A sealed expression tree in Java
Java interview exercise on Records, classes & interfaces, difficulty 2 of 3. Solve it in the editor beside a worked example, or cold in Interview mode with the example hidden; the tests run the moment you do.
Write sealed interface Expr permits Num, Add, Mul with records Num(int value), Add(Expr left, Expr right) and Mul(Expr left, Expr right), and eval(expr) that computes the value with a switch over the sealed type (recursive for Add and Mul).
Each exercise pairs a worked example with a twin problem to solve in a real editor; the tests run the moment you do. Interview mode hides the example: solve it from memory, without the hint or the solution, and it counts as solved cold. Progress is saved in your browser, and in your account if you sign in.