager().beginTransaction();
            ft3.replace(R.id.frame_content, fragment);
            ft3.commit();
            getSupportActionBar().setTitle(item.getTitle());
            break;
        case R.id.rate:
            Intent face = new Intent(Intent.ACTION_VIEW);
            face.setData(Uri.parse("fb://profile/" + "100000914715145"));
            startActivity(face);
            break;
        default:
            return true;
    }
    DrawerLayout drawerLayout = (DrawerLayout)findViewById(R.id.drawer_layout);
    drawerLayout.closeDrawer(GravityCompat.START);
    return true;
}
}
And my MapFragment.java:
package com.example.alex.testing_map;
import android.Manifest;
import android.content.Context;
import android.content.pm.PackageManager;
import android.location.Criteria;
import android.location.Location;
import android.location.LocationManager;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.content.ContextCompat;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import com.google.android.gms.maps.CameraUpdateFactory;
import com.google.android.gms.maps.GoogleMap;
import com.google.android.gms.maps.OnMapReadyCallback;
import com.google.android.gms.maps.SupportMapFragment;
import com.google.android.gms.maps.model.BitmapDescriptorFactory;
import com.google.android.gms.maps.model.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;
public class MapFragment extends Fragment implements OnMapReadyCallback {
private GoogleMap mMap;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup viewGroup, Bundle savedInstanceState) {
    View view = inflater.inflate(R.layout.map_fragment, viewGroup, false);
    FragmentManager manager = getFragmentManager();
    FragmentTransaction transaction = manager.beginTransaction();
    SupportMapFragment fragment = new SupportMapFragment();
    transaction.replace(R.id.mapView, fragment);
    transaction.commit();
    fragment.getMapAsync(this);
    return view;
}
@Override
public void onMapReady(GoogleMap googleMap) {
    mMap = googleMap;
    if (ContextCompat.checkSelfPermission(getContext(), Manifest.permission.ACCESS_FINE_LOCATION)
            == PackageManager.PERMISSION_GRANTED) {
        mMap.setMyLocationEnabled(true);
    } else {
        if (ContextCompat.checkSelfPermission(getContext(), Manifest.permission.ACCESS_COARSE_LOCATION)
                == PackageManager.PERMISSION_GRANTED) {
            mMap.setMyLocationEnabled(true);
        } else {
        }
    }
    mMap.getUiSettings().setCompassEnabled(false);
    mMap.getUiSettings().setIndoorLevelPickerEnabled(false);
    mMap.getUiSettings().setMapToolbarEnabled(false);
    mMap.getUiSettings().setRotateGesturesEnabled(false);
    mMap.getUiSettings().setZoomControlsEnabled(false);
    // Primer poso les coordenades i estableixo els punts (numerats amb lletres en castellà)
    // i després afegeixo els marcadors, posant de localització
    // el nom dels punts definits amb anterioritat
    LocationManager locationManager = (LocationManager)getActivity().getSystemService(Context.LOCATION_SERVICE);
    Criteria criteria = new Criteria();
    String provider = locationManager.getBestProvider(criteria, true);
    Location location = locationManager.getLastKnownLocation(provider);
    LatLng latLng = new LatLng(location.getLatitude(), location.getLongitude());
    LatLng p1 = new LatLng(41.39355, 2.15473);
    mMap.addMarker(new MarkerOptions().position(p1).icon(BitmapDescriptorFactory.fromResource(R.drawable.pointer_bici1)));
   mMap.moveCamera(CameraUpdateFactory.newLatLngZoom(latLng, 18));
}
}