File: Expr.java

sealed interface Expr
    permits Expr.Add, Expr.Sub, Expr.Mul, Expr.Div, Expr.Const {

    record Add(Expr left, Expr right) implements Expr {
        public double eval() {
            return left.eval() + right.eval();
        }
    }

    record Sub(Expr left, Expr right) implements Expr {
        public double eval() {
            return left.eval() - right.eval();
        }
    }

    record Mul(Expr left, Expr right) implements Expr {
        public double eval() {
            return left.eval() * right.eval();
        }
    }

    record Div(Expr left, Expr right) implements Expr {
        public double eval() {
            return left.eval() / right.eval();
        }
    }

    record Const(double value) implements Expr {
        public double eval() {
            return value;
        }
    }

    double eval();

    default String str() {
         return switch(this) {
            case Add(Expr l, Expr r) ->
                String.format("(%s + %s)", l.str(), r.str());
            case Sub(Expr l, Expr r) ->
                String.format("(%s - %s)", l.str(), r.str());
            case Mul(Expr l, Expr r) ->
                String.format("(%s * %s)", l.str(), r.str());
            case Div(Expr l, Expr r) ->
                String.format("(%s / %s)", l.str(), r.str());
            case Const(double value) ->
                String.format("(%f)", value);
        };
    }

    public static void main(String[] a) {
        var expr = new Add(
            new Mul(new Const(23.44), new Const(Math.PI)),
            new Div(new Const(3.9), new Const(11.33)));
        System.out.printf("%s = %f\n", expr.str(), expr.eval());
    }
}


$ ./javac --enable-preview --source 19 Expr.java
Note: Expr.java uses preview features of Java SE 19.
Note: Recompile with -Xlint:preview for details.
$ ./java --enable-preview Expr
Error: LinkageError occurred while loading main class Expr
java.lang.ClassFormatError: Method $proxy$left in class Expr has illegal modifiers: 0x1008