A sealed shape family (Java 17 style)
A sealed class or interface lists exactly which classes may extend or implement it, with permits. The family is closed: the compiler and JVM reject any other subclass, and (from Java 21) a switch over a sealed type can be checked for completeness without a default.
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Practice questions
Write the code in the editor, run it, then open the model answer to compare.
Model sealed interface Expr permits Num, Add, Mul with records, and write a Java 17-compatible eval(Expr) using instanceof patterns. Evaluate (2 + 3) * 4.
Model traffic-light states as a sealed interface with three records, and write next(Light) that cycles Red → Green → Amber → Red. Print four steps from Red.
Write a sealed class Account with final subclasses Savings and Current, then print Account.class.getPermittedSubclasses() names.
Explain it without notes
What do sealed, permits, final and non-sealed each do?
Where must permitted subclasses live?
How do sealed types make switch statements safer?
How is sealing enforced at run time?
Expected output
Circle[r=1.0] -> 3.14
Square[side=2.0] -> 4.00
Rectangle[w=2.0, h=5.0] -> 10.00
Shape is sealed: true
permits Circle
permits Square
permits Rectangle