class Main{
    void main(){
        IVisitor v = new Visitor();
        FooBar f = new FooBar();
    
        f.accept(v);
    }
}

interface IVisitor{
    void visit(Foo f);
    void visit(FooBar fb);
}

interface IAcceptor{
    void accept(IVisitor v);
}

class Visitor implements IVisitor{

    @Override
    public void visit(Foo f) {
    }

    @Override
    public void visit(FooBar fb) {
    }
}

class Foo implements IAcceptor{

    @Override
    public void accept(IVisitor v) {
        v.visit(this);
    }
}

class FooBar implements IAcceptor{

    @Override
    public void accept(IVisitor v) {
        v.visit(this);
    }
}