Dear Sir, as you know, we can see "CalculatorDisplay.java" in git. if you open the file, you can see line 59, 60(calc.adjustFontSize((TextView)getChildAt(0)) in here, I have a question. I searched for getChildAt, which it is in ViewGroup class. both android.widget.ViewGroup and android.view.TextView are sibling and two class received inheritance from android.view.View in addition, until now, I know that it is possible that typecasting is inheritance relative. so I can not understand why (TextView)getChildAt(0) is Ok?? if someone know this reason, can you explain for me?