about typecasting in android source

[proandceo]

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?