Visitor in Java - Code Recipes

Design patterns / Behavioral patterns

interface Element {
    void accept(CarVisitor v);
}

//ConcreteElement
class Engine implements Element {
    public void accept(CarVisitor v) {
        v.visit(this);
    }
}

//ConcreteElement
class Wheel implements Element {
    private int number;

    public Wheel(int number) {
        this.number = number;
    }

    public int getNumber() {
        return number;
    }

    public void accept(CarVisitor v) {
        v.visit(this);
    }
}

//ConcreteElement
class Car implements Element {
    private Element[] items = {
        new Engine(),
        new Wheel(1), new Wheel(2),
        new Wheel(3), new Wheel(4)};

    public void accept(CarVisitor v) {
        for (Element e : items) {
            e.accept(v);
        }
        v.visit(this);
    }
}

//Visitor
interface CarVisitor {
    void visit(Engine engine);
    void visit(Wheel wheel);
    void visit(Car car);
}

//ConcreteVisitor
class TestCarVisitor implements CarVisitor {
    public void visit(Engine engine) {
        System.out.println("test engine");
    }

    public void visit(Wheel wheel) {
        System.out.println("test wheel #" +
            wheel.getNumber());
    }

    public void visit(Car car) {
        System.out.println("test car");
    }
}

//ConcreteVisitor
class RepairCarVisitor implements CarVisitor {
    public void visit(Engine engine) {
        System.out.println("repair engine");
    }

    public void visit(Wheel wheel) {
        System.out.println("repair wheel #" +
            wheel.getNumber());
    }

    public void visit(Car car) {
        System.out.println("repair car");
    }
}

//Client
var car = new Car();
var v1 = new TestCarVisitor();
var v2 = new RepairCarVisitor();

car.accept(v1);
car.accept(v2);

Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

The problem. A document is made of paragraphs, images and tables. The program must export it, count its words and check its spelling. If each operation is added as a method to every kind of part, the part classes grow with every new operation. The code of one operation is also spread over many classes.

The idea. Each operation becomes a separate object, called a visitor. A visitor has one method for each kind of element. Each element has one method that takes a visitor and calls the visitor's method for its own kind. So the right code runs for each pair of element and operation, without checks of the type. To add a new operation, you write a new visitor; the element classes do not change.

How to read the example. The elements are of several kinds. Each concrete visitor is one operation, with a separate method for each kind of element. The client code gives a visitor to the elements, and each element sends it to the method for its own kind. In most examples one element holds the others and gives the visitor to each of its parts. Two visitors then pass over the same parts, each with its own operation. In some languages the elements are plain data with a type tag, and a visitor is one function with a version for each kind.

When to use it. Use Visitor when the set of element classes is stable, but new operations on them appear often. Do not use it when new kinds of element are added often: each new kind needs a new method in every visitor.

A common mistake. Checking the type of the element with conditions inside one general visitor method. A new kind of element is then missed without any warning. Give each kind its own method.