How would you save Activity state during a screen rotation?

When your orientation changes, you don’t have to manually change to the landscape layout file. Android does this automatically for you. When orientation changes, Android destroys your current activity and creates a new activity again, this is why you are losing the text.

Basically, whenever Android destroys and recreates your Activity for orientation change, it calls onSaveInstanceState() before destroying and calls onCreate() after recreating. Whatever you save in the bundle in onSaveInstanceState, you can get back from the onCreate() parameter.

private TextView mTextView;
private static final String KEY_TEXT_VALUE = "textValue";
@Override
protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  mTextView = (TextView) findViewById(R.id.main);
  
  if (savedInstanceState != null) {
      CharSequence savedText = savedInstanceState.getCharSequence(KEY_TEXT_VALUE);
      mTextView.setText(savedText);
  }
}

@Override
protected void onSaveInstanceState (Bundle outState) {
   super.onSaveInstanceState(outState);
   outState.putCharSequence(KEY_TEXT_VALUE, mTextView.getText());
}