VisitorPattern

1ItemVisitor.java

public interface ItemVisitor<T> {
    T visit(Box box);

    T visit(Container container);
}

public interface Item {
    <T> T visit(ItemVisitor<T> itemVisitor);
}

public record Box(int size) implements Item {
    @Override
    public <T> T visit(ItemVisitor<T> itemVisitor) {
        return itemVisitor.visit(this);
    }
}

public record Container(Item[] items) implements Item {
    @Override
    public <T> T visit(ItemVisitor<T> itemVisitor) {
        return itemVisitor.visit(this);
    }
}

2PrinterVisitor.java

public class PrinterVisitor implements ItemVisitor<Void> {
    @Override
    public Void visit(Box box) {
        System.out.print("*Box");
        return null;
    }

    @Override
    public Void visit(Container container) {
        System.out.print(" Container[");
        for (Item item : container.items()) {
            item.visit(this);
        }
        System.out.print("]");
        return null;
    }
}

3WeightVisitor.java

public class WeightVisitor implements ItemVisitor<Integer> {
    @Override
    public Integer visit(Box box) {
        return box.size();
    }

    @Override
    public Integer visit(Container container) {
        int sum = 2;
        for (Item item : container.items()) {
            sum += item.visit(this);;
        }
        return sum;
    }
}