show an alert dialog box on android

AlertDialog.Builder builder = new AlertDialog.Builder(YourActivity.this)
    .setTitle("Alert Title")
    .setMessage("Alert Message")
    .setCancelable(false)
    .setPositiveButton("ok", new OnClickListener() {
      @Override
      public void onClick(DialogInterface dialog, int which) {
            // Code before AlertDialog closed
      }
    }).show();