InlinedVisitor.java

abstract class Query {

    class Sum {

    }

    class Add {

    }

    void process(Consumer<Sum> onSum, Consumer<Add> onAdd) {
        Scanner s = new Scanner(System.in);
        int n = s.nextInt();
        for (int i = 0; i < n; i++) {
            String word = s.next();
            switch (word) {
                case "A":k
                    onAdd.accept(new Add());
                    break;
                case "S":
                    onSum.accept(new Sum());
                    break;
                default:
                    throw new IllegalStateException("Bad input: " + word);
            }
        }
    }

}

Visitor.java

package com.company;

abstract class Query {
    interface Visitor<T> {
        T visitSum(Sum sum);

        T visitAdd(Add add);
    }

    abstract <T> T accept(Visitor<T> visitor);

    final class Add extends Query {
        final int amount;
        final int left;
        final int right;

        Add(int amount, int left, int right) {
            this.amount = amount;
            this.left = left;
            this.right = right;
        }

        @Override
        <T> T accept(Visitor<T> visitor) {
            return visitor.visitAdd(this);
        }
    }

    final class Sum extends Query {
        final int left;
        final int right;

        Sum(int left, int right) {
            this.left = left;
            this.right = right;
        }

        @Override
        <T> T accept(Visitor<T> visitor) {
            return visitor.visitSum(this);
        }
    }

}