The Visitor design pattern in Java

VPT.java

import static java.util.Arrays.asList;

interface BaseVisitor {

    default void visit(Base base) {
        if (base != null) {
            base.foo();
            System.out.println();
        }
    }

    default void visit(Derived derived) {
        if (derived != null) {
            derived.foo();
            derived.bar();
            System.out.println();
        }
    }
}

class Base {

    private static int counter;

    final int id = ++counter;

    void foo() { System.out.printf("Base(%d)::foo%n", id); }

    void accept(BaseVisitor v) { v.visit(this); }
}

class Derived extends Base {

    void foo() { System.out.printf("Derived(%d)::foo%n", id); }

    void bar() { System.out.printf("Derived(%d)::bar%n", id); }

    void accept(BaseVisitor v) { v.visit(this); }
}

public class VPT {

    public static void main(String[] args) {
        BaseVisitor v = new BaseVisitor() {};
        asList(new Base(), new Derived(), new Derived(), new Base()).stream().forEach(b -> b.accept(v));
    }
}