Add Marker to Google Map (Google Maps Android API v2)

To add marker to Google Map of Google Maps Android API v2:

                        LatLng targetLatLng = new LatLng(lat, lon);
                        MarkerOptions markerOptions =
                                new MarkerOptions().position(targetLatLng).title(strTitle);
                        mMap.addMarker(markerOptions);

Modify from last example "Set map type for Google Maps Activity using Google Maps Android API v2" to add feature to add markers on Google Maps Activity.

Edit values/strings.xml to add string resource of "menu_addmarkers"

<resources>
    <string name="app_name">AndroidStudioMapApp</string>
    <string name="title_activity_maps">Map</string>
    <string name="menu_legalnotices">Legal Notices</string>
    <string name="menu_about">About</string>
    <string name="menu_addmarkers">Add Markers</string>
</resources>

Edit menu/activity_main.xml, to add menu option of Add Markers:

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/menu_about"
        android:orderInCategory="100"
        app:showAsAction="ifRoom"
        android:title="@string/menu_about"/>
    <item
        android:id="@+id/menu_addmarkers"
        android:orderInCategory="100"
        app:showAsAction="ifRoom"
        android:title="@string/menu_addmarkers"/>
    <item
        android:id="@+id/maptype"
        android:orderInCategory="100"
        app:showAsAction="never"
        android:title="Map Type...">
        <menu >
            <group android:id="@+id/groupmaptype"
                android:checkableBehavior="single">
                <item android:id="@+id/maptypeNORMAL"
                    android:title="NORMAL" />
                <item android:id="@+id/maptypeSATELLITE"
                    android:title="SATELLITE" />
                <item android:id="@+id/maptypeTERRAIN"
                    android:title="TERRAIN" />
                <item android:id="@+id/maptypeHYBRID"
                    android:title="HYBRID" />
                <item android:id="@+id/maptypeNONE"
                    android:title="NONE" />

            </group>
        </menu>

    </item>
    <item
        android:id="@+id/menu_legalnotices"
        android:orderInCategory="100"
        app:showAsAction="never"
        android:title="@string/menu_legalnotices"/>


</menu>

Modify MapsActivity.java

package com.blogspot.android_er.androidstudiomapapp;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.net.Uri;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.text.InputType;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.EditText;
import android.widget.LinearLayout;
import android.widget.Toast;

import com.google.android.gms.common.GoogleApiAvailability;
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.LatLng;
import com.google.android.gms.maps.model.MarkerOptions;

public class MapsActivity extends AppCompatActivity implements OnMapReadyCallback {

    private GoogleMap mMap;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_maps);
        // Obtain the SupportMapFragment and get notified when the map is ready to be used.
        SupportMapFragment mapFragment = (SupportMapFragment) getSupportFragmentManager()
                .findFragmentById(R.id.map);
        mapFragment.getMapAsync(this);

    }

    /**
     * Manipulates the map once available.
     * This callback is triggered when the map is ready to be used.
     * This is where we can add markers or lines, add listeners or move the camera. In this case,
     * we just add a marker near Sydney, Australia.
     * If Google Play services is not installed on the device, the user will be prompted to install
     * it inside the SupportMapFragment. This method will only be triggered once the user has
     * installed Google Play services and returned to the app.
     */
    @Override
    public void onMapReady(GoogleMap googleMap) {
        mMap = googleMap;
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.activity_main, menu);
        return super.onCreateOptionsMenu(menu);
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch (item.getItemId()) {
            case R.id.menu_addmarkers:
                addMarker();
                return true;
            case R.id.maptypeHYBRID:
                if(mMap != null){
                    mMap.setMapType(GoogleMap.MAP_TYPE_HYBRID);
                    return true;
                }
            case R.id.maptypeNONE:
                if(mMap != null){
                    mMap.setMapType(GoogleMap.MAP_TYPE_NONE);
                    return true;
                }
            case R.id.maptypeNORMAL:
                if(mMap != null){
                    mMap.setMapType(GoogleMap.MAP_TYPE_NORMAL);
                    return true;
                }
            case R.id.maptypeSATELLITE:
                if(mMap != null){
                    mMap.setMapType(GoogleMap.MAP_TYPE_SATELLITE);
                    return true;
                }
            case R.id.maptypeTERRAIN:
                if(mMap != null){
                    mMap.setMapType(GoogleMap.MAP_TYPE_TERRAIN);
                    return true;
                }
            case R.id.menu_legalnotices:
                String LicenseInfo = GoogleApiAvailability
                        .getInstance()
                        .getOpenSourceSoftwareLicenseInfo(MapsActivity.this);
                AlertDialog.Builder LicenseDialog =
                        new AlertDialog.Builder(MapsActivity.this);
                LicenseDialog.setTitle("Legal Notices");
                LicenseDialog.setMessage(LicenseInfo);
                LicenseDialog.show();
                return true;
            case R.id.menu_about:
                AlertDialog.Builder aboutDialogBuilder =
                        new AlertDialog.Builder(MapsActivity.this);
                aboutDialogBuilder.setTitle("About Me")
                        .setMessage("http://android-er.blogspot.com");

                aboutDialogBuilder.setPositiveButton("visit",
                        new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        String url = "http://android-er.blogspot.com";
                        Intent i = new Intent(Intent.ACTION_VIEW);
                        i.setData(Uri.parse(url));
                        startActivity(i);
                    }
                });

                aboutDialogBuilder.setNegativeButton("Dismiss",
                        new DialogInterface.OnClickListener() {
                    @Override
                    public void onClick(DialogInterface dialog, int which) {
                        dialog.cancel();
                    }
                });

                AlertDialog aboutDialog = aboutDialogBuilder.create();
                aboutDialog.show();

                return true;
        }
        return super.onOptionsItemSelected(item);
    }

    private void addMarker(){
        if(mMap != null){

            //create custom LinearLayout programmatically
            LinearLayout layout = new LinearLayout(MapsActivity.this);
            layout.setLayoutParams(new LinearLayout.LayoutParams(
                    LinearLayout.LayoutParams.MATCH_PARENT,
                    LinearLayout.LayoutParams.WRAP_CONTENT));
            layout.setOrientation(LinearLayout.VERTICAL);

            final EditText titleField = new EditText(MapsActivity.this);
            titleField.setHint("Title");

            final EditText latField = new EditText(MapsActivity.this);
            latField.setHint("Latitude");
            latField.setInputType(InputType.TYPE_CLASS_NUMBER
                    | InputType.TYPE_NUMBER_FLAG_DECIMAL
                    | InputType.TYPE_NUMBER_FLAG_SIGNED);

            final EditText lonField = new EditText(MapsActivity.this);
            lonField.setHint("Longitude");
            lonField.setInputType(InputType.TYPE_CLASS_NUMBER
                    | InputType.TYPE_NUMBER_FLAG_DECIMAL
                    | InputType.TYPE_NUMBER_FLAG_SIGNED);

            layout.addView(titleField);
            layout.addView(latField);
            layout.addView(lonField);

            AlertDialog.Builder builder = new AlertDialog.Builder(this);
            builder.setTitle("Add Marker");
            builder.setView(layout);
            AlertDialog alertDialog = builder.create();

            builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
                @Override
                public void onClick(DialogInterface dialog, int which) {
                    boolean parsable = true;
                    Double lat = null, lon = null;

                    String strLat = latField.getText().toString();
                    String strLon = lonField.getText().toString();
                    String strTitle = titleField.getText().toString();

                    try{
                        lat = Double.parseDouble(strLat);
                    }catch (NumberFormatException ex){
                        parsable = false;
                        Toast.makeText(MapsActivity.this,
                                "Latitude does not contain a parsable double",
                                Toast.LENGTH_LONG).show();
                    }

                    try{
                        lon = Double.parseDouble(strLon);
                    }catch (NumberFormatException ex){
                        parsable = false;
                        Toast.makeText(MapsActivity.this,
                                "Longitude does not contain a parsable double",
                                Toast.LENGTH_LONG).show();
                    }

                    if(parsable){
                        LatLng targetLatLng = new LatLng(lat, lon);
                        MarkerOptions markerOptions =
                                new MarkerOptions().position(targetLatLng).title(strTitle);
                        mMap.addMarker(markerOptions);
                        mMap.moveCamera(CameraUpdateFactory.newLatLng(targetLatLng));
                    }
                }
            });
            builder.setNegativeButton("Cancel", null);

            builder.show();
        }else{
            Toast.makeText(MapsActivity.this, "Map not ready", Toast.LENGTH_LONG).show();
        }
    }
}

Download the files (Android Studio Format) .

Next:
- initialize map in xml

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~ Step-by-step of Android Google Maps Activity using Google Maps Android API v2, on Android Studio