The visitor design pattern

The visitor design pattern provides a method of separating an algorithm on an object and the object's actual class implementation. This allows the programmer to easily follow the open/closed principle;

software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification

Object-Oriented Software Construction, Bertrand Meyer

That is, modifying an entity's behaviour without modifying the underlying source code. Following the open/closed principle provides many quality-related benefits as the original code never changes.

Benefits

• Follows the open/closed principle
• Allows a new operation to be defined without changing the implementation of the class
• A visitor object can have state

Drawbacks

• If a new visitable object is added then all visitors need to be modified

Double dispatch

The visitor pattern is one way of implementing double dispatch in a language that doesn't support it. Double dispatch is a method of dispatching a function call to a particular concrete function based on two object parameters. To understand this better, it's basically a method of implementing the following:

(A, B).func();

It may seem like function overloading can accomplish this same task, and in some cases that is true. However, properly implementing double dispatch solves a problem that can occur due to the compile-time evaluation of calls. For example, consider this code:

class Paint { }
class RedPaint extends Paint { }

class Car {
    public void paint(Paint paint) {
        System.out.println("Car painted");
    }

    public void paint(RedPaint paint) {
        System.out.println("Car painted red");
    }
}

public class Program {
    public static void main(String[] args) {
        Car car = new Car();
        Paint paint = new Paint();
        Paint redPaint = new RedPaint();

        car.paint(paint);
        car.paint(redPaint);
    }
}

The output for the above program is not as expected:

Car painted
Car painted

The second line should say Car painted red since RedPaint is used but instead uses the implementation of its super type Paint. This is because the function overloading on Car.paint(...) is done at compile-time and uses the redPaint variable's boxed type Paint.

To solve this problem the visitor pattern can be applied which has a single method Car.paint(Paint) that calls the Paint.paint(Car) interface. The implementation chosen depends on what the type is at runtime, not compile-time.

class Car {
    public void paint(Paint paint) {
        paint.paint(this);
    }
}

class Paint {
    public void paint(Car car) {
        System.out.println("Car painted");
    }
}

class RedPaint extends Paint {
    public void paint(Car car) {
        System.out.println("Car painted red");
    }
}

Producing the expected output:

Car painted
Car painted red

This works because instead of relying on function overloading on Car.paint(Paint) to dispatch the call which is evaluated statically at compile-time, the function Paint.paint(Car) is used which is virtual and evaluated dynamically at runtime.

UML diagram

Code

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public interface ElementInterface {
    public void accept(VisitorInterface visitor);
}

public class ConcreteElementA implements ElementInterface {
    public void accept(VisitorInterface visitor) {
        visitor.visit(this);
    }
}

public class ConcreteElementB implements ElementInterface {
    public void accept(VisitorInterface visitor) {
        visitor.visit(this);
    }
}

public interface VisitorInterface {
    public void visit(ConcreteElementA element);
    public void visit(ConcreteElementB element);
}

public class ConcreteVisitor1 implements VisitorInterface {
    public void visit(ConcreteElementA element) {
        System.out.print("1 visits A");
    }

    public void visit(ConcreteElementB element) {
        System.out.print("1 visits B");
    }
}

public class ConcreteVisitor2 implements VisitorInterface {
    public void visit(ConcreteElementA element) {
        System.out.print("2 visits A");
    }

    public void visit(ConcreteElementB element) {
        System.out.print("2 visits B");
    }
}

Usage examples

Here are a few example of where you could use the visitor pattern:

• A 'screen painter' object (the visitor) that paints several widgets to the screen (the elements).
• A HTML parser (visitor) that parses HTML nodes (the elements).