How to write files from Asset folder to Sd card in Android

Call copyAssets() method from MainActivity.

public class MainActivity extends Activity{
@Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

 copyAssets();
}

//Method to copy filename.ext from assets to sdcard.
private void copyAssets()
{
      AssetManager assetManager = getAssets();
      String[] files = null;
      InputStream in = null;
      OutputStream out = null;
      String filename = "filename.ext";
      try
      {
            in = assetManager.open(filename);
            out = new FileOutputStream("/sdcard/" + filename);
            copyFile(in, out);
            in.close();
            in = null;
            out.flush();
            out.close();
            out = null;
      }
      catch(IOException e)
      {
            Log.e("tag", "Failed to copy asset file: " + filename, e);
      }     
}
private void copyFile(InputStream in, OutputStream out) throws IOException
{
      byte[] buffer = new byte[1024];
      int read;
      while((read = in.read(buffer)) != -1)
      {
            out.write(buffer, 0, read);
      }
}

}

Note: Don't forget to add permission in AndroidManifest.xml

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