How can I enable or disable the GPS programmatically on Android?

the GPS can be toggled by exploiting a bug in the power manager widget. see this xda thread for discussion.

here's some example code i use

private void turnGPSOn(){
    String provider = Settings.Secure.getString(getContentResolver(), Settings.Secure.LOCATION_PROVIDERS_ALLOWED);

    if(!provider.contains("gps")){ //if gps is disabled
        final Intent poke = new Intent();
        poke.setClassName("com.android.settings", "com.android.settings.widget.SettingsAppWidgetProvider"); 
        poke.addCategory(Intent.CATEGORY_ALTERNATIVE);
        poke.setData(Uri.parse("3")); 
        sendBroadcast(poke);
    }
}

private void turnGPSOff(){
    String provider = Settings.Secure.getString(getContentResolver(), Settings.Secure.LOCATION_PROVIDERS_ALLOWED);

    if(provider.contains("gps")){ //if gps is enabled
        final Intent poke = new Intent();
        poke.setClassName("com.android.settings", "com.android.settings.widget.SettingsAppWidgetProvider");
        poke.addCategory(Intent.CATEGORY_ALTERNATIVE);
        poke.setData(Uri.parse("3")); 
        sendBroadcast(poke);
    }
}

use the following to test if the existing version of the power control widget is one which will allow you to toggle the gps.

private boolean canToggleGPS() {
    PackageManager pacman = getPackageManager();
    PackageInfo pacInfo = null;

    try {
        pacInfo = pacman.getPackageInfo("com.android.settings", PackageManager.GET_RECEIVERS);
    } catch (NameNotFoundException e) {
        return false; //package not found
    }

    if(pacInfo != null){
        for(ActivityInfo actInfo : pacInfo.receivers){
            //test if recevier is exported. if so, we can toggle GPS.
            if(actInfo.name.equals("com.android.settings.widget.SettingsAppWidgetProvider") && actInfo.exported){
                return true;
            }
        }
    }

    return false; //default
}

ENABLE GPS:

Intent intent=new Intent("android.location.GPS_ENABLED_CHANGE");
intent.putExtra("enabled", true);
sendBroadcast(intent);

DISABLE GPS:

Intent intent = new Intent("android.location.GPS_ENABLED_CHANGE");
intent.putExtra("enabled", false);
sendBroadcast(intent);

All these answers are not allowed now. Here is the correct one:

For all those still looking for the Answer:

Here is how OLA Cabs and other such apps are doing it.

Add this in your onCreate

if (googleApiClient == null) {
    googleApiClient = new GoogleApiClient.Builder(this)
            .addApi(LocationServices.API).addConnectionCallbacks(this)
            .addOnConnectionFailedListener(Login.this).build();
    googleApiClient.connect();
            LocationRequest locationRequest = LocationRequest.create();
    locationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);
    locationRequest.setInterval(30 * 1000);
    locationRequest.setFastestInterval(5 * 1000);
    LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder()
            .addLocationRequest(locationRequest);

    // **************************
    builder.setAlwaysShow(true); // this is the key ingredient
    // **************************

    PendingResult<LocationSettingsResult> result = LocationServices.SettingsApi
            .checkLocationSettings(googleApiClient, builder.build());
    result.setResultCallback(new ResultCallback<LocationSettingsResult>() {
        @Override
        public void onResult(LocationSettingsResult result) {
            final Status status = result.getStatus();
            final LocationSettingsStates state = result
                    .getLocationSettingsStates();
            switch (status.getStatusCode()) {
            case LocationSettingsStatusCodes.SUCCESS:
                // All location settings are satisfied. The client can
                // initialize location
                // requests here.
                break;
            case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:
                // Location settings are not satisfied. But could be
                // fixed by showing the user
                // a dialog.
                try {
                    // Show the dialog by calling
                    // startResolutionForResult(),
                    // and check the result in onActivityResult().
                    status.startResolutionForResult(Login.this, 1000);
                } catch (IntentSender.SendIntentException e) {
                    // Ignore the error.
                }
                break;
            case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
                // Location settings are not satisfied. However, we have
                // no way to fix the
                // settings so we won't show the dialog.
                break;
            }
        }
    });
}

These are the implmented methods:

@Override
public void onConnected(Bundle arg0) {
    // TODO Auto-generated method stub

}

@Override
public void onConnectionSuspended(int arg0) {
    // TODO Auto-generated method stub

}

@Override
public void onConnectionFailed(ConnectionResult arg0) {
    // TODO Auto-generated method stub

}

Here is the Android Documentation for the same.

This is to help other guys if they are still struggling:

Edit: Adding Irfan Raza's comment for more help.

@Override protected void onActivityResult(int requestCode, int resultCode, Intent data) {
     if (requestCode == 1000) {
         if(resultCode == Activity.RESULT_OK){
             String result=data.getStringExtra("result"); 
         } if (resultCode == Activity.RESULT_CANCELED) {
             //Write your code if there's no result 
         } 
    } 
}