Save bitmap to file function

You need an appropriate permission in manifest.xml:

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>

out.flush() check the out is not null..

String file_path = Environment.getExternalStorageDirectory().getAbsolutePath() + 
                            "/PhysicsSketchpad";
File dir = new File(file_path);
if(!dir.exists())
    dir.mkdirs();
File file = new File(dir, "sketchpad" + pad.t_id + ".png");
FileOutputStream fOut = new FileOutputStream(file);

bmp.compress(Bitmap.CompressFormat.PNG, 85, fOut);
fOut.flush();
fOut.close();