android.content.res.Resources$NotFoundException: String resource ID Fatal Exception in Main

Move

Random pp = new Random();
int a1 = pp.nextInt(10);
TextView tv = (TextView)findViewById(R.id.tv);
tv.setText(a1);

To inside onCreate(), and change tv.setText(a1); to tv.setText(String.valueOf(a1)); :

@Override
protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);

  Random pp = new Random();
  int a1 = pp.nextInt(10);

  TextView tv = (TextView)findViewById(R.id.tv);
  tv.setText(String.valueOf(a1));

}   

First issue: findViewById() was called before onCreate(), which would throw an NPE.

Second issue: Passing an int directly to a TextView calls the overloaded method that looks for a String resource (from R.string). Therefore, we want to use String.valueOf() to force the String overloaded method.