$ javac src/SealedDemo.java && java SealedDemo     (17.0.20.1)
Circle[r=1.0]              area = 3.142
Square[side=2.0]           area = 4.000
Rect[w=2.0, h=3.0]         area = 6.000
permitted       : [Circle, Square, Rect]
isSealed        : true