recordandsealed

package com.example.expression;
​
public sealed interface Expr
    permits ConstantExpr, PlusExpr, TimesExpr, NegExpr {...}
​
public record ConstantExpr(int i)       implements Expr {...}
public record PlusExpr(Expr a, Expr b)  implements Expr {...}
public record TimesExpr(Expr a, Expr b) implements Expr {...}
public record NegExpr(Expr e)           implements Expr {...}